Guest Post by the Economist 🙂
The following article was not written by myself (AKA Whisker Flowers) but by our Economist in the house. It is a very comprehensive article on how we got our household solar ready (installed in 2014 and still running! ). This is a full length article but well worth the effort for all those interested in going solar or even for those that just want to lighten the electricity bill. At the end of the article the savvy economist even included a Solar System Viability Calculator available from my Google Drive account (link below article). This is a nifty little excel spreadsheet that basically shows you what type (wattage wise) solar system to implement for your current usage and how long it will take you to pay off the system over its lifetime. Remember that this is just an example of what we did; each household will be different and some adjustments will have to be made for your specific needs.
Solar articles written during the course of installation (2014) and 2015. Thus cost estimates for purchasing solar may be less (and you’ll get better tech). Municipal charges have increased exponentially too!
Solar Power in South Africa
A real life true journey of a South African family to go off-grid on sustainable energy supply and usage.
We first looked at the potential to cross over to solar power in November 2009. Eskom had its first blackouts and load shedding activity in 2008 and the writing was on the wall. All indications were that electricity supply in South Africa was a problem, which will probably become increasingly problematic in the future.
It was somewhat bewildering to try and understand how to go about transitioning to off-grid electrical usage sustainable living. The question was not just how to go from Eskom power to Solar Power. That could be achieved in 2009 without much thought and planning if we were prepared to pay about R650,000-00 for a system which would marginally be able to sustain the transition. It was simply not economically viable to do.
So what, we concluded is that solar is significantly under development and will become more efficient and achieve international economies of scale (it will become cheaper as technology improves and more people starts buying it) at some point in the future. The cost of buying electricity was set to increase significantly with Eskom making requests to the government for huge and cumulative price increases. It was logical that the improvements in price and efficiency in supply on the one hand and the rising cost of electricity will probably ensure an economic viable transition point in the future. We did not know when, but we started a process of getting ready to make the transition if it happens.
We are not green fanatics but do strive for responsible living within economic viable parameters.
Preparing to go to Solar
We had no real road map of how to go about it and mostly did our own path finding. Here is what we did:
- Know what you use.
We established how much electricity we were using. The easiest way is to just look on your electricity bill and see what your average daily usage is. Take the last 12 month average to make sure you are getting an average which accounts for all four seasons as consumption can increase quite dramatically in winter.
We were at around 40kWh average per day in summer and around 50kwh average in winter. Convert that to a 30 day period, 30×40=1200kWh for a 30 day period in summer or 30×50=1500kWh for a 30 day period in winter. Hereunder is a cut-out copy of our electricity usage portion to show you what to look for on your electricity bill. Note that we have used an average of 14.389kWh from Dec 2014 to Jan 2015 which is 14.389×30=431.67kWh for a 30 day period in high summer. We will tell you how we reduced consumption from 1200kWh per 30 day period in summer to 432kWh.
- Understand your usage.
Understanding what kWh generally means to us “normal consumers” as opposed to electrical specialists is important. The “w” stands for watts which is the standard measurement for electricity as is a litre the standard unit of measure for liquids such as water. Watts are important to us consumers and City Power charges us per watt but as we use a lot of watts per month it is easier to measure in kilo watts which is 1000 watts.
Electricity flows similar to water so to measure it we need to know how much flow we used over a specific period of time. So we will take watts per hour (Wh) or kilo watts per hour (kWh) to measure how much we used of the flow. Electrical specialists would also use volts, load, kWA, kWV, etc. but to keep it relatively easy to follow we will try to stick to watts, kilo watts and kilo watts per hour (kWh).
It is important to understand how to measure as we measure a volume (w or kW) and over time (an hour). So if I use a kettle rated at 3000w then I still need to measure how long it takes to boil the water. Say it boils the water in 5 minutes then I will use 3000w for 5 minutes. In terms of a kWh quantity used it will be 3000w x (5minutes/60minutes) = 250Wh or 0.25kWh. If my geyser use 4kW and it takes 2 hours per day to keep hot water available, then my geyser will use 4x2hours=8kWh per day. A light may use 100w which is, compared to the kettle and the geyser, a small usage but if that light is on overnight for 12 hours then it will consume 100×12=1.2kwh which is a lot more than the kettle usage of 0.25kwh.
The important point to understand is that it is the combination of (a) how long you use an item and (b) how much electrical power it uses which will drive all your decisions if you chose to attempt to go off-grid.
- Understand electrical usage spikes and electrical usage hogs.
Electrical equipment each have unique ways to use the electricity flows. I have an electric lawnmower which is rated at 1500w but the moment it is turned on (when the blade is turning to mow the lawn), in that moment it draws the full 1500w but once it is running it drops back to about 600w. So every time it starts up it has an electricity spike (start-up power surges). We need to understand electricity spikes more-or-less as it is important once one switches over to solar but it is often over played by sellers of solar energy. We have many electrical appliances and units in our house. Not all spike at the same time nor do we use everything at once and we are fully capable of planning our electricity usage of energy hogs to avoid them overlapping. Energy hogs are items such as geysers, stoves, ovens, tumble dryers, dishwashers, kettles, hair blow dryers and laser printers, basically all appliances which can use 1500 watts or more. More about this in the discussions on solar.
- Reduce total monthly usage.
The principle here is that we all generally have some hugely energy inefficient equipment in our houses as we only start gaining efficiency once we have developed an energy efficiency awareness. Target energy hogs first and particularly energy hogs which are used often or for long spells during the day. We’ll deal with the items in our house and on our journey. You will probably have different needs and a different time table so we hope that our journey will help you plan your journey but as a rule of thumb try to get your house to 200kWh per adult/teenager and 100 kWh per child per 30 day period (so two adults and 2 children will be 600kWh per month or an average daily use of 20kWh for the family). The simple equipment rule is to make sure you have A+ to A+++ (EU energy efficiency standards) energy efficient rated appliances wherever you can and change old inefficient equipment sooner rather than later as the energy savings generally will pay for the replacement of older inefficient appliances.
See the label hereunder for a refrigerator downloaded from Wikipedia, it tells you that you can expect the refrigerator to use 280kWh per annum. So average daily usage in kWh can be calculated at 280kWh divided by 365 = 0.77kWh per day and this refrigerator is rated A++.
This is usually the number one energy hog in your house. A rough rule of thumb for Gauteng South Africa is 2hours at 4kWh per geyser per day. We had two 150 L geysers to deal with, one servicing the bathrooms and one servicing the kitchen. We have one other small 50 L geyser servicing outbuildings. That’s 16kWh per day, plus another 2kWh for the smaller one accounting for 18kWh or 45% of 40kWh per day.
Possible solutions considered by us was solar geysers, heat pumps, gas geysers or water heaters, induction geysers and in line induction tankless water heaters. Our solutions were to install a heat pump for the geyser servicing the bathrooms. The heat pump uses about 1kWh for about two hours per day to give us a 6kWh saving per day on that geyser. We experimented in the kitchen to see if the kitchen really needed hot water and found that there is hardly any need for hot water when all dish washing is done by a dishwasher (laundry in cold water in a washing machine). We switched off the geyser in the kitchen and have not bothered to look for another answer as we do not find a need for hot water there. We do have a small need for hot water in the outbuildings and have chosen a 3000w in line induction tankless water heater, which we have ordered from China and will install when (if?) it arrives. The induction water heater is rated a bit on the high side at 3000w but we have found that induction appliances usually use less than half the stated max rating. Induction heating is also very efficient so it will only be in operation for very short spells during the day and expected usage is around 0.5kWh per day.
Our geyser solutions got rid of 15.5kWh per day or 465kWh per month. These solutions may not work for you, what is important is to assess each energy hog separately and find a suitable solution for each. The heat pump and the induction in line water heater are compatible with solar power if properly planned for. We have programmed the heat pump to switch on at 10h00 and the water usually reach the desired temperature by 11h30 (in winter it will probably take twice as long). The heat pump will not run at night and we will have to provide a solar blanket for the geyser in winter to prevent excessive water heat loss overnight. We lose about 3 degrees per shower and about 4 degrees overnight.
Our first solution was to use gas for cooking. Gas is an efficient cooking alternative but we just did not like it and always felt that there are risks with gas cooking. When we initially investigated induction cooking we found that induction stove hubs were rated between 7500w and 9000w which was simply not compatible with our long term plans to convert to solar power. We reconsidered induction cooking just before taking the final steps with solar and questioned the 7500w rating. The fact is that single induction plates are rated between 1500w and 2000w but the user decides how many watts to deploy as the induction plate normally have settings from 200watts to 2000watts in increments.
We bought a single induction plate and started experimenting. Using the 1300watts as displayed on the LED screen we boiled water in about 3minutes, made mince curry in less than 15 minutes at about 600w, rice in around 15 minutes at around 800w and steamed vegies in around 8 minutes at an average 400w (cooking is not at a constant wattage so the references are averages). A cooked meal of meat, starch and vegies generally would be within 1500w usage and takes less than 30 minutes to prepare so a full meal would consume less than 0.75kWh (1.5kw divided by 30 minutes out of 60 minutes).
The experiments with induction cooking were a success and it was clear that induction cooking is compatible with solar provided it is used with understanding. We removed the gas stove and the electric stove and replaced it with three induction plates. We switched over to solar and so far the induction cooking is a breeze. We need to use the induction cooking a bit longer before we make a decision about an induction stove hub. We can control the usage better for now by using single induction plates.
Refrigerators can sometimes hang around for a long-long time. They are sneaky energy hogs as they are on-off 24 hours per day so even if they use low wattage they can use a lot per day. We had an old one of indeterminable wattage usage which we bought some time in the previous century. Older refrigerators can easily use in excess of 2kWh per day. We retired it and got an A+++ one in its place which uses less than 500w per day. Each appliance that we addressed had an impact on our electricity bill and I will later show our usage over the past two years.
4.4. Deep freeze
Same as with the refrigerator. It runs 24 hours per day on-off and old energy inefficient ones can be an energy hog using in excess of 2kWh per day while hiding in plain sight. Once again, we sold the old one off on ebay and got an A++ rated one.
4.5. Winter heating
We sold or threw away oil, bar and plate electric heaters and replaced them with gas heaters. We have no better energy efficient solution for deep winter blues for now but avoid the gas in favour of electric blankets (energy efficient!) and winter busting clothing where appropriate. South African winters are not that cold.
The oven is usually the number 2 energy hog after the geysers because it normally uses 2500kWh and one would have it on for hours at a time. Our old oven was one of the first appliances that we replaced with an A+ energy efficient oven with fans, plug in watts at 2500w. Fully compatible with solar provided one keeps the use mostly during mid-day when we have excess solar electricity input.
We retired all the kettles and use an induction friendly kettle on the induction plate at 1300w to boil water for tea and coffee. Water boils usually in under 3 minutes. The solar inverters indicate that the induction plate is actually only using around 800w during operation in spite of it showing 1300w on its LED screen setting. So we are boiling water at around 800w using induction as opposed to 3000w using an electric kettle using more-or-less the same time.
4.8. Microwave oven
We have an A rated microwave oven of 1000w but as it is used only for very short periods and often only occasionally per week we do not consider the microwave as an energy hog in our house.
We have outside spotlights (150w to 300w) for security with movement sensors which we have replaced with LED spotlights of 10w and 20w without movement sensors. A 300w spotlight if left on for 10 hours will consume 3kWh which will be a serious energy hog.
4.10. Room lights
We have even before 2009 replaced all lights with florescent lights. Lights are not an energy hog in our home even when we have all of them on at the same time. We do have a rule, if nobody is in a room then the light must be off because “nobody” has no need for light. We will probably change lights to LED lights over time but it is not a priority.
4.11. Computers with screens, modems and sound as well as TV’s with decoders.
Modern computers and computer screens are energy efficient. Judging from the activity on the solar inverters we estimate that a 400w powered computer with a 68cm LED flat screen and sound actually only use about 75-100w per hour in use. Latest LED flat screens for computers are powered with 9v similar to tablets and cell phones, all of which uses so little electricity it is not worth managing. Replace/retire older energy inefficient computer screens and TV’s.
4.12. Electric Lawnmower
Energy efficiency has not yet impacted sufficiently on lawnmowers and we have a large lawn to maintain. We judged that the lawnmower will only be used during the day when it is not raining so it will draw power when power supply from solar would be abundant. Our electric lawnmower is an energy hog with power spikes but we can manage all other larger energy using appliances not to compete with it when it is in use once a week , often only once every 2 weeks.
4.13. Washing machine
We replaced an old 8kg energy hog Speed Queen with an A++ washing machine and saved on both electricity and a lot of water.
Here is another huge energy hog hiding in plain sight. Our dishwasher is used often and it runs for long hours when in use. Dishwashers generally heats the water it uses and older models can easily use 2-3kWh per complete wash, rinse and dry. Replace with an A+ or better rated dishwasher and look for one capable of using heat exchange and a passive drying cycle (dries dishes without using electricity to “blow dry” the dishes).
4.15. Tumble dryer
This is another monster energy hog, usually the number 3 after a geyser and an oven but an old energy inefficient tumble dryer can easily be the number one energy hog. It uses a lot of energy and have long cycles but as drying clothes happens less often than washing dishes it would still rate after a dishwasher in monthly consumption in most homes. Same principles, get an A+ or better rated tumble dryer looking for heat exchange and condensation capabilities.
4.16. Vacuum cleaner
We have only about 140 square meters in carpets so the vacuum cleaner is not a material contributor to energy consumption in our home. It is rated at 1200w and does power spikes on start-up. Same rules, A+ or better rated for energy efficiency and watch for overlapping other energy hogs due to the energy spikes at start-up if planning for solar.
4.17. Pool pump
We do not have a pool or pool pump. It is generally the number one energy hog, worse than a geyser due to the long hours in operation, which can take 1.5-2kWh per hour for many hours a day (2kWh for 10 hours is 20kWh per day which is almost 40% more than our current average total electricity usage per day). You will probably have to deal with this one separately from your other electricity use but as we have not addressed this problem, we have no further comment on this item.
4.18. Other smaller electric appliances (irons, blow dryers, toasters, etc.)
Generally manageable with the A+ or better energy rating rule and avoiding too much overlapping for use under solar power conditions.
Your house should be “solar ready” once you have dealt with the energy hogs. Look for solutions which do the same tasks smarter and faster while using less absolute energy and taking into consideration the wattage rating, the actual wattage usage and the average watts used per hour.
We addressed each of the above mentioned items since 2009 while waiting for the cost of solar systems to drop.
Hereunder is a chart of our energy usage as per City Power who often estimate usage rather than reading the meters and then adjust the usage when they do have a reading (will not comment on very high estimations in high tariff months June, July and August with low estimations in lower tariff months in April and May as well as Sept, Oct. The seasonal effect is probably not nearly as dramatic as the estimations would have it). It is an erratic chart but the trend is down.
Our electricity usage has dropped from about 35kWh average per day or 1050kWh per 30 day month to 14.4kWh average per day or 435kWh average per month. We have implemented the process of appliance energy efficiency over a number of years as there was no need to accelerate the process given that solar economies of scale was and still is improving. We picked the oldest appliances first for retirement. Then the least expensive to replace. Then the appliances which if replaced will give the best energy saving. Each household and family will have their own needs and can implement an energy efficiency plan which suit their lifestyles and budgets. We had an objective to get consumption below 20kWh average per day with a very specific aim to go off-grid. Our economic viability assessment indicated that the payback period for a solar system is significantly increased if we have to continue to pay all the admin and other fees that the Metro adds to our electricity bill and therefore planned to go off-grid.
Evaluating your Solar needs
We have found that four significant variables drive the solar decision. We view a suitable solar system as one which will provide for our needs without having to make material lifestyle changes or suffering inconveniences while still being economically viable enough to repay the expense of installing the whole system within an 8 year period. We calculated our expected electricity costs for the next 10 years taking into account expected Metro and Eskom price increases. The total outlay for the solar system must be less than the number we get. Its economic implication is that if the solar system lasts mostly intact for say 15 years, it will be repaid in say 8-10 years and we will enjoy 5 years of “free” electricity. Say the batteries fail after 8-10 years then we will replace them and viability will not be effected though the quantum of economic benefit will be less. We must receive some economic compensation for the risk of capital outlay and investment made by installing the system.
- Monthly average usage measured in kWh (How many solar panels?).
We recognised that we probably have a higher electricity consumption in winter and concluded that we have a rounded off need for 15kWh average per day summer need and an expected 20kWh average winter need. The daily need indicated that we must generate at least 20kWh per day from solar panels under perfect conditions. It is expected that electricity production under overcast weather conditions will drop by at least 50% and at times can easily drop to only 10% of capacity. It is rare in Gauteng to have 3-5 days of heavy overcast conditions so we felt that production capacity at around 150% of 20kWh will be required. That meant we needed to install panels which at maximum capacity will generate 30kWh per day. The solar industry uses a 6 hour day as a measure so we calculated that we must produce 5kW electricity per hour (30kWh divided by 6 hours is 5kWh). Panels are rated in watts per hour. It is also expected that panels will lose some capacity over time (about 10% over 10 years) so we need to do a bit more than 5kWh to provide for inefficiencies during installation and degradation. That brought us to a choice of 24 solar panels of 250watts each, 24×250=6000watts or 6kWh. Reality is different but I will discuss that below.
- Maximum kW usage at any given point in time (How much inverter capacity?).
The basic rule of thumb is that the more appliances you wish to run at the same time the higher your “draw” capacity must be and it must include power spikes of your appliances. For example: Use a dishwasher at 1500w, a heat pump at 1500w, an induction plate at 1300w, a refrigerator at 200w, computer at 200w and a deep freeze at 400w all together and your “draw” will be 5.1kW (1500+1500+1300+200+200+400=5100w). Your system must allow that draw or else it will trip or you could even damage the system. You must also realise that each of those appliances may spike and while statistically highly improbable that they will all spike at the same time some appliances may very well spike at the same time. Another advantage of rated energy efficient appliances is that they generally have a much lower probability of spikes and the spikes are much reduced in intensity. Say provide another 1500w for spikes to the 5.1kW and it would indicate that you would need to cater for at least 6.6 “draw” in inverter capacity.
With energy efficient equipment in your house you can turn the problem on its head. Can you manage your appliance and electricity usage to ensure that the draw will probably never approach 80% of your draw capacity? If you can then that capacity will serve. We satisfied ourselves that we will be able to operate within an 8kW “draw” capacity made up of 0.5kW in core household needs (computers, refrigerator, deep-freeze, TV, decoder and standby appliances); heat pump 1kW, dishwasher 200w average operating usage with spikes giving a high tendency need of 1.7-2kW; and other appliances when needed. We will avoid drawing excessively when the lawnmower is in operation as it spikes a lot and often. We will avoid operating the oven, the three induction plates and the tumble dryer at the same time. That is what overlapping implies, be aware of the overlapping of energy hogs and manage that. Do not unduly stress the “draw” capacity of your system.
We have switched everything and all the lights on in our house excluding only the tumble dryer and the lawnmower and we were at a “draw” of 6.3kW so we know that we should not have any problem with a draw capacity unless someone does something really stupid. We have heard of a welder arriving at another solar powered home who just plugged in an 8kW welding machine and blew up the system with its draw and power spike. Running a welding machine with a huge load and spike off a solar system is “doing something stupid”.
Most households will struggle with a 4kW draw capacity. A 6kW draw capacity is probably fine but you will have to be extra careful with energy hogs. An 8 kWh draw is adequate but still requires some care with energy hogs. A system of 12kWh draw or higher will probably be carefree for most energy efficient households.
- Overnight kWh usage (How much battery capacity do I need?).
We need to draw all our electricity from a battery bank from sunset to sunup. We have alluded to “core needs” above. The question is which appliances draw power overnight or parts of the night? Some management is called for. We set the heat pump only to operate between 10h00 and 16h00 so it will never impact on our overnight need. A silly example is that the lawnmower will never operate at night. We will mostly use our energy efficient dishwasher overnight and we will use lights, computers, TV, decoder, the induction plates for cooking and making coffee, the toaster, small appliances, the microwave, the refrigerator and the deep-freeze. The refrigerator and the deep-freeze are core needs as they will run all night. Both are energy efficient and together they draw around 150-200w per hour. We found that after 12pm use of power drops to this level (around 200w) and remains there until early morning when we rise. The energy hogs are the inverter plates used for cooking and for coffee. The TV and computers can make a play for electricity hog status if used over long spells.
Every household will be different. We concluded that about 30% of our average daily use will take place at night which meant that we had to provide for at least 4.32kWh in battery power availability for summer (14.4×30%) and 6kWh for winter. As this is only a guestimate we felt that we would need to leave some spare capacity of say 1kWh for “just-in-case” indicating that we would need at least 7kWh. Solar batteries may not be drawn more than 50% of total capacity so we would need a 14kWh battery bank. We selected 24 solar batteries of 300Ah which configured for 48v would translate into a 14.4kWh battery bank (the calculation is a bit more complex and we use Ah to kWh conversion calculators available on the Internet or ask your solar provider to make the calculation – in the 48v configuration the total Ah x 2 gives kWh but other configurations will be different).
- Distances which electricity travels in your household. (Batteries voltage configuration?)
The batteries will be configured to suit your needs and as this is a bit more technical we will just mention that the basic rule is the longer the distances that the electricity must travel the higher the voltage configuration will be with 12v, 24v or 48v categories. There are other technical considerations also but for us a 48v system was required.
- Outliers define capacity needs (Robust test?)
This is where for instance weather starts playing a role. Our system is suitable for all our needs with some tolerances for above average use but what if we have days of overcast conditions, will our system be robust enough to cope?
The reality of the solar panels are that they start producing in excess of our core needs already around 06h00 (6am) in the morning and only switch over to battery power at around 19h00 (7pm). That means the panels are generating some electricity even if not at peak performance as early as 6am and as late as 7pm in mid-February in Gauteng. Hereunder is a simulated chart of the electricity production of the panels.
When we looked at the design we chose panels which will produce 36kWh per day, 6kWh for 6 hours. The reality is we will get closer to 54kWh under perfect conditions, 6kWh for 6 hours (36kWh) and an average of 3kWh for another 6 hours (18kWh). At 50% of perfect conditions we will produce 27kWh which is still more than sufficient and at 25% of perfect conditions the panels will produce 13.5kWh which is 94% of summer needs and 67.5% of winter needs.
We have had an opportunity to test the panels and batteries in real time under heavy overcast with rain conditions and estimate that we produced about 10.5kWh for that day which is 73% of our average daily use. The choice here is to avoid running energy hogs such as the oven, tumble dryer and washing machine which will result in below average usage and 10.5kWh would probably be sufficient or draw extra power from the batteries. Our current 14.4kWh battery bank will not cope with additional draw down as taking 4kWh for the day will leave only 3.2kWh for the night. It is a to-the-wire use of the capacity and a 2nd rainy day will mean that we would have to then significantly cut power consumption to say 4kWh for that day to leave spare capacity to charge batteries again. So we will have to provide spare capacity to overcome overcast and rainy conditions. The importance of overcast conditions requires more discussion, see further down.
We believe at this stage (we still need to live through a 2014 winter off-grid) that we have sufficient generating power.
The battery bank capacity is very tight at 14.4kWh and under prolonged bad weather conditions will certainly not be sufficient so we have decided to add another battery bank, which at this stage meant that we had to double the battery bank to 28.8kWh. We will now probably be over supplied from batteries generally but will still only have limited protection against overcast conditions.
Gauteng seldom has 3 or more heavy overcast days in succession. The same cannot be said for coastal towns for example. It is not economically viable to add another battery bank for the perhaps 3 or 4 times a year when three or more days of overcast conditions have depleted our available battery capacity (50% 0f 28.8kWh = 14.4 kWh). We have therefore added the ability to power the system with a 4kW petrol generator which we already own, having bought it to cope with load shedding. We have previously used gas at a cost of around R1200 per annum for cooking. We now use the solar system for cooking and do not expect that we would need a budget in excess of R1200 to cater for petrol to run the generator as back-up a few times per year. The generator will therefore not add a cost to the solar system and the previous gas budget will now be allocated to a generator back-up “fund”.
Testing the solar system against reality.
The first few days off-grid in 2014.
We have switched off the main power supply and went off-grid on the 14th of February 2014. The weather had been sunny and clear skies with only intermittent light cloud cover for most days, but we had had a few heavy overcast and rainy days. One full dark, rainy and heavy overcast day allowed us to stress test the system under very weak power generation conditions.
We seem to have had a general peak “draw” need of around 2.2kW mostly when we were preparing major meals (against an 8kW “draw” capacity). We could push the draw capacity to about 75% by using the oven, washing machine, tumble dryer and heat pump together with the core use (computers, refrigerator, deep freeze & TV) all at the same time. We would be pushing our luck if we were to add more energy hogs, for example the dishwasher or lawnmower into this mix.
We had been using around 3.6kWh for overnight needs at about 25% of our current 14.4kWh battery bank. The additional battery bank will mostly be deployed preserving the battery life (the lower the overnight draw cycles on the batteries, the longer they will last) and to provide a battery back-up bridge for less than perfect solar conditions. The second battery bank as installed soon after, but until then we were vulnerable to overcast conditions.
The batteries were recharged by about 10h00 in the morning under perfect conditions where after most of the power generation goes to waste (given our need for 14.4kWh we would could have excess electricity of 40kWh per day during sunlight hours in perfect conditions which we could sell back to the Metro if ever they have a viable and attractive arrangement in place).
We have to manage energy hogs as energy overlapping risks are mostly concentrated around the major energy hogs such as the oven, the tumble dryer, the lawnmower, two or more induction plates at one time and the dishwasher. We can responsibly have up to two major energy hogs running while the heat pump is on and can run three major energy hogs when the heat pump is done. Perfect and good conditions excess energy have opened the door to opportunities to put some of the excess power generation and supply to other uses such as using the oven more often for baking, providing air conditioning during the day, etc.
We will add updates if any significant event or changes were to take place (up-coming posts!).
Overcast conditions can dramatically impact solar power generation. Just light intermittent cloud movement can easily drop generation to 80% of perfect conditions, light cloud cover drops generation to 50% and all day long heavy cloud cover will drop generation to around only 10% of perfect conditions. Combine that with a need to bridge nights and it can cumulatively deplete the double spare capacity in just two- three days.
We used around 4kWh per night so we start with a need first thing in the morning to charge the batteries by 4kWh. We will also needed at least 7kWh for minimal electricity demand during the day. Perfect conditions for us is 54kWh electricity generation from the PV panels. It follows that our minimal PV panel generation need is 11kWh which is around 20% of perfect conditions (11/54=20%). We easily drop below 20% if we have a heavy cloud cover day. We can generate sufficient electricity at between 17% and 20% of perfect condition to survive a number of days if we conserved power. We were in trouble with the current 14.4kWh battery bank when generation drops to between 17% and 14% of perfect conditions. We could survive 1 day of heavy overcast conditions, but we will have to use back-up generator supply below 14% solar generation. A battery bank of 28.8kWh would give us 3 days if we managed consumption, but there after we would have to tap into back-up generator supply.
Gauteng conditions are solar friendly. You may have to increase your battery capacity should you experience a greater number of consecutive heavy overcast days from time to time. Heavy overcast periods of a week or more will be a challenge and you will probably be better served by a grid-tie solar system or grid back-up system should you have such weather in your area. You will experience energy shortage and hardship if you run out of power without back-up.
Testing our living conditions
We defined our conversion to solar power as solar power without significant lifestyle changes. I have read many blog posts of people who explained that they were using 25kWh average per day and went off-grid just to implement a solar system capable of supplying under perfect conditions around 10kWh per day with an overnight capacity of around 2kWh and a draw capacity of 4kW. Then they compare the cost of the system with their old usage of 25kWh and claim that they will repay the solar system in 5 years or less. Those are not accurate evaluations as they have significantly less electricity than before and though they often claim that they are living “normally” they would have had to make significant lifestyle changes.
We did not make significant lifestyle changes even though we acknowledge that we have to manage energy hogs overlapping and have self-imposed energy efficiency. Our cooking change from gas to induction had in fact been an improvement on our on-grid methods. We were not unduly budgeting our solar electricity consumption or how long we take a shower. We still mow the lawn with the energy hog lawnmower. We use the dishwasher at night and with the additional battery bank will also be fully capable (with care) of using the tumble dryer and oven (this one is a huge consumer so only sparingly) at night and on rainy days.
There is however not a right or wrong with regards to how you go about it particularly as budgets matter. We could afford a fully off-grid compatible solar system. Not everybody can afford such a charge and often people are quite content to make the lifestyle changes to fit into a smaller than previous on-grid usage implied. It does not matter, but the evaluation of the cost and payback must be done on a like for like basis. We were satisfied that we were true to our defined objective and that we had a like-for-like conversion to off-grid. In fact we have some surpluses due to the fact that we have to provide for outliers. Thus we have met our objective and have valuable economic utility beyond our objectives.
Testing economic viability
Here is our system budget:
We defined the system as suitable for up to 27kWh average usage per day or around 800kWh monthly usage. Factually we would mostly use around 14-18kWh for half the year and 20-24kWh for the other half of the year. The solar system must provide for the highest use and will be functional under stressed conditions provided we have generator back-up.
The economic viability model in excel is available at this link [Solar Power Viability Model] for download. These were our results back in 2014:
Our system will pay back the cost outlay in just under 7.5 years and even if we were to borrow the money it would still be economically viable. The present value of the system is R201,500 vs a lifetime value of the electricity purchased estimated at R689,068.
We share our experience in the hope that our evaluation and journey will assist you in your decision to properly assess the viability and practicality of sustainable solar off-grid electricity supply. We hope also that our explanations will demystify a lot of disinformation surrounding the use of sustainable solar electricity.
Having a fully functional solar system installed is a significant advantage to a homeowner. It adds significant value and appeal to your property. It is expected that you will be compensated in property price even if you do not stay in your house for the life-time of your solar system.
Our system was designed and installed with the assistance of Jurie Venter, cellphone 083 557 6031 and email firstname.lastname@example.org .