How’s your Solar in Winter? (Author: The Economist)
Winter sneaked up on our solar electricity system during July 2015. We hardly took notice of the change in the length of a day before installing solar and now it matters. We lose up to 6 hours in daylight from the longest day in summer to the shortest day in winter. That is a lot. The days shrink by almost 2 minutes per day, that’s about 20 minutes every 10 days! The result is that the supply of solar electricity shrinks as the days get shorter and the demand for reservoir electricity increases as the nights get longer.
The daylight and night time effects combine with a third negative, a drop in temperatures for a triple punch to the solar electricity system. The drop in temperature means that all heat based electricity needs increase and here I exclude home heating. The heat pump takes 3 hours to get the geyser temperature to 60 degrees when in summer it would take between 1.5 and 2 hours to do the same job. The kettle takes a few minutes more every time it is used. The dishwasher has to contend with much lower temperature water inflows to be heated up and the tumble dryer also has to work harder to dry the clothes. It all adds up to increased electricity demand when days get shorter and reservoir needs increase.
Winter is when you find out how robust (or not) your solar electricity system is. So, let’s look at supply of electricity from the solar panels.
There is one additional observation with regards to supply in winter which matters for us here in Gauteng South Africa. Winter sees atmospheric interference which causes hazy mornings and afternoons. So the solar electricity production is muted even with good sunshine at around 08h30. Once the hazy conditions clear up around 09h30 we experience a steep rise in electricity production. Sadly the loss of electricity generation due to haziness has a significant effect on total daily electricity production in winter and haziness is the norm not the exception, so we show hazy conditions on the graph as the “best” case. The electricity generation in winter is about 55% of summer generation with hazy conditions, but on clear winter days we can get around 65%-70% of summer electricity production.
Overcast days in winter are terrible for solar electricity production but fortunately Gauteng is a summer rainfall area with very few overcast winter days. We’ve discussed generator back-up in our first article here, Part 1: How to go off grid permanently (The System Set-up). The change is season requires a resource management response from us. We have to use the electricity with more care with the most important effect having to spread the use of electricity. An example is not to do the washing only once a week, which requires the tumble dryer to run 4 times during the day but to spread the washing over two non-consecutive days. Thus, the tumble dryer runs only twice on those days. Demand management will be the subject of our next post.
Here is a table of our summer and winter electricity supply and demand data. Keep in mind that it applies to our system and our household needs, which for each household would differ.
Our solar electricity system can supply up to 56.5kWh electricity per day in high summer. We usually only need around 20kWh per day in summer on high use days, which means that we have surplus supply of more than double our needs. Here the “use it or lose it” principle applies so one can store it in ever larger battery banks or use it for unusual needs such as cooling the house (aircon) or processing of excess fruit and vegetables into jams or for drying (which uses the oven), etc.
In winter the picture changes fairly dramatically. We produce only around 31kWh electricity per day while a high use winter day can easily get to 28kWh. Our winter surplus now falls to only about 3kWh per day slightly more than a 10% surplus margin. Our daylight supply in deep winter is tight and our overnight reservoir is also under pressure. I’ve discussed the battery banks here, Part 2: Living with Solar in Gauteng, South Africa. Our solar electricity supply is adequate albeit tight as the few high winter weeks are manageable and soon after we are back to ever increasing surpluses as the length of days increase by 20 minutes every 10 days.
The solar experience so far is a positive adventure with no load shedding and with significant economic benefits!