May-October – Our solar battery data

So, here’s our 6 month update of life with Bert our Wattstor battery (May to October). Every month we learn new things.

As well as the shorter days, October has been rather misty and murky but still our 4 kW PV panels are often generating enough during the middle of the day to cover our energy demand in real time and, after dark, Bert-the-battery is still often able to service our evening cooking (which is mostly done on electric).

The first chart shows daily Wh averages – our daily demand, the amount generated by the solar PV, the amount we’ve had to import from the grid and the amount we’ve exported to the grid because it’s more than the battery can absorb at any point.


Not surprisingly, our daily demand has now crept up to an average of 8.5 kWh/day. This increase (we think) is a combination of  lighting and the pump for the boiler. We’ve also had two lots of visitors in October which always makes a difference!

The solar is now dropping significantly (5.2 kWh/day) because of shorter daylight hours, the sun lower in the sky and it disappearing behind the trees and October being rather dull.

We’ve imported almost twice the amount from the grid than we did in September (5.o kWh/day). One big difference is that our Twizy (which also has a 6kWh battery) and charges at 2kW can no longer charge reliably directly from solar during the day so we have switched to charging the Twizy from the grid after dark to avoid pointlessly taking energy from one battery to fill another. Looking at these data certainly brings home to you what it would be like if we owned a full sized electric vehicle. They are energy hungry beasts and no domestic solar system is going to be able to cater for them all year around. Fortunately, the Twizy (which manages all our local journeys) operates at 140 Wh/mile which is why we chose it.

And, finally, the system has rarely been in enough solar credit during the day to export to the grid rather than filling the battery. So, the amount we have exported has dropped to 1.2 kWh/day which means we are not currently contributing much to our fellow citizens! (Neither incidentally are the many domestic hydros around our area as the mountain streams have dried to a trickle given the lack of rain for a month or more).

Another big change is one that has been programmed by the Victron system. The lead acid batteries only ever operate between 50% and 100% capacity. In the summer, the batteries would often fill to 95-100% during the sunshine hours and then empty each night down to ~50% before switching off. Once the solar stopped providing that many Wh to the battery, the battery started switching off at around 70% – 75% . It appears to be estimating the amount it’s likely to manage top up by the following day and aiming to cycle around in the top end of its limit (rather than the bottom) to best preserve the battery.

The 2nd chart shows the proportion of our daily electricity provided by each source and how that is changing each month:-


As already mentioned, the amount we are now taking from the grid has almost doubled from September but we are still getting 40% of our demand via solar and this would be down to  21% without a battery. It’s interesting that the split between solar (direct) and solar time-shifted via the battery remains roughly equal. Will that still be true in mid-winter?

About Alison Kidd

Research Psychologist
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