Having the Powerwall installed has made two things very obvious. First, just how much solar energy is available even on a short sunny winter day. And second, how much of that goes to waste with just a solar PV array.
The days are slowly, slowly getting longer, and just four weeks after solstice there is a noticeable difference in generation. Last Friday was a day of clear blue skies from dawn to dusk, my 3.8kW solar PV peaked at 2.5kW generation, and by the end of the day the Powerwall had reached 50% charge starting from empty.
Chart 1 Shows PV generation (yellow) starting about 8am. Soon after grid draw (white) drops to zero and stays there for the rest of the day. (How excellent is that?) While the sun shines the PV runs the house and also charges the Powerwall (green). When the sun sets the Powerwall runs the house for the rest of the day.
That’s not the end of it because a 50% charge is enough to (almost) run the house through the night until the morning. I say almost because you can see four little blips between about 2am and 6am where there is a very small draw from the grid. The Powerwall will be at a very low charge state by then and I’ve seen before how discharge appears to gracefully decline rather than just stop.
I’m still not sure what those 1kW spikes are at night. I suspect the chest freezer but am not sure how to prove it.
That 50% charge very nearly carried the house through a full 24 hours. Had the next day been a sunny one that would have carried on and we’d have as near as dammit been off-grid in January. This being winter in England there’s been nothing but grey skies, sleet and rain since then, with very little PV generation.
Even so, I’m impressed. 50% is not quite enough, but 60% should do it. Give it another few weeks and cold clear February skies, maybe a 60% charge, and we should be there. It’s going to happen and I now believe it will be much sooner in the year than I first expected. A 100% charge is some way off yet, but when that happens I am excited to see how the other gadget I had installed along with the Powerwall behaves.
Watch this space.