Some Solar Numbers

Looking at some rough numbers to size out a solar system for my house. We are probably on the high end of power usage since we have a relative large house and a pool pump. I also did not build this house myself, and there are lots of choices, like many American houses, that I would not have made. That said we have done a bit of work to save energy with efficient appliances, including air conditioners and some solar shading, which really made a big difference.

Of course, we won’t be putting solar on this house, since we intend to leave Texas in a few months. This is just to get a feel for the sort of numbers I would be looking at for a complete, 100%, more or less off grid solar installation.

From August 2019 tbrough July 2020, a year of relatively normal use (no ice storms / blackouts) we used a somewhat surprising 35,390 kWh for the year. This comes to just under 100 kWh per day and about 4 kW constantly over the year. Back of the envelope I would figure 3x to 4x this value for the panels, since the sun doesn’t shine all day. Let’s be really generous and call if 15 kW of solar panels.

Even at 400W a panel that is about 40 panels. And I have been working from a yearly average. Probably best to look at summer peak. It is interesting that summer peak in Texas is about twice what we use in the winter. We have propane heat so this says that half of our bill in the summer is air conditioner.

Lets say summer is 4,000 kWh over 30 days or about 133 kWh per day or 5.56 kW continuous. About 40% more than the average. Now we are looking at over 55 x 400W panels. That is quite a bit. But we could trim that down a bit.

Battery storage would be minimally enough to get through the night. Here is where it gets tricky. You probably aren’t going to run the air conditioner as much at night. And the pool pump will also be off. In fact that 5.5 kW might be too rough here. I need to see if I can get daily data from my power company. But let’s soldier on. Batteries need to supply 5.5 kW for 16 hours, or 88 kWh of storage. A Tesla Powerwall is under 14 kWh, so that is over six Tesla power walls. I’m thinking that isn’t going to happen. Probably need to look at day vs night consumption. Somebody check my math 😀.

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