Using *over* 100% of our solar battery capacity in one day!

Please imagine I have an extreme-YouTube-shocked-face as this post's thumbnail image.

We recently got a 4.8kWh solar battery installed. Batteries are still somewhat complex beasties. In order to prevent damage to the internal structure, a BMS (Battery Management System) ensures that each cell in the battery gets a fair share of wear and tear. One side effect of this is that our battery never dips below 10% charge.

So our usable capacity is 90% of 4.8kWh - which is 4.32kWh.

The other day my solar panels sent me a message on Mastodon saying:

5.9kWh? Oh buggeration, my shonky Python code has messed up. Right? Wrong!

Here's what the battery did over a 24 hour period:

We started at midnight with a battery about 30% full. Throughout the night it discharged into our circuits, getting down to 10%.

Come sunrise, the battery started charging. It had a short discharge as we turned on temporary loads like the kettle.

By about 1200 it was fully charged - only to start discharging again when we started cooking in lunch, using the dishwasher, etc. Luckily there was enough residual sunshine in the afternoon to top it up to 100% again.

Then, throughout the afternoon and evening, the battery did its job and powered our WFH laptops, TVs, game consoles, electric showers, ovens, etc. Here's what our energy use looked like:

And that's how our 4.32kWh usable became 5.9kWh actual in a single day.

The battery's maximum charge/discharge rate is 2.4kW. So theoretically we could do a charge/discharge cycle 6 times per day and use 28.8kWh - but that is unlikely to make economic sense unless electricity costs fluctuated wildly throughout the day.

So, depending on your usage patterns and the variability of energy prices, you don't necessarily need a battery spec'ed to 100% of your average daily use.