This post looks at what our solar PV system has actually done. A reminder that it is a relatively small system (compared to others reported here 
) of 5.84kW. Our house was continually occupied for this entire period, to put these results in context.
In those 4 months, our house in total consumed 2479 kWh, but we generated 3007 kWh Yay! We are in front already by 21%.
935 kWh of solar was used directly, 1071 kWh of solar was taken from the battery (i.e. when solar was not enough to meet demand). So having the battery has more than doubled our solar self-consumption. The battery is charged only from solar – there are a few blips where the grid puts some charge into the battery, but they are miniscule compared to the overall picture.
473 kWh was consumed from the grid – only 19% of all consumption. Critically, thanks to the battery, only 9% of that occurred at peak ToU rate (2pm-8pm weekdays). 26% of grid use was in the shoulder period, and 65% offpeak – which is mostly charging our LEAF and/or running the dishwasher (if we don’t or can’t do it during the day). We exported 847 kWh to the grid. We buy 100% Green Power, but even if we didn’t, we have injected 80% more green power back into the grid than what we’ve taken.
Looking at the battery, it spent 18.2% of its time empty (well, at the minimum charge state I have set of 10%) and 12.5% of its time full – and the rest in-between. The charge state distribution is shown below.
I calculate the battery round-trip efficiency to be 93.5% and the solar PV system efficiency to be 94.9%.
The most solar we have generated in a day was 36.0 kWh. The least was 1.56 kWh. Here’s the distribution of the daily power flows:
If I add up the off-grid times (i.e. grid draw zero), we have spent exactly 100 days (of the 123 days) off-grid. Meaning we drew no grid power for a total of 2400 hours. The longest continuous time off-grid was 68 hours (2.8 days). On 9 occasions we were off-grid for 48 hours or more, and on 25 occasions off-grid for 24 hours or more. The longest continuous time on-grid was exactly 1 day.
My next post next week will compare electricity rate plans – which one would be the best for this usage pattern? And the final one will be the what-ifs… what would this have looked like with no battery? With a larger batter? Larger solar array? That will answer the question of “payback time” and various optimisations.
So much fun to come…
About Author
