Looking at Solar, Grid, and House Load for the Summer Solstice June 20, 2025 and Winter Solstice December 21, 2025. Looking at a month gives a 30-31 day sample for each month. SOLAR 460.7 kWh vs 222.7 kWh - 683.4 kWh 67.4% = 460.7 / 683.4 32.6% = 222.8 / 683.4 Longest Summer is ~2x the Shortest Winter (including one extra day) GRID 195 kWh vs 603.5 kWh - 798.5 kWh 24.4% = 195/798.5 75.6% = 603.5/798.5 About a 3 to 1 ratio in grid power needed in winter An extended power outage required 2.3 kWh from the emergency generator Grid electrons cost ~$0.12 / kWh HOUSE LOAD 883.4 kWh vs 947.7 kWh - almost identical electrical load June or December CONCLUSION The Summer or Winter electrical loads were nearly identical. The solar production significantly avoided grid load in the Summer and still helped in the Winter. This suggests that adding roof-top, wind powered generator(s) might significantly improve year round performance. Bob Wilson
My location has a far more pronounced PV seasonal production difference: (The 2025 summer spikes are from a couple microinverters having communication problems, delaying some reports until the next day or so.) The seasonal differences here are a combination of the northerly latitude, the local climate zone having much more cloudcover in winter than summer, and tree shade being worse in winter when more neighbors' tree shadows reach my system.
This discussion points to interests that converge in photovoltaics and plant photosynthesis (PS). Bringing to readers attention: ERA5 hourly time-series data on single levels from 1940 to present To reach that page one needs to go 'upstream' and register for a free ECMWF account. It requires disclosing an email address, but no other hurdles. There, one can download .csv files including "Surface solar radiation downwards" at 0.25 degree lat/lon and hourly resolution, from 1940 to now! It is a remarkable resource for both teams PV and PS. Teams blue and the green, if you will. Readers aware of any similar datasets available would please make that known. Team green would like to know == As a pup in college decades ago I was told that chemistry can be defined as 'pushing electrons'. It is also so for PV but where electron are pushed externally. == Team blue (PV) can be constrained by heat. Team green (PS) can also be constrained by external dryness because plants close stomates when unable to provide adequate water from soil. This isn't the place to discuss plants' stomates. But if anyone is curious ... ==Pushing electrons
Returning home, it was overcast and the solar roof was generating less than 1 kW of power charging the home battery: "green line" - charge being put on the solar battery charge-discharge efficiency ~86%, so 100 Wh in gives 86 Wh out. "blue line" - the EV charging at the minimum, 6 A, 240 VAC * 6 A = 1,440 W 100% of the solar stored in the EV battery for future trips About 4 PM, the clouds and low sun angle stopped solar cell production set the charge rate to maximum 16 A to complete the charge with grid power This manual, load management maximized solar energy in low light conditions. In effect, tailoring the EV charging load to match the available solar power stored in the EV battery. Once solar production stopped, the rate was increased to top off the EV using the grid. I plan to automate this. I have replaced the controller of a former JuiceBox 40 Pro with an "OpenEVSE" controller. There are several ways to read the solar panel output and adjust the "OpenEVSE" to match that current. This will maximize low-light power generation and storage in my EVs and bypass the 86% efficient, solar roof battery. Bob Wilson
