Seasonal variation in solar output is ignored in LOCE and LCOC models, by everyone that publishes models. I am going to use St. Louis, MO to show this example. You can do the same for your location and your bills using PVWatts as a starting point. (Put your city in and then change the size of the system to 1kW for ease of math).
Let’s say that for ease of use you average 2 kW is usage over the whole year and have to build both storage and solar to support your load. What you don’t need is exported to the grid.
2kW is 48 kilowatt hours per day.
In, July 1 kW of solar produces 1kW of solar produces 148 kWh of energy or 3 days of energy, so you need10 kW of solar on the roof.
In December it produces 73 kWh during the month and so you need 20 kW of solar.
This assumes that heating is non-electric and there are no electric vehicles. So in December you need double for an average winter. Storage needs to be 20kW (winter max production by solar) by 36 kWh (18 hours when the sun does not produce enough energy).
Now if you add a cold climate heat pump (CCHP) on a maximum day a 3-ton unit draws 6.6 kW. If you are in a polar vortex that drops temperatures down to -6 F ( -6 was the low in 2022, the record is -21). Then you draw the full 6.6 kW continuously. Say you need that extra energy for an hour after dark. 1 kW of PV produces 2.4 kWh, so you need not 20, but 23 kW of solar and an extra 6.6 kWh of battery. Of course cold seldom lasts for just one hour.
So for the summer you need 10 kW of solar and batteries (10kkw by 24 kWh).
For a cold winter day you need 23 kW and 43 kWh of storage (for one very cold hour, more for a longer cold snap).
Summer costs are $30,000 for solar and $24,000 for battery = $54,000
Winter adds at least 13kW@$3,000 for solar and 19 kWh of storage @ $1,000 = $58,000 for the home.
Total cost = $112,000
Average cost of a home in St. Louis $235,000.
