Installation costs for a solar power installation in SF is about $2.5 per kW, so the cost of an 8.042kW system would be about $20000.
In the calculation below, I’ll work out how many years it will take to recover that capital cost:
San Francisco residential electricity cost per kWh = 25.7 cents/kWh
Yearly savings = energy usage x electricity cost = 12000kWh x 25.7 = $3084/year
Solar payback time San Francisco = solar cost/yearly savings = $20000/3084 = 6.5 years