Understanding your new annual total
Why work from an annual figure
An annual total naturally smooths out seasonal swings between summer and winter bills, which can make it a more stable number to plan around than any single month. A $1,800 annual bill with an 85% offset saves $1,530 a year, leaving a new annual total of about $270, or roughly $22.50 a month on average. The US Department of Energy's homeowner's guide to going solar walks through the same decision step by step.
Annual and monthly give the same answer
Since the math scales linearly, working from your annual total or your monthly average produces the same underlying savings figure. Use whichever number is easier to pull from your records. The monthly bill reduction estimator works identically if a single recent bill is all you have on hand. If you have not sized the system yet, start with the system size by electric bill amount and bring its kilowatt figure back here.
Why the new total rarely hits exactly $0
Most utilities apply a minimum monthly service or connection charge regardless of offset, which adds up over a full year even at 100% offset. Expect a small residual annual charge rather than a literal $0 total in most cases. The production figure behind this estimate comes from the annual kwh output estimator, which models output for your location and roof.
Turning this into a longer-term view
This tool gives a single-year snapshot. For how that annual savings figure compounds over the life of your system, see the 25 year lifetime savings calculator, or the rate inflation projection tool if you want to factor in rising utility rates. Utility rates drive the whole savings calculation, and the US Energy Information Administration's electricity data track them state by state.