Future household-load sensitivity
Keep the proposal's first-year solar-production forecast fixed and test what happens if annual household electricity use rises. These are transparent what-if rows, not forecasts of future consumption.
| Household load scenario | Annual usage | Energy offset | Production balance |
|---|---|---|---|
| Current annual load | 12,000 kWh/yr | 110.0% | +1,200 kWh/yr |
| +10% annual load | 13,200 kWh/yr | 100.0% | -0 kWh/yr |
| +20% annual load | 14,400 kWh/yr | 91.7% | -1,200 kWh/yr |
| +30% annual load | 15,600 kWh/yr | 84.6% | -2,400 kWh/yr |
Examples of future load can include EV charging, a heat pump, electric water heating, pool equipment or a home addition. Actual load changes should be modeled from the equipment and usage you expect, not assumed from these percentages.
Review notes
- The stated solar offset aligns with the simple annual production ÷ annual usage calculation within 1 percentage point.
- A calculated energy offset above 100% means modeled annual solar production exceeds annual household consumption. It does not mean the utility bill will be reduced by more than 100% or that exported kWh are credited at the retail rate.
- Solar energy offset is an annual kWh ratio, not a bill-savings percentage. Electricity prices, time-of-use periods, fixed charges, minimum bills, net metering or net billing, export compensation and monthly production/usage timing affect actual utility-bill savings.
What solar offset means
For this checker, annual solar offset is:
first-year solar production ÷ annual household electricity usage × 100
If a home used 12,000 kWh over the last 12 months and the proposal forecasts 13,200 kWh in Year 1, the simple annual energy offset is 110%.
Why 110% solar offset does not mean 110% bill savings
Energy offset compares annual kWh. Utility bills depend on much more than annual kWh totals: when the home imports electricity, when solar exports to the grid, fixed or minimum charges, time-of-use pricing and the utility's net-metering or net-billing rules all matter.
A system can produce more kWh than the home consumes over a year and still leave a utility bill. Exported electricity may also be credited at a different rate than imported electricity.
Which annual usage number should you use?
Use a recent 12-month electricity-usage total from the utility when possible. A proposal can show a different offset if the installer used an older billing period, estimated missing months, adjusted usage for a future EV or heat pump, or used a different annual-consumption baseline.
If the proposal's offset does not match the arithmetic, ask the installer to show the exact annual usage number and first-year production number used in the calculation.
Check the production number too
An offset calculation can be arithmetically correct while still depending on an aggressive solar-production forecast. After checking the percentage, compare the proposal's first-year production with a ZIP-specific PVWatts reference and the installer's detailed roof/shade model.
Solar offset FAQ
What does 100% solar offset mean?
A 100% annual solar offset means the proposal's first-year solar production equals the home's annual electricity usage when both are measured in kWh. It is an annual energy ratio, not a promise that every monthly utility bill will be zero.
What does 110% solar offset mean?
A 110% annual solar offset means the proposal forecasts first-year production equal to 110% of the annual household electricity usage used in the calculation. The extra annual kWh do not automatically receive the same value as electricity imported from the grid.
Does 100% solar offset mean a zero electric bill?
No. Fixed charges, minimum bills, time-of-use prices, monthly import and export timing, net-metering or net-billing rules and export compensation can leave a utility bill even when annual solar production equals annual consumption.
Why can my calculated solar offset differ from the proposal?
The installer may have used a different 12-month usage period, estimated missing bills, included planned EV or heat-pump load, rounded the usage or production values, or based the percentage on a different first-year production forecast. Ask for the exact annual usage and production numbers behind the stated percentage.
Can an offset percentage be correct even if the production forecast is aggressive?
Yes. The offset arithmetic can be internally correct while the first-year production forecast itself still needs review. Check the proposal production against a ZIP-specific PVWatts reference and the installer's detailed roof and shade model separately.
Scope of this checker
This tool checks the arithmetic of an annual energy-offset claim. It does not estimate hourly self-consumption, utility-bill savings, export compensation, future load growth or system performance after Year 1. Use it as one proposal-review signal, not as a savings guarantee.