A transparent planning model
Solar Panel Calculator uses annual energy balances to compare assumptions. It does not simulate an hourly roof, automatically obtain your bill, fetch current utility tariffs or provide an installer quote. You can change inputs to see how the result changes.
Data provenance and dates
| Input | Status | How to replace it |
|---|---|---|
| State electricity rates | Illustrative defaults inherited from v4; exact EIA release not established | Enter your marginal energy rate, excluding fixed fees; cross-check EIA reporting. |
| Monthly bills | Illustrative household inputs, not verified state averages | Use the past 12 months of your own bills. |
| Peak sun hours | Approximate planning inputs; exact irradiance dataset not established | Use an address-specific PVWatts or installer production estimate. |
| Rooftop cost: $2.75/W | Editable modeling assumption, not a verified 2026 national average | Use total cash price divided by panel DC wattage. |
| Battery cost: $0.55/Wh | Editable model convention for rooftop mode, not a market quote | Compare actual battery and installation costs; use kit total in plug-in mode. |
| Tax credit: 0% §25D | IRS post-2025 rule checked October 1, 2026 | Review the IRS reference for your installation and tax circumstances. |
EIA’s Electric Power Monthly page identifies July 2026 as its data period and September 24, 2026 as the release date when checked for this edition. We have not imported that release into the state defaults, and we do not claim the inherited figures match it.
How the calculations work
We first remove fixed monthly charges before converting a bill into estimated electricity consumption. This prevents the model from treating all charges as avoidable through solar.
Rooftop capacity is rounded up to 0.1 kW to offset estimated annual use under the selected solar resource. This is a planning target, not a recommended engineering design. Roof space, shading and interconnection limits may constrain capacity.
Annual rooftop production = kW × peak sun hours × 365 × 0.86
Plug-in production uses a 0.85 performance factor.
Each later year reduces output using the degradation input. Self-consumption is the share of generation used at home after losses, capped by annual household consumption. Remaining production is valued at the export rate, which defaults to zero.
Net savings = accumulated gross savings − upfront cost − maintenance − replacements
Payback, replacements and scenarios
Modeled break-even is the first point at which accumulated net savings reaches zero; a later replacement may temporarily make net savings negative again. Replacement costs recur at the chosen interval. Rate growth compounds both retail and export values; real export tariffs may change differently. Maintenance and replacement costs are constant dollars. The model does not discount future cash flows.
The conservative case reduces production by 15% and increases upfront cost by 15%; the optimistic case increases production by 10% and reduces upfront cost by 10%. These are sensitivity illustrations, not forecasts or statistical confidence ranges.
Battery and time-of-use limitations
Battery shifting requires an entered capacity and an enabled time-of-use option. Shiftable energy is limited by modeled self-used energy, battery capacity at 80% usable capacity and 90% efficiency, peak-window duration and estimated peak-period household demand. This simplified annual budget is not a battery dispatch optimizer. Self-consumption must already reflect storage losses.
Worked example
With a $150 bill, $20 fixed charge, 17¢ retail rate and 4.5 peak sun hours, annual use is approximately 9,176 kWh and modeled rooftop capacity is 6.5 kW. At 50% self-consumption and zero export compensation, year-one gross savings are about $780. Using full retail compensation for exported energy raises that estimate to roughly $1,561. The difference comes from tariff assumptions, not different panels.
What the model excludes
- Hourly weather, roof geometry, snow, shading and inverter clipping.
- Tariff caps, seasonal true-up, net-metering credit expiry and minimum bills.
- Financing, taxes, inflation-adjusted maintenance, incentives and discount rates.
- Battery chemistry, temperature, capacity fade, warranty limits and backup wiring.
Update record
October 1, 2026: separated retail and export rates; added degradation, fixed charges, maintenance, replacement intervals, gross/net labels and sensitivity cases; clarified inherited data provenance.
Sources and verification
- EIA Electric Power Monthly: electricity prices
- PVWatts: location-specific solar production estimates
- IRS: Residential Clean Energy Credit
Links provide primary references or verification tools. Linking a source does not mean every calculator default is reproduced from it. See our data limitations.