Solar & Battery Savings Payback Calculator
Turn a solar quote, local electricity price and realistic energy use into a clear annual savings and payback estimate.
Your energy and quote
Estimated outcome
How to use this calculator
- Start with your bills. Add the most recent 12 months of electricity use. Use the energy charge per kWh, and enter a separate export credit if your utility pays for surplus solar.
- Copy the proposed system. Enter the panel capacity, installer’s annual-production assumption or local peak-sun-hours estimate, and a conservative performance factor.
- Describe when energy is used. Direct self-use is the share of annual solar consumed while it is generated. The battery can shift some remaining solar to later hours.
- Use all-in prices. Include equipment, labor, permits and required electrical or roof work. Subtract only incentives you reasonably expect to receive.
- Compare scenarios. Save the solar-only result first by setting battery cost and usable battery size to zero. Then add the battery and compare the change in annual value with its extra cost.
Methodology and formulas
Annual solar (kWh) = system kW × peak sun hours/day × 365 × performance factor
The performance factor is a simplified allowance for inverter and wiring losses, heat, dirt, shading and downtime. It is not panel efficiency. Use an installer’s location-specific production estimate when available.
Direct solar = the lesser of annual electricity use and annual solar × direct self-use percentage
Battery charge input = the least of solar surplus, usable battery kWh × annual cycles, and remaining household demand ÷ battery efficiency
Battery delivered = battery charge input × efficiency
Battery loss = charge input − delivered energy
Grid export = solar surplus − battery charge input
This order matters: battery conversion losses are neither household consumption nor grid exports. The calculator now keeps the lost energy separate so export income is not overstated.
Annual energy value = (direct solar + battery-delivered energy) × retail rate + grid export × export rate
Simple payback = net installed cost ÷ annual energy value
Worked example using the default inputs
A 6 kW solar system with 4.5 peak sun hours and an 80% performance factor produces 7,884 kWh/year. At 35% direct self-use, 2,759 kWh is consumed immediately. The 10 kWh battery, cycled 300 times, accepts up to 3,000 kWh of surplus solar and delivers 2,700 kWh after 90% efficiency, with 300 kWh reported as conversion loss.
The home therefore uses 5,459 kWh of solar and exports 2,125 kWh. At $0.22/kWh retail and $0.07/kWh export credit, first-year energy value is about $1,349.79. With $21,000 of installation costs and $3,000 of incentives, net cost is $18,000 and simple payback is about 13.3 years. The 20-year simple projection is about $8,995.80 before the costs and changes excluded below.
These figures are an illustration of the arithmetic, not a claim about what a particular home will save.
How to interpret the results
| Result | What to check |
|---|---|
| Annual solar | Compare it with the installer’s production guarantee or a location-specific model. A generic sun-hours estimate is only a first pass. |
| Home-used solar | This receives the retail-rate value. If it exceeds plausible daytime and evening demand, lower direct self-use or battery cycles. |
| Grid export | Confirm whether exports are paid, capped or credited differently by time. Enter zero when no monetary credit applies. |
| Simple payback | Use it to compare quotes under the same assumptions. It is not ROI, NPV or a guarantee that equipment lasts beyond the payback year. |
Run at least three cases. For a conservative case, lower solar production and direct self-use, keep quoted costs intact and avoid assuming future rate increases. For an optimistic case, change only assumptions supported by the quote or utility tariff.
Limits and safety checks
- The model is annual, so it does not match hourly solar output with hourly household demand. Time-of-use pricing, demand charges and seasonal tariffs need interval-data analysis.
- The 20-year figure holds first-year savings constant. It excludes panel and battery degradation, replacement, maintenance, insurance, financing, taxes and changing electricity prices.
- Battery cycles are limited by both available solar surplus and remaining household demand; entering a high cycle count cannot create usable energy.
- Backup power has resilience value but that value is not priced here. Usable backup also depends on inverter design, essential-load circuits and minimum state-of-charge settings.
- Do not use this estimate to size wiring, protection devices or structural supports. A licensed installer must check the roof, shading, fire and electrical codes, permits, interconnection and equipment compatibility.
Privacy: calculations run in your browser. TrendPromptLab does not ask for an address, utility account number or installer contact details.
Sources, assumptions and related tools
The production structure is a simplified annual version of concepts used in the U.S. National Renewable Energy Laboratory’s photovoltaic model technical reference. For a more location-specific public example, Australia’s official SunSPOT calculator considers installation cost, annual savings and payback. Rules, incentives and export tariffs vary by location, so this page deliberately treats them as user-entered assumptions.
Continue planning with the appliance electricity cost calculator, EV charging cost calculator, or ROI calculator. Review our editorial policy for how TrendPromptLab explains estimates and limitations.
Frequently asked questions
Does payback include financing interest or battery replacement?
No. It is simple payback: net installed cost divided by first-year energy value. Add financing, maintenance and replacement costs separately before making a purchase decision.
How do I compare solar-only with solar plus a battery?
First set battery cost and usable battery size to zero and record the result. Then enter the battery quote and specifications. Compare the extra annual value with the extra installed cost; do not compare savings alone.
Why does the calculator show battery loss?
A battery returns less energy than it receives. Charging input is removed from solar surplus, the efficiency is applied to the delivered energy, and the difference is shown as conversion loss rather than being counted as an export.