Solar ROI Calculator

How ROI, payback, and lifetime savings are computed

The return figures come from the same sizing model as the main solar calculator, with the money side brought to the front. Using the defaults of a $180 monthly bill, $0.17 per kWh, 4.5 sun hours, 90% offset, 400 W panels, and $2.75 per watt, the model sizes an 8.93 kW system that produces 11,435 kWh a year, then works through three outputs.

• Yearly savings are the kWh offset multiplied by your rate: 11,435 × $0.17 = $1,944, or $162 a month. The model assumes every one of those kWh replaces a kWh you would otherwise buy at retail.
• Payback is net cost divided by yearly savings. Gross cost is $24,546, the 30% federal credit is $7,364, and the net cost is $17,182. $17,182 ÷ $1,944 = 8.8 years.
• Savings over 25 years are yearly savings × 25 = $48,600. Subtracting the net cost gives a gain of $31,418, which is a 183% return on the money invested, or about 2.8 dollars of savings for every dollar spent.

Payback in this model does not depend on the size of your bill. A bigger bill produces a bigger system, and cost and savings grow together, so the ratio is set by price per watt, sun hours, electricity rate, and the tax credit alone. That is why the levers discussed below are about price, sun, and rate rather than system size.

Simple payback versus 25-year return

Payback answers one question: when do I break even? It says nothing about what happens afterwards, and for an asset that keeps working for 25 to 30 years that is most of the story. Because the model uses flat savings, the two figures are tied together: total savings equal 25 divided by the payback, expressed as a multiple of net cost. An 8.8-year payback returns 2.8× the investment, a 12.5-year payback returns 2.0×, and a 5-year payback returns 5.0×.

The 25-year return is not the same as an annualized investment return. It is undiscounted and it ignores what the cash could have earned elsewhere. Spread evenly, a 183% gain over 25 years works out to roughly 7% a year in simple terms, tax-free and tied to your electricity bill rather than to markets. The payback calculator lets you explore the break-even year in more detail.

Payback sensitivity to electricity rate

The rate you pay per kWh is the strongest driver of return. The table holds the default 8.93 kW system fixed at its $17,182 net cost and changes only the rate. In the calculator itself, changing the rate while leaving the bill at $180 also resizes the system, but the payback column comes out the same either way.

8.93 kW, 11,435 kWh/yr, $2.75/W, 4.5 sun hours, 30% federal credit applied. Ratings use the 7-year and 11-year thresholds shown on the result card.

The $0.12 row is roughly Washington, Utah, or Idaho; $0.17 is close to the national average; $0.25 to $0.30 covers California, Massachusetts, Connecticut, and Rhode Island. Sun hours and installed price shift the rows up or down, which is why the cost by state page shows modeled payback for each state.

What improves solar ROI

• High electricity rates. Every cent per kWh raises savings directly. This is why the Northeast and California show short paybacks despite the highest install prices.
• Good sun. Moving from 4.0 to 5.5 sun hours cuts payback by about 27%, since the same panels produce that much more energy. Roof pitch and a south-facing orientation matter for the same reason.
• Incentives. The federal credit alone cuts payback by 30%. State credits, rebates, and SRECs lower net cost or add income the calculator does not count.
• Self-consumption. Using solar power while it is being produced, by running the dishwasher, laundry, water heater, or EV charger during the day, avoids buying retail power and avoids exporting at a discount.
• Competitive quotes. Each $0.25 per watt shaved from the price shortens payback by roughly 9% at the default inputs.

What hurts solar ROI

• Net billing. Where exports are credited at a wholesale or avoided-cost rate, often a quarter to a half of retail, the kWh you send to the grid earn much less than the calculator assumes. A battery or a lower offset can partly compensate.
• Low electricity rates. Below about $0.14 per kWh the default system falls into the longer-payback band, and it takes cheap installation or strong sun to pull it back.
• Shading and orientation. Trees, chimneys, or an east-west roof can cut production 10–30% below what your sun-hours figure suggests.
• Financing interest and fees. Loan interest and dealer fees add to the cost without adding savings. The solar loan calculator shows the monthly payment and total interest for a given rate and term.

Degradation and rate inflation

Two effects work in opposite directions and are both left out of the model. Panels lose about 0.5% of their output each year, so year-25 production is roughly 88% of year one and the true 25-year savings are about 6% lower than the flat figure. Utility rates, meanwhile, have historically risen faster than that, and a 2.5% annual increase would raise the same 25-year savings by about a third. On balance the flat-rate estimate on this page is more likely to understate your return than overstate it, provided your utility credits exports near the retail rate.