Savings and payback
See the total value your system creates in today's dollars, after discounting every year of future savings back to the present.
Current status: the 30% federal residential solar tax credit (Section 25D) expired for systems placed in service after December 31, 2025. If you're buying with cash or a loan in 2026 or later, use your full system price with no credit subtracted. Only subtract 30% if your system was placed in service by the end of 2025.
How it works
Step 1
Use the result from the payback period calculator if you haven't worked these out yet.
Step 2
The tool discounts each year of savings within your horizon back to today's dollars, then subtracts your net cost to get NPV. See the discounted payback comparison for the related year-by-year breakeven view.
A $22,000 net cost system with $1,800 in annual savings, a 6% discount rate, and a 25 year horizon has an NPV of about $1,010. That means the system is worth roughly $1,010 more, in today's dollars, than simply keeping the original cash uninvested. A positive NPV signals a genuinely worthwhile investment at your chosen discount rate, though the margin is much tighter than under the historical 30% credit. Utility rates drive the whole savings calculation, and the US Energy Information Administration's electricity data track them state by state.
Both use identical discounting math. Discounted payback tells you the year the investment breaks even in present-value terms, currently about 12.4 years for this example on the discounted payback comparison. NPV tells you the total value created by your chosen end date. Not coincidentally, NPV crosses exactly zero at that same discounted payback year. If you have not sized the system yet, start with the solar panel calculator and bring its kilowatt figure back here.
A negative NPV means the discounted value of your savings doesn't cover your upfront cost within the chosen horizon, so the investment underperforms the return implied by your discount rate. This tends to happen with high discount rates or shorter horizons than the system's real payback needs, not because solar itself is a poor investment in absolute terms. The internal rate of return calculator approaches this same question from the opposite direction, solving for the rate at which NPV hits exactly zero. The US Department of Energy's homeowner's guide to going solar walks through the same decision step by step.
25 years is the standard choice, matching typical panel warranties, the same period used in the 25 year lifetime savings calculator. A shorter horizon like 10 or 15 years is more conservative, useful if you're unsure how long you'll stay in the home, since it only counts savings you're confident you'll actually collect. This estimate assumes normal upkeep, and the maintenance cost vs savings net calculator breaks down what that upkeep costs year by year.
Questions
A positive NPV means your system creates real financial value even after discounting future savings back to today's dollars. A $22,000 net cost system with $1,800 in annual savings, a 6% discount rate, and a 25 year horizon has an NPV of about $1,010, meaning it's worth roughly that much more than simply keeping the cash.
A negative NPV means the discounted value of your savings doesn't cover your upfront cost over the chosen horizon, so the investment underperforms the return implied by your discount rate. This tends to happen at high discount rates or with a shorter time horizon than the system's real payback needs.
They use the same discounting math but answer different questions. Discounted payback tells you when the investment breaks even in present-value terms, while NPV tells you the total value created by a specific end date. NPV crosses zero at exactly the discounted payback year.
25 years is standard, matching typical panel warranties. A shorter horizon, like 10 or 15 years, is more conservative and useful if you're unsure how long you'll stay in the home, since it only counts savings you're confident you'll actually receive.
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