Simple payback and why it is not the number
Simple payback is net cost divided by first-year saving. It is the figure everyone quotes and it is wrong in two directions at once: it ignores that electricity will probably cost more later, which shortens the real payback, and it ignores that the array will produce less later, which lengthens it. The two do not cancel, because rate escalation compounds at a few percent a year while degradation runs at half a percent.
The modelled figure on this page runs the twenty-five years year by year. Each year's production is the previous year's less the degradation rate, each year's rate is the previous year's plus the escalation rate, and the cumulative saving is tracked against the net cost until it crosses. The break-even year comes out of that crossing, interpolated within the year rather than rounded to it.
The four inputs that decide everything
| Input | Why it moves the answer | Where to get it |
|---|---|---|
| Rate escalation | The largest single lever. It compounds, so small differences become large by year 20. | Your own bills over ten years, not a forecast in a proposal. |
| Electricity rate | Sets year-one savings directly. Payback is roughly inversely proportional to it. | Total bill divided by kWh used, all charges included. |
| Annual production | Same proportional effect as the rate, and more often overstated. | A site-specific irradiance estimate for your tilt and azimuth, with realistic losses. |
| What you paid | Fixed on day one and the only one you fully control. | Multiple quotes. Cost per watt installed varies enormously for identical hardware. |
The sensitivity block in the results exists so you can see this rather than take it on faith. Set escalation to zero and watch the break-even move by years. That gap is the size of the assumption you are being asked to accept.
Incentives are an input, deliberately
This calculator will not tell you what credits or rebates you qualify for, and you should be suspicious of any tool that does. Programmes are national, regional, state, municipal and utility-level all at once; they have eligibility rules about ownership, tax liability, equipment origin and installer certification; they get amended, capped, extended and withdrawn; and the terms that applied to a system commissioned two years ago are frequently not the terms available now. A number baked into a web page is a number that was true on the day it was written.
Find out what actually applies to you, at the time you are buying, from the programme administrator or a tax professional, and put that figure in the field. If you are not certain of it, run the model with it and without it. If the decision only works with an incentive you have not confirmed in writing, you do not have a decision yet.
Exports, and the part that quietly changes
How much a kilowatt-hour is worth depends on whether you use it as it is generated or send it back. Self-consumed energy is worth the full retail rate including delivery charges, because it is energy you did not buy. Exported energy is worth whatever your utility credits, which ranges from full retail down to a wholesale rate that is a fraction of it, and which can be structured as net metering, net billing, a fixed export tariff or a time-varying one.
This distinction has been the subject of active regulatory change in many places, generally moving away from full retail credit for new systems. It is also frequently grandfathered, so what a neighbour receives may be irrelevant to what you would receive. Ask the utility what a new interconnection would be offered, in writing, before the offset field on this page gets set to 100 percent.
What a projection cannot know
Everything above compounds four guesses over a quarter of a century. Utility pricing is set by regulators, fuel markets and infrastructure spending, none of which are predictable in a straight line. Production depends on weather that varies year to year around whatever average you used. Degradation is real but its rate is a specification, not an observation of your array. And maintenance is a small number that is occasionally a large one, because inverters fail, roofs need work under arrays, and squirrels have opinions about DC wiring. Run the model at the pessimistic end of each and see whether the decision still holds. If it does, the arithmetic has told you something useful. If it only works at the optimistic end of all four, the arithmetic has told you something more useful still.
Questions people ask
What is a typical solar payback period?
The honest answer is that the range is wide enough that a typical figure is not much use. Payback is driven by your electricity rate, your irradiance, what you paid per watt and what incentives you could claim, and all four vary by a factor of two or more between locations. A high-rate, high-sun location with a good price and a substantial incentive can land under eight years; a low-rate, cloudy location with an average price and no incentive can be past twenty, or never within the term. That is why this page asks for all four rather than quoting an average. Run it with your own numbers and then run it again with pessimistic ones.
Does the incentive figure in this calculator reflect current programmes?
No, and that is deliberate. The incentive field is an input with no default assumption about what you qualify for, because credits and rebates differ by country, state, utility and individual circumstance, and they change often enough that any figure written into a page would be out of date before long. Eligibility can also depend on things a calculator cannot know, such as whether you have tax liability to offset, whether you own rather than lease the system, and whether the installer holds a particular certification. Confirm what applies to you with the programme administrator or a tax professional at the time you buy, then enter that number.
Should I include the loan interest if I am financing the system?
Yes, and it changes the picture materially, which is why financed proposals are often presented as a monthly payment comparison rather than as a payback period. A financed system has no single break-even point in the way a cash purchase does; what it has is a monthly payment compared against a monthly saving, and a total cost of borrowing that has to be added to the system cost before any payback figure means anything. To approximate it here, add the total interest you expect to pay to the system cost. To do it properly, compare the loan schedule against the year-by-year savings, which is a different calculation from this one.
Does solar add to the value of the house?
Studies generally find owned systems add value, with the amount varying widely by market, system age and local electricity prices, and this calculator does not attempt to model it. Two things are worth knowing regardless of the figure. Owned and leased systems behave completely differently at sale: an owned system is an asset, while a lease or power purchase agreement is a contract the buyer has to be willing to assume, and that has complicated sales. And a system near the end of its life adds less than a new one, since the buyer inherits the remaining production rather than the original. If resale is part of the reason for buying, the ownership structure matters more than the payback year.