Solar Panel & Battery Payback Calculator
Estimates how many years solar panels, and a home battery added to them, take to repay their cost over 25 years. Each year, solar used as it is generated saves the electricity price and the rest earns the export rate, with output falling 0.5% a year and prices rising at your rate. The battery stores spare solar on the days it charges, capped by its capacity, the spare output and the uncovered use, less round-trip losses.
Enter the size of your solar array, how much each kW of it produces in a year, what it costs, how much electricity your home uses, and what you pay and are paid per kWh. The calculator works out how many years the panels take to repay their cost, whether a battery added to them pays for itself within its life, and how long the two take together. With the defaults, a 6 kW array costing 9,000 pays back in 10.0 years, a 10 kWh battery costing 5,000 falls 1,071 short over its 10-year life, and together they pay back in 11.1 years.
How the payback is worked out
Each year the panels' output is split in two. The share you use as it is generated replaces electricity you would otherwise buy, so it saves the full price. The rest goes to the grid and earns the export rate, whatever your supplier calls it: a feed-in tariff, an export credit or a net-metering credit.
savings = used × price + exported × export rate
A battery moves part of the exported share into the evening. On each day it charges from solar, it can hand back no more than its usable capacity, no more than an average day's spare output after its losses, and no more than an average day's use that the panels left uncovered:
stored = days × min(capacity, spare × efficiency ÷ 365, uncovered use ÷ 365)
Each kWh it hands back saves the price, but storing it gave up more than a kWh of exports, because charging and discharging lose energy:
value per stored kWh = price − export rate ÷ efficiency
Output falls 0.5% a year, the median that Dirk Jordan and Sarah Kurtz of the US National Renewable Energy Laboratory found across nearly 2,000 degradation rates measured on modules and whole systems, so year 25 produces 88.7% of year one. Both the price and the export rate rise by the yearly rate you enter. The battery's savings stop at the end of its life; it is not replaced. The payback is the point at which the running total of savings first reaches the cost, counted to a fraction of the year in which that happens. A total that never gets there within 25 years, or within the battery's life, is reported in words rather than as a number.
A worked example
The defaults describe a 6 kW array that produces 1,000 kWh per kW, so 6,000 kWh in year one. With 30% used as it is generated, 1,800 kWh replace electricity bought at 0.28 and the other 4,200 kWh earn 0.08: 504 plus 336 makes 840 in the first year. Against a cost of 9,000 that takes 10.0 years to repay, and over 25 years the panels come out 16,221 ahead.
The home uses 4,500 kWh a year, so 2,700 kWh are still bought from the grid, 7.4 kWh on an average day. That, not the battery's 10 kWh, is the binding limit: on its 250 days the battery hands back 1,849 kWh a year, the equivalent of 185 full cycles. Each stored kWh is worth 0.28 − 0.08 ÷ 0.9, about 0.19, so the battery saves 353 in its first year, and over its 10-year life it falls 1,071 short of its 5,000 cost. Together the two pay back in 11.1 years and finish 15,150 ahead, and the battery lifts the share of the home's use covered by solar from 40% to 81% in the first year.
Why the export rate decides the battery
A battery earns the gap between what you pay for a kWh and what you are paid to export one, after its losses. With a price of 0.28 and 90% efficiency, that gap closes at an export rate of 0.252: from there up, exporting pays at least as much as storing, and the calculator reports that the battery saves nothing. Under full net metering, where exports are credited at the retail price, a battery cannot cut the bill at all, and its case rests on backup power or time-of-use savings that this page does not model. At the default prices, the 10 kWh battery would pay for itself within its 10-year life if it cost 3,929 or less, and a 5 kWh battery costing 2,000 does so in 7.8 years.
Size matters less than it seems. A 20 kWh battery scores exactly the same as a 10 kWh one here, because an average day leaves only 7.4 kWh of use to cover. Real batteries meet a similar ceiling. Modelling about 2,000 Swedish households for a 2016 paper in Applied Energy, Emil Nyholm and colleagues found that batteries of 0.15 to 100 kWh could raise the share of solar used at home by a practical maximum of 20 to 50 percentage points, a range set by the seasonality of solar output at Swedish latitudes. The 250-day default is this page's assumption, not a measured figure: lower it for a long, dark winter and raise it where the sun shines most of the year.
From a lab curiosity to the roof
Alexandre-Edmond Becquerel saw the photovoltaic effect in 1839, when metal electrodes in a solution produced a small current as light fell on them, an effect he could not explain. Practical solar power came 115 years later. On 25 April 1954 Bell Labs announced a silicon cell, made by Daryl Chapin, Calvin Fuller and Gerald Pearson, that turned about 6% of the sunlight on it into electricity, as APS News recounts, and within a few years cells like it were powering satellites.
Payback on a home system became a practical question once governments paid for the output. Germany's Renewable Energy Sources Act, in force from 1 April 2000, obliged grid operators to connect solar systems, take their electricity and pay a fixed minimum price for it, guaranteed for 20 years. Prices then fell: Berkeley Lab's Tracking the Sun data show the installed price of US systems dropping by 9 to 11 dollars per watt over two decades, a 70 to 80% fall after inflation, with the steepest declines through 2014.
Storage followed the lithium-ion battery, whose foundations were laid during the oil crisis of the 1970s. Akira Yoshino created the first commercially viable lithium-ion battery in 1985, the cells first entered the market in 1991, and Yoshino shared the 2019 Nobel Prize in Chemistry with John Goodenough and Stanley Whittingham for their development. Tesla unveiled its Powerwall home battery at its design studio in Hawthorne, California, on 30 April 2015, in a 7 kWh version meant for daily cycling and a 10 kWh version meant for weekly cycling. In 2025, Berkeley Lab counted 37% of new US home solar systems paired with a battery, up from 25% a year earlier, with a median battery of 13.5 kWh.
Enter costs after incentives
Use what you will actually pay, after grants, rebates and tax relief, and the payback will reflect it. In the United States the residential clean energy credit paid 30% of the cost of solar panels from 2022 through 2025, and of batteries of at least 3 kWh from 2023, but the IRS says it is not available for any property placed in service after December 31, 2025. Great Britain charges no VAT on installing solar panels in homes until 31 March 2027, and has included batteries since 1 February 2024. Germany's VAT law sets a rate of zero on solar modules, their essential components and storage installed on or near homes.
What the estimate leaves out
The model is deliberately simple. It uses one flat price and one export rate, so it misses time-of-use tariffs, where a battery can earn more by charging cheaply and discharging at the peak, and it ignores standing charges, which solar does not reduce. It does not fade the battery's capacity or replace it at the end of its life, it does not budget for replacing the inverter, and it charges no loan interest. Savings years from now count at face value, without discounting. Net-metering schemes that settle credits once a year, or cap them, will differ. And a battery's value as backup during a power cut is real but has no price here.
Results are estimates from a simplified yearly model, not a quote or financial advice. See the site disclaimer.
Frequently asked questions
How long do solar panels take to pay for themselves?
It depends mostly on what each kWh is worth to you. With the defaults, a 6 kW array costing 9,000 saves 840 in its first year, pays back in 10.0 years and ends 16,221 ahead after 25 years. If every exported kWh earned the full 0.28 price, as under full net metering, the same array would pay back in 5.2 years. Enter your own output, price and export rate to see where you stand.
Is a home battery worth it?
Only when the gap between your electricity price and your export rate repays it within its life. At the defaults a 10 kWh battery costing 5,000 saves 353 in its first year and falls 1,071 short over its 10-year life, while a 5 kWh battery costing 2,000 pays for itself in 7.8 years. Backup power during a cut is a separate benefit that this calculator does not price.
Why doesn't a bigger battery save more?
A battery can only supply the use your panels leave uncovered, and it refills only from what they have spare. The default home has 7.4 kWh of uncovered use on an average day, so a 20 kWh battery stores exactly what a 10 kWh one does, 1,849 kWh a year. Size a battery to your evening and overnight use rather than to your panels.
Does the 30% federal solar tax credit still apply?
No. The IRS says the residential clean energy credit is not available for any property placed in service after December 31, 2025. It paid 30% of the cost of solar panels installed from 2022 through 2025, and of batteries of at least 3 kWh from 2023. Enter your costs after whatever grants, rebates or tax relief you can still claim, and the payback will reflect them.
How much output do solar panels lose each year?
About 0.5% a year. That is the median Dirk Jordan and Sarah Kurtz of the National Renewable Energy Laboratory found across nearly 2,000 degradation rates measured on modules and whole systems, and the calculator uses it. A 6 kW array that makes 6,000 kWh in its first year makes 88.7% of that, about 5,320 kWh, in its 25th.
What export rate makes a battery pointless?
Any rate at or above your electricity price times the battery's round-trip efficiency. At 0.28 per kWh and 90% efficiency the cut-off is 0.252: from there up, exporting spare solar earns at least as much as storing it, and the calculator reports that the battery saves nothing. Tesla's datasheet gives 89% for its Powerwall 3 in typical solar shifting.
Sources
- Photovoltaic Degradation Rates: An Analytical Review, National Renewable Energy Laboratory, 2012
- Solar photovoltaic-battery systems in Swedish households: Self-consumption and self-sufficiency, Applied Energy, via Chalmers Research, 2016
- Powerwall 3 Datasheet, Tesla
- Residential Clean Energy Credit, Internal Revenue Service
- Energy-saving materials and heating equipment (VAT Notice 708/6), HM Revenue and Customs, GOV.UK
- Umsatzsteuergesetz, § 12 Steuersätze, Bundesministerium der Justiz, Gesetze im Internet
- April 25, 1954: Bell Labs Demonstrates the First Practical Silicon Solar Cell, APS News, American Physical Society, 2009