Solar Panel Calculator
How many solar panels you need, what they cost and the payback time, for every US state and UK region, from your bill or appliances.
By Syed Mubashir Ali · Last updated
How to Use Solar Panel Calculator
- 1Choose your US state or UK region. That sets how much electricity a kilowatt of panels produces there and the local electricity price. Anywhere else, pick 'Somewhere else' and enter your peak sun hours.
- 2Size it from your electricity bill (monthly kWh, or the bill amount and your price per kWh) or from a list of appliances with their watts and daily hours. Quick-add buttons fill in typical figures.
- 3Set how much of your use you want solar to cover and the panel wattage you're considering (most home panels are 400–450 W).
- 4Under 'Cost and payback', check the installed price per watt against your quote, add any rebates you've confirmed, and set how much of the solar you'll use at home and what exported power earns.
- 5Read the system size, number of panels, roof space, yearly output, first-year savings, payback time and 25-year return, with a chart and a year-by-year table. Copy or share the result as a link.
Frequently Asked Questions
How many solar panels do I need?
Divide your yearly electricity use by what 1 kW of panels produces where you live, then by the panel size. A US home using 860 kWh a month (10,320 kWh a year) in Texas, where 1 kW produces about 1,348 kWh a year, needs about 7.7 kW: 18 panels of 430 W. The same home in Arizona needs about 6 kW (14 panels); in Washington state, about 9.3 kW (22 panels). A typical UK home using 2,700 kWh a year in London needs about 2.7 kW, or 7 panels.
How long does solar take to pay for itself?
It depends mostly on your electricity price. For that 860 kWh-a-month home at $2.60 a watt, with prices rising 2% a year and every kWh credited at the full rate: about 4.5 years in California (33¢/kWh), 6.3 in New York and Massachusetts, 9.1 in Arizona, 11.1 in Texas and Florida, and 14.6 in Washington (14¢/kWh). In London, a 3 kW system using half its output at home (26.32p) and exporting the rest at 15p pays back in about 8.7 years.
Is solar worth it in 2026?
Where electricity is expensive, usually yes: panels last 25 years or more, and in the examples above they save about 2 to 6 times what they cost over 25 years. It's weakest where power is cheap and sunshine modest, and under net billing, where exported power earns much less than you pay. The 30% US federal tax credit no longer applies to systems installed after 2025, so payback is longer than older guides suggest. Check state and utility rebates.
What is net metering, and why does 'solar used at home' matter?
With full net metering your utility credits every kWh you export at the retail price, so all solar is worth the same: set 'solar used at home' to 100%. Under net billing (California's NEM 3.0, for example) or the UK's Smart Export Guarantee, exported power earns less, so the share you use yourself matters. The California example above pays back in 7.7 years instead of 4.5 if half the output is exported at 5¢. A battery, or running the dishwasher and EV charger in the day, raises the share.
How much do solar panels cost?
In the US, EnergySage's marketplace averages about $2.60 a watt before incentives, so a 7.7 kW system is about $20,000. In Great Britain, DESNZ figures from MCS installations put domestic retrofits at about £1,820 a kW (with 0% VAT), so a 3 kW system is about £5,500. Your roof, panel brand and installer change this a lot; enter your quote.
How much roof space do I need?
A 400–450 W panel is about 1.7 m by 1.1 m, so allow about 2 m² (21 sq ft) per panel. 18 panels need roughly 35 m² (380 sq ft) of unshaded roof, ideally facing south (in the northern hemisphere).
Where do the sunshine figures come from?
From the European Commission's PVGIS model, using 19 years of weather data (2005–2023) for each state's largest city and a UK city per region: south-facing panels tilted 30° in the US and 35° in the UK, with 14% system losses. Shade, a different roof angle or an east or west roof lowers output. Enter your installer's production estimate if you have one.
About Solar Panel Calculator
This solar panel calculator answers the three questions people ask before going solar: how big a system do I need, what will it cost, and how long until it pays for itself. Size the system from your electricity bill or from a list of appliances, for any US state or UK region, or anywhere else using peak sun hours.
For each of the 50 US states and Washington DC it uses the average residential electricity price from the US Energy Information Administration and the yearly output of 1 kW of panels modelled by PVGIS for the state's largest city. For the UK it uses the Ofgem price cap and regional solar output from Exeter to Glasgow. It turns your usage into a system size and number of panels and estimates the roof space.
The payback section works year by year: panels lose about 0.5% of output a year, electricity prices rise at the rate you choose, solar you use yourself saves your full price, and exported power earns your utility's credit or your Smart Export Guarantee rate. It shows the year the savings overtake the cost, the 25-year gain and the return on what you spent, with a chart and a table.
Incentives change often, so none is assumed. In the US the 30% federal credit doesn't apply to systems installed after 2025, while state and utility rebates still exist in many places. In Great Britain, solar panels carry 0% VAT until March 2027. Treat the results as a planning estimate. A survey of your roof and two or three installer quotes give the real numbers.