How to use the solar panel & battery calculator
- Enter how much electricity you use: the kWh figure on your monthly bill, or a daily number.
- Pick the nearest city for sun hours (or type your own), and check the panel wattage and loss figures.
- Read the array size, panel count, roof area and yearly output; adjust the battery and cost sections for your setup.
How the array size is worked out
The calculation is the one installers use for a first estimate, and the one the US Department of Energy's PVWatts model is built around:
Array kW = daily kWh ÷ (peak sun hours × (1 − losses))
A home using 30 kWh a day in a city with 4.5 peak sun hours, with 14% losses, needs 30 ÷ (4.5 × 0.86) = 7.75 kW. At 400 W per panel that is 20 panels, which take about 39 m² (420 ft²) of roof. The same array produces 7.75 × 4.5 × 0.86 × 365 ≈ 10,950 kWh a year.
The sun-hour figure is the single biggest lever. It is an annual average: a London array makes roughly three times as much in June as in December, so a system sized to cover 100% of annual use will over-produce in summer and under-produce in winter. That is normal, and it is why export rates matter so much in the UK and Australia.
Choosing the inputs
Panel wattage. Residential panels sold in 2026 are mostly 400–450 W, roughly 1.1 × 1.8 m (about 2 m²). Commercial panels run to 600 W but are larger, so the roof area per watt barely changes.
Losses. 14% is PVWatts' default and covers soiling (2%), shading (3%), mismatch (2%), wiring (2%), connections (0.5%), light-induced degradation (1.5%), nameplate tolerance (1%), availability (3%). A roof with morning shade from a tree can lose 10% more. An array facing east or west instead of the equator loses 10–20% on top; north-facing in the northern hemisphere (south-facing in Australia) is usually not worth it.
Roof orientation and tilt. The sun-hour values assume panels tilted near the local latitude and facing the equator. A flat roof with panels laid flat loses about 10% in the UK and 5% in Singapore, where the sun is overhead anyway.
Battery sizing
A battery is sized in three steps: how many kWh you want to cover when the sun is not shining (usually overnight: 8–12 kWh for an average home, or just the essentials: fridge, lights, router, about 3 kWh), how many days of that you want in reserve, and how deep the chemistry can be discharged. LiFePO₄ (lithium iron phosphate), which is what almost every home battery sold today uses, is safely cycled to 90%. Lead-acid and AGM should not go below 50% if you want them to last. Round-trip efficiency, the energy lost charging and discharging, takes another 5–15%.
Battery kWh = kWh per day × days ÷ (depth of discharge × round-trip efficiency)
The amp-hour figure is the same energy at your system voltage (Ah = Wh ÷ V). A 10 kWh bank is 208 Ah at 48 V but 833 Ah at 12 V, which is why anything over a few kWh is built at 48 V.
Cost, incentives and payback
Installed cost per watt before incentives is roughly: US $2.5–3.5, Canada C$2.5–3.5, UK £1.4–1.9, Australia A$0.9–1.3 (the cheapest residential solar in the world, after the federal STC rebate is applied at the point of sale), Singapore S$1.8–2.5. Battery storage adds roughly $800–1,200 per kWh installed in the US and £500–800 in the UK.
The US federal residential clean energy credit was 30% through 2025 and has been changing under 2025–26 legislation; the calculator defaults to 30% but check the current rule for your install date. Canada has no federal percentage credit but a 0% Greener Homes loan; Australia's incentive is already in the price; the UK charges 0% VAT on installations. Set the incentive field to match.
Payback is net cost divided by yearly savings. Savings depend on how much solar you use at home (worth the retail price) versus export (worth the feed-in rate). Without a battery, a working household typically self-consumes 30–45%; with one, 70–90%. In the UK, where export pays 4–15p against a 25–30p retail price, that difference halves the payback time.
Worked example: Brisbane, 600 kWh a month
Daily use 19.7 kWh; Brisbane 5.2 sun hours; losses 14%. Array = 19.7 ÷ (5.2 × 0.86) = 4.4 kW → 12 × 400 W panels (4.8 kW), 24 m² of roof, 7,800 kWh a year. At A$1.10 per watt installed the system costs about A$5,300. With 50% self-use at 32c and export at 5c, yearly savings are about A$1,450, so simple payback is under four years, which is why Australia has the highest rooftop solar penetration on earth.
What this tool does not do
It does not model shade from specific objects, hourly weather, time-of-use tariffs or panel-level electronics, and it does not know your roof's real pitch and azimuth. For a bankable figure, run the same inputs through PVWatts (US), PVGIS (Europe) or the Australian PV Institute's SunSPOT tool, and compare with quotes from installers who have seen the roof.
Questions and answers
How many solar panels do I need for a 2,000 sq ft house?
Floor area is a poor guide; what matters is the kWh on your bill. A typical US home using about 900 kWh a month in a 4.5-sun-hour city needs roughly a 7.5–8 kW array, which is 19–20 panels of 400 W. The same house in Phoenix needs about 13 panels; in Seattle about 25.
What are peak sun hours?
The number of hours per day at a standard 1,000 W/m² that would deliver the same energy as the real, varying sunshine. London averages about 2.9, Sydney 4.8, Phoenix 6.5. It already accounts for cloud, season and day length, so you multiply array kW by sun hours to get daily kWh.
Why is there a 14% system loss?
Panels are rated at 25 °C in a lab. In practice heat, dirt, wiring, inverter conversion, mismatch between panels and shading each take a few percent. The US National Renewable Energy Laboratory's PVWatts tool uses 14% as its default and that is what this calculator uses; shaded or very hot roofs lose more.
How big a battery do I need?
Decide how many kWh you want to cover without sun and for how many days, then divide by the depth of discharge the chemistry allows. To cover 10 kWh for one night with LiFePO₄ (90% usable) you need about 11.7 kWh of rated capacity; with lead-acid (50% usable) about 23.5 kWh.
Is the payback figure reliable?
It is an estimate from your inputs. Installed cost per watt, incentives, electricity price and how much solar you use yourself versus export all vary a lot by country and by installer. Check the figures against two real quotes and your tariff; the calculator shows the arithmetic so you can see what moves the result.
Does it work for the UK, Australia, Canada and Singapore?
Yes. Cities for all five countries are built in with typical sun hours, you can switch between m² and ft², and the cost section takes any currency. Export rates differ most: the UK Smart Export Guarantee pays little, Australian feed-in tariffs vary by state, and some US utilities credit exports at the full retail rate.
Free to use, runs in your browser, nothing uploaded. Last updated 2026-10-07. Made by HandyToolkits; questions to [email protected].