What this calculator works out
Solar quotes usually start from a system size in kilowatt-peak, kWp, a rating for how much the panels can generate under standard test conditions, not what they actually produce on your roof in a British or American winter. This tool starts from the other end: how much electricity you actually use in a year, and how much of that you want a system to cover, then works backwards to a kWp figure, a panel count and the roof area that needs.
It is a sizing tool, not a savings calculator. The only promise it makes about money is a simple payback figure, and only once you type in your own install quote; everything else, generation, self-consumption and battery sizing, is shown as a clearly labelled estimate built from published industry conventions, named below, rather than a guess dressed up as precision.
How the kWp arithmetic works
Every solar yield convention, UK or US, boils down to the same shape: a location-and-orientation figure in kWh generated per kWp per year, multiplied by the system size, gives annual generation. Run that backwards from a target annual generation figure and you get the kWp needed.
Panels are only sold whole, so the panel count is always rounded up, which nudges the actual system size very slightly above the raw kWp figure the first line of that formula gives. That small rounding-up is exactly the same principle the Insulation Calculator on this site uses for rolls and the Roofing Calculator uses for sheets: buy in whole units, and let the real system land at or just past the target rather than short of it.
UK roofs: south, or east and west
The MCS Standard Estimation Method, set out in MIS 3002 and used by every MCS-certified installer to quote a generation figure, derives its yield number, referred to as "Kk", from irradiance tables built on the European Commission's PVGIS dataset, split across 21 UK zones by roof pitch and compass orientation. A well-tilted, unshaded south-facing roof typically lands in the 900-1,100 kWh per kWp per year range; an east- or west-facing roof, still perfectly usable, sits meaningfully lower. This calculator uses 950 kWh/kWp for south-facing and 850 for east- or west-facing, two representative points inside that convention, rather than the full 21-zone table, since an exact postcode figure is what an MCS quote is for.
US roofs: sun hours by region
NREL's PVWatts calculator is the tool the US solar industry treats as standard, and its own regional comparisons show why a single national yield figure would be misleading: a 1kW system in the sunny Southwest can produce nearly two-thirds more energy a year than the same system in the cloudier Pacific Northwest. This calculator offers four region tiers built on that spread, 1,700 kWh/kWp for the Sun Belt Southwest down to 1,200 kWh/kWp for the Pacific Northwest and Great Lakes, rather than one figure that would flatter one region and understate another. PVWatts itself gives an address-specific number once you have a rough size in mind from here.
Panel wattage and the roof area it needs
Residential panels sold today mostly sit in a 400-500W band. Qcells' own Q.PEAK DUO BLK ML-G10+ 400W datasheet lists dimensions of 1,879 x 1,045mm, about 1.95 square metres; independently published 2026 panel-size guides show the same figure for 400W panels, and put the larger half-cut-cell format used at 425W and up, the format Jinko's Cheetah Plus and Tiger Neo ranges use among others, at roughly 2.0 to 2.3 square metres as wattage rises toward 480-500W. This calculator multiplies the panel count by the chosen wattage's typical area for a roof-area estimate; real footprints vary a little by manufacturer, and a proper survey also accounts for rafters, chimneys and rack spacing.
Without a battery, only some of it gets used on site
Generation and demand rarely line up. Solar output peaks around midday; most homes draw the most power morning and evening, when the sun has dropped or hasn't yet climbed high enough to matter. Studies of UK households without batteries, including a University of Reading smart-grid project and National Energy Action's own review, put typical self-consumption at 35-50% of what's generated. This calculator assumes 40%, the middle of that range, and shows the split as a chart rather than folding it silently into a savings number.
A battery, as a rough sizing aside
A battery's job is to catch some of the midday surplus that would otherwise be exported and release it in the evening, raising typical self-consumption from that 35-50% range toward 60-80% once one is fitted. This calculator turns the estimated daily surplus into a rough capacity suggestion, clamped to the 2-13.5kWh range most home batteries are actually sold in (Tesla's own Powerwall tops out at 13.5kWh), as a starting point for an installer conversation rather than a spec to buy against; real sizing depends on your evening usage pattern, not just your generation.
A worked example
Take a UK home using 2,700 kWh a year, a south-facing roof and a target of covering all of it with 425W panels. Target generation is 2,700 kWh; divided by the 950 kWh/kWp yield gives a raw 2.84kWp; 2.84 x 1,000 ÷ 425 = 6.69, rounded up to 7 panels, an actual system of 2.975kWp generating around 2,826 kWh a year from about 14 square metres of roof, comfortably past the 2,700 kWh target because whole panels rarely land exactly on it.
As a US aside, a 10,000 kWh home in the Midwest, Northeast or Mid-Atlantic tier, at 1,300 kWh/kWp, targeting 75% of its usage, needs 7,500 kWh of generation, a raw 5.77kWp, and with 400W panels that's 5,770 ÷ 400 = 14.43, rounded up to 15 panels, an actual 6.0kWp system generating about 7,800 kWh a year.
Payback is an estimate, not a promise
The payback figure here is deliberately narrow: install cost, divided by the annual saving from the self-consumed share of generation at your own unit price. It leaves out export payments, unit-price changes, degradation and any battery cost, because folding all of those in would make it look far more precise than a rough sizing tool can honestly claim. Typical UK installs run around £1,200-£1,800 per kWp; typical US installs run $2.30-$3.60 per watt before incentives, and the federal residential solar tax credit that used to offset a chunk of that expired at the end of 2025, so a US figure today reflects the full cost.
Panels lose a little output every year
Crystalline solar panels degrade slowly and predictably rather than failing outright: industry data, including NREL's own analysis, puts average degradation at around 0.5% a year, leaving a panel at roughly 87-88% of its first-year output after 25 years, in line with most manufacturer performance warranties, typically 85-90% at year 25. It's a reason to expect generation to ease gradually over a system's life, not a cliff-edge drop.
Export payments: the Smart Export Guarantee, honestly
UK suppliers with more than 150,000 customers are required to offer at least one export tariff under the Smart Export Guarantee, paying for exported electricity a smart export meter records, but they are free to set the rate anywhere above zero. In practice that means a wide spread, from a few pence up to around 30p/kWh, with most of the better-known fixed tariffs sitting around 12-15p/kWh. It's worth shopping the export tariff separately from your import supplier, since you're not obliged to use the same one, but it's genuinely variable enough that this calculator leaves it out of the payback line rather than assuming a rate that may not match what you're offered.
Questions people ask
How much does south-facing really matter compared with east or west?
A real difference: this calculator's south-facing figure, 950 kWh/kWp a year, sits around 100 kWh/kWp above its east-or-west figure of 850, following the MCS standard estimation method's irradiance tables. East- or west-facing is still a sensible install; it just needs a slightly larger system to hit the same target. A north-facing or heavily shaded roof needs an installer's exact site figure rather than either number here.
Should I add a battery, or skip it?
That depends on your evening usage and budget, not on this calculator. What it can show is the trade-off: without a battery, typical UK self-consumption sits at 35-50% of generation, since solar peaks at midday while demand peaks morning and evening; a battery can lift that toward 60-80%. The battery-sizing aside on this page is a rough starting capacity, not a specification.
What can I actually expect from export payments?
Genuinely variable. UK suppliers with over 150,000 customers must offer an export tariff under the Smart Export Guarantee, but they set the rate themselves, anywhere from a few pence to around 30p/kWh, with most well-known fixed tariffs around 12-15p/kWh. Because it varies so much, this calculator leaves it out of the payback estimate rather than assuming a figure for you.
Do solar panels lose output as they get older?
Yes, slowly. Industry data puts average degradation at around 0.5% a year, so a panel is typically producing around 87-88% of its first-year output after 25 years, in line with most manufacturer performance warranties. It's a gentle decline, not a sudden drop-off, and this calculator's generation figures are first-year estimates.
Is the payback figure a guarantee I'll break even by then?
No, and it's shown as an estimate for that reason. It only divides install cost by the saving from the self-consumed share of generation at today's unit price; it leaves out export income, future price changes, degradation and any battery cost, since assuming values for all of those would make the figure look more certain than it can honestly be.
Why does the panel count sometimes generate more than my target?
Because panels are only sold whole. The raw system size rarely lands on an exact number of panels, so the panel count always rounds up, which means the actual system, and its generation, comes out at or just past your target rather than short of it.
UK yield figures follow the MCS Standard Estimation Method (MIS 3002) convention, cross-checked against published MCS-data yield tools. US yield figures follow NREL's PVWatts convention, given as regional tiers rather than an address-specific number. Panel dimensions follow Qcells' own datasheet and independently published 2026 panel-size guides. Self-consumption, degradation, export-payment and install-cost figures are drawn from National Energy Action, NREL, Ofgem's Smart Export Guarantee rules and recent UK and US installer cost surveys. Real figures vary by exact site, panel and installer; treat every number here as a sizing estimate, not a quote.