Result 1 Recommended solar plant size
Sized to cover the 5,333 kWh a year your answer implies, at 1,012 kWh per kWp a year in London, United Kingdom.
Over a year this generates about 100% of the electricity you use. Not all of it arrives when you want it, which is what the next sections account for.
Hours of full-strength sun are not daylight hours. A panel only makes its rated output under 1,000 watts per square metre — roughly midday, clear sky, sun overhead. The rest of the day it makes less: at sunrise and sunset the light crosses far more atmosphere and strikes the panel at a glancing angle. So a twelve-hour day delivers the energy of about five hours at full strength, and that figure is what every generation number here is built on.
Result 2 Electricity generation
Total kWh this plant would produce. The 25-year figure allows for panels slowly losing output, so it is not simply twenty-five times the first year.
The twelve figures
| Month | kWh |
|---|---|
| Jan | 139 |
| Feb | 216 |
| Mar | 432 |
| Apr | 631 |
| May | 685 |
| Jun | 702 |
| Jul | 730 |
| Aug | 632 |
| Sep | 522 |
| Oct | 326 |
| Nov | 198 |
| Dec | 119 |
| Total | 5333 |
Monthly figures are for an average day in each month, scaled by its length. A single cloudy week will not match them; a year will come close.
Result 3 Financial savings
What that generation is worth to you. Electricity you use as you make it saves the full unit rate; the rest earns your export rate, which is usually a good deal less.
At 55% used in the home, an average generated unit is worth 21.6p/kWh — blending what you avoid buying with what you are paid for the surplus. The 25-year total is in today's money and assumes no price rises, which makes it a cautious figure rather than an optimistic one.
Result 4 Estimated project cost
Your installed price per kWp, multiplied by the size above and then divided into the parts a quote usually itemises.
- Panels £2,952
- Inverter £1,107
- Mounting and racking £738
- Wiring, protection and meter £591
- Installation labour £1,402
- Design, permits and inspection £591
- Total installed cost £7,381
Result 5 Payback and return
How long the savings take to cover the cost, and what the system returns each year as a percentage of what it cost.
Payback walks year by year with panel output declining, rather than dividing the cost by the first year's saving — that shortcut flatters the answer, because later years earn less than the first. The return figure is first-year saving over installed cost: a simple yardstick, not an internal rate of return, and it ignores both inflation and the cost of borrowing.
Result 6 Environmental impact
Every unit you generate is a unit the grid does not have to. What that is worth in carbon depends entirely on how your grid makes electricity, which is why it is a figure you set.
Displaced grid emissions only. It does not net off the carbon of making and shipping the panels, which a rooftop system typically repays in one to three years of operation — a real figure, and one this page does not model rather than estimate loosely.
What this does not know
- Shading. It assumes a clear sky in every direction. A chimney or a tree can cost far more than its share of the roof — the shading calculator models that properly.
- Your money, not ours. Every figure in currency on this page — the unit rate, the installed price, the export rate — is one you typed or one we left as a starting point, and the starting points are UK examples. Nothing here knows what solar costs in your country, and nothing here should pretend to. Change the currency and the numbers are relabelled, never converted; where a country quotes tariffs in whole units rather than in cents or pence, the same figure also means something different, so replace it rather than trusting it.
- When you use power. The self-use share is a single percentage, not a real load profile. If most of your usage is in the evening, the true figure is lower — the battery calculator is where that argument lives.
- Price rises. Savings are in today's money throughout. If electricity gets dearer, the real return is better than shown.
For a system you have actually been quoted, the output calculator takes the panel count and wattage from the quote instead of estimating them.
Generation is modelled with the same physics engine as every other tool in this section, validated against PVGIS and reported openly on the methodology page. Money is not modelled at all — it is arithmetic on figures you supplied. That difference is deliberate, and it is why the two are kept apart.
Frequently asked questions
How many solar panels do I need?
Panels are the last step, not the first. This calculator works out the system size in kilowatts peak (kWp) from what you actually use: your bill divided by your unit rate is your annual units, and your units divided by how much a kilowatt generates a year where you live is the kilowatts you need. Only then does it divide by the wattage of one panel to count them, rounding up because half a panel cannot be fitted.
So the count moves with the module you are quoted — a 5 kWp system is about twelve 450-watt panels or eight 700-watt ones. For scale, EnergySage put the average US home near a 12 kW system in 2026, while the Energy Saving Trust puts a typical UK home nearer 3.5 to 4.5 kWp. Enter your own bill and panel wattage above to size yours.
So whether you search for a solar panel calculator, a solar array calculator or a photovoltaic (PV) panel calculator, this is the same sum: how many kilowatts, and how many panels, cover the electricity you use.
Sources: EnergySage, average US solar prices (retrieved 2026-09-04); Energy Saving Trust, solar panels (retrieved 2026-09-04).
Is this a solar power calculator or a solar system size calculator?
Both, in sequence. It sizes the system first — the kilowatt-peak and the number of panels that cover the bill, the units or the roof you gave it — and then simulates what that array generates month by month at your location, which is the power side. The cost, savings and payback figures are built on those two results.
Can I use this as a solar calculator by address?
Search for your town, city or postcode in step 3 and the generation figures use the sun geometry and climate for that location. It is not a satellite view of your particular roof: the tilt and the direction it faces are yours to set, and a shaded roof needs the shading calculator.
How much do solar panels cost?
This page holds no prices. It multiplies the installed price per kWp that YOU enter by the system size it works out, then splits the total into the parts a quote itemises — so the cost is always traceable to a figure you can check against a real quote rather than a national average.
For orientation only, and not what the calculator uses: EnergySage reported US residential solar at about $2.60 per watt before incentives in 2026 — roughly $16,000 for a 6 kW system and $31,000 for a 12 kW one. In the UK the Energy Saving Trust puts a typical domestic system near £7,000. Prices vary widely by country, installer and roof, which is exactly why the tool asks for yours instead of guessing.
Sources: EnergySage, average US solar prices (retrieved 2026-09-04); Energy Saving Trust, solar panels (retrieved 2026-09-04).
What is the solar payback period, and how is it calculated?
Payback is the year your accumulated savings cover what the system cost. This calculator does not take the usual shortcut of cost divided by the first year’s saving — that flatters the answer, because panels lose a little output every year, so later years save less. It walks year by year with that decline built in and marks the year the savings pull level.
Typical residential payback runs from several years to around fifteen, driven mostly by your electricity price and any incentives — the Energy Saving Trust cites roughly 11 to 15 years for UK homes. Yours depends on the numbers you enter above.
Sources: Energy Saving Trust, solar panels (retrieved 2026-09-04).
How much money can solar panels save?
Two rates, not one. Electricity you use as you generate it saves the full unit rate you would have paid; the surplus you export earns a lower export rate. This calculator blends the two by the self-use share you set, so the saving is honest about the fact that midday solar and evening demand do not line up without a battery.
As an anchor, the Energy Saving Trust estimates UK households can save up to about £610 a year in London under the Smart Export Guarantee. Your figure depends on your usage, your tariff and where you are — which the calculator takes from what you enter.
Sources: Energy Saving Trust, solar panels (retrieved 2026-09-04).
Is there still a 30% federal solar tax credit in the US in 2026?
Not for a system you buy yourself. The 30% Residential Clean Energy Credit (Section 25D) ended for equipment placed in service after 31 December 2025 under the 2025 budget law, so homeowner-owned systems installed in 2026 no longer qualify. A lease or power-purchase agreement can still reach the credit indirectly, because the company that owns the panels claims the business version (Section 48E) — a benefit scheduled to run into 2027.
This calculator applies no incentives of any kind. Enter your NET price, after anything you actually qualify for, and it prices the system from that.
Sources: SEIA, clean-energy provisions of the 2025 budget law (retrieved 2026-09-04).
What is net metering, and does this calculator include it?
Net metering — or a Smart Export Guarantee in the UK — is the credit you get for electricity you send back to the grid. This calculator models it through the export rate in the fine print: set it to what your supplier actually pays, or to zero if you are paid nothing. A stronger export rate shortens the payback, which is why it is an editable figure here rather than a hidden assumption.