Are solar panels worth it?
Put the quote you have been given against what a roof in your country actually generates, and see the payback, the lifetime return, and what a unit of your own electricity really costs.
Using the US average of 18.44¢/kWh — the U.S. Energy Information Administration, May 2026.
The size most households are quoted, on a clear roof facing the right way, with nobody home during the day.
Common cases
What drives the cost
Four kilowatt-peak is the size most households end up quoted, because it is roughly what fits on half a typical roof and roughly what the average home uses in a year. That coincidence is misleading, though. Matching your annual generation to your annual consumption sounds tidy but means very little in practice, because the generation arrives in summer daylight and the consumption happens on winter evenings. What matters is not the annual totals lining up but how much of the output is wanted at the moment it appears.
With nobody home in the day, that is typically only a quarter to a third of it. The rest is exported at a fraction of what it would have saved you. This is why two identical arrays on two identical roofs can have paybacks years apart, and why the single most useful thing you can do before signing is work out honestly what your household does at two o'clock on a weekday afternoon.
How this is worked out
A solar system is worth the electricity it saves you buying, plus the little it earns for what you send back. Those two are not worth the same, and that is the whole subtlety of the sum. A unit you use as it is generated saves the full retail price; a unit exported earns whatever your export tariff pays, typically a fraction of it. So the share you use yourself moves the answer more than the size of the array does — which is why being home during the day, or adding a battery, changes the payback so much more than adding panels. What the roof itself generates is the easy part: a kilowatt-peak of panels delivers a known amount in a given place, adjusted for which way the roof faces and what stands in the way of the sun.
What this assumes
- Yield comes from PVGIS, the European Commission's Joint Research Centre model, for a building-mounted array at the optimal tilt facing the equator, with 14% system losses. The figure is population-weighted across each country's largest cities, so it describes where people actually live rather than the sunniest corner of the map.
- Orientation factors relative to that best case are: 96% at 45° off, 83% east or west, 66% at 135° off, and 58% facing away from the equator. These are representative of mid-latitudes — an east-west roof loses less in southern Spain, where the sun is high and the day long, than in Scotland where it is neither.
- Shading is applied as a flat 8%, 20% or 35% loss. Real shading is far more specific than that, and worse than a simple percentage on a string inverter, where one shaded panel drags down every panel wired with it. Treat a shaded result as a reason to ask the installer for a proper shade analysis, not as an answer.
- Panels are assumed to lose 0.5% of their output each year, which is the usual warranted rate. Payback is worked out year by year rather than dividing cost by first-year benefit, because later years generate less and the simple division quietly flatters the answer.
- The default export rate is a rule of thumb — about a third of the import rate — and not a published figure. Export schemes differ enormously: Germany has a statutory tariff, Britain leaves it to each supplier, the Netherlands is unwinding net metering, Australia varies by state. Put your own tariff in.
- Nothing is counted for inverter replacement, which most systems need once in 25 years, for cleaning or maintenance, or for any grant, loan interest or tax treatment where you live. Nor for electricity prices changing, which over 25 years they certainly will — rising prices make solar pay back faster than this shows.
- The rate filled in for you is an average that spreads fixed charges across a year's usage, so it sits above what one extra unit really costs. Entering the unit rate from your own bill gives a closer answer.
Questions
How many panels is 4 kWp?
Around 8 to 13, depending on their rating. Modern domestic panels are commonly 350 to 450 watts each, so a 4 kWp array is roughly ten of them, needing somewhere around 20 square metres of roof. Panel counts in quotes are less comparable than the kWp figure, which is why that is what this asks for.
Is my quote a fair price?
The calculator answers this better than a price comparison does. Change the installed cost until the payback looks like what you would accept, and you have your own ceiling. As a sanity check, the cost per unit generated is the number to watch: if it lands comfortably below what you pay the grid, the quote is doing its job whatever the headline price.
Are solar panels actually worth it?
Put your own quote in above and the answer stops being a matter of opinion. The test that matters is the cost per unit generated: if the system produces electricity for less than you currently pay the grid, it pays for itself and then keeps going. In most of Europe a fairly quoted array on a decent roof comes out well under the retail rate. What kills a specific case is nearly always one of three things — a heavily shaded roof, a price well above the going rate, or exporting almost everything because nobody is home.
Why does using the electricity myself matter so much?
Because the two things a generated unit can do are worth very different amounts. Used at home, it saves you the full retail price. Exported, it earns the export tariff, often around a third of that. Moving a unit from the export column to the self-use column is therefore worth the gap between the two, every time. That single fact is why a battery, a timed immersion heater, or simply running the washing machine at midday all improve the payback more than another panel would.
Does a battery pay for itself?
Sometimes, and it is much closer than the sales pitch suggests. A battery generates nothing — it only moves units from export to self-use, so its value is capped at the gap between those two rates multiplied by how much it shifts. Compare the two presets above: storage lifts self-consumption from about 35% to about 70%, which is real money, but it also adds thousands to the quote and most batteries are warranted for fewer cycles than the panels' 25 years. It is far more likely to pay in a country where export pays badly and electricity is dear.
How long do solar panels last?
Longer than the payback, which is the point. Panels are typically warranted to still produce around 80 to 85% of their original output after 25 years, and most keep working well beyond that — the 25 years used here is a warranty period, not a lifespan. The inverter is the part that usually needs replacing once along the way, which this calculator does not count and you should budget for.
Is an east-west roof worth bothering with?
Usually yes, and more than the raw numbers suggest. An east-west array generates around 83% of what an equator-facing one would, which sounds like a serious loss until you notice when it arrives: morning and evening rather than concentrated at midday. That matches household demand better, so a higher share gets used at home at full retail value instead of exported cheaply. A worse generator can be a better investment.
Should I fill the whole roof with panels?
It depends on what happens to the extra output. Bigger arrays cost less per kilowatt, because scaffolding and a day's labour do not shrink, so the generation is cheap. But each additional panel is more likely to be exported than used, and exported units are worth a fraction of the ones you use. If your export tariff is decent, filling the roof is usually right; if it pays badly, there is a size beyond which extra panels barely earn anything.
Do solar panels work in cloudy countries?
Yes, and the figures above show it: Britain sits around 944 kWh per kilowatt-peak a year against Spain's 1,523. That is a real difference, but it is a difference of about a third rather than the order of magnitude people imagine. Panels run on daylight, not heat, and in fact lose a little efficiency when they get hot — which is why a cool bright day in northern Europe is close to ideal conditions for them.
Related tools
- How much does it cost to run an appliance? Work out what any appliance costs to run per use, per month and per year on your own electricity rate — including what it draws on standby. Rates for all 50 states.
- What does it cost to heat this room? Work out what a room actually loses in heat, the heater size it needs, and what holding it at a comfortable temperature costs per hour, per month and across a winter.
Sources
- PVGIS 5.2, EC Joint Research Centre — grid-connected PV performance tool — checked 2026-08-28
- PVGIS — data sources and calculation methods — checked 2026-08-28
Rates from U.S. EIA for May 2026. Last reviewed August 28, 2026.