How Many Solar Panels Do I Need?
You can answer this from one electricity bill and two lookups. It is four divisions, and the only place people go wrong is forgetting that a panel never delivers its rated output. Here is the calculation with every factor shown.
Step 1 — Find your daily consumption
Everything starts with kilowatt-hours, not with panels. Take your electricity bill and find the number of units consumed and the number of days billed. Divide one by the other.
Use twelve months of bills if you can get them, because a single month misleads badly. A summer bill in a hot climate is inflated by air conditioning; a winter bill in a cold one is inflated by heating. Add the twelve monthly figures, divide by 365, and you have a defensible annual daily average.
For our worked example, take a household using 900 kWh per month, which is 10,800 kWh per year:
Step 1 result: 900 kWh ÷ 30 days = 30 kWh per day.
Step 2 — Find your peak sun hours
Peak sun hours is the single most misunderstood input here. It does not mean hours of daylight. One peak sun hour is one hour of irradiance at 1,000 watts per square metre — the laboratory standard a panel's wattage is measured against. A location with twelve hours of daylight might have only five peak sun hours, because early morning and late afternoon sun is far weaker than the standard.
Look this up rather than guessing. The Global Solar Atlas and NASA's POWER dataset both give free annual averages for any coordinate, and the figure you want is normally reported as kWh/m²/day, which is numerically the same as peak sun hours.
| Location type | Peak sun hours | Effect on array size |
|---|---|---|
| Sunny subtropical | 5.2–5.8 | Smallest array |
| Mediterranean | 4.5–5.2 | Small |
| Temperate maritime | 2.6–3.4 | Roughly double a sunny site |
| High-latitude | 2.2–2.8 | Largest array |
REPLACE these bands with figures you have looked up and cited for the regions your readers live in. They are the right order of magnitude but they are broad, and this input has more leverage on the final answer than any other — halving peak sun hours doubles the array you need.
Our example uses 5.3 peak sun hours.
Step 3 — Apply a performance ratio
A 1 kWp array in a location with 5.3 peak sun hours does not produce 5.3 kWh per day. Several losses sit between the rating and the meter, and skipping them is what makes online calculators optimistic. The combined factor is called the performance ratio.
| Loss | Typical | Multiplier | Why |
|---|---|---|---|
| Cell temperature | −10 % | 0.90 | Panels run far above their 25 °C rating temperature |
| Inverter conversion | −4 % | 0.96 | DC to AC is never lossless |
| Soiling | −4 % | 0.96 | Dust, pollen, bird droppings between cleans |
| Cabling & connectors | −2 % | 0.98 | Resistance in DC and AC runs |
| Module mismatch | −2 % | 0.98 | Nominally identical panels are not identical |
| Performance ratio | — | ≈ 0.80 | Product of the above |
Use 0.80 for a well-designed roof-mounted system with an air gap beneath the panels. Drop to 0.75 if the array is flush-mounted in a hot climate, dusty, or partly shaded. Ground-mounted arrays with good airflow in a mild climate can reach 0.85. If shading is a factor at all, read our guide on inverter topologies before settling on a number — the right inverter choice can recover several points of it.
Multiplying gives the number that matters:
Effective daily yield: 5.3 peak sun hours × 0.80 = 4.24 kWh per kWp per day.
Step 4 — Array size, then panel count
Two divisions remain. First, daily consumption divided by effective daily yield gives the array size in kilowatts-peak:
30 kWh/day ÷ 4.24 kWh/kWp/day = 7.08 kWp
Second, array size divided by the wattage of the panel you intend to buy gives the count. A common residential module in 2026 is 550 W, which is 0.55 kW:
7.08 kWp ÷ 0.55 kW = 12.9 panels → 13 panels
Always round up, then re-check. Thirteen 550 W panels is a 7.15 kWp array, which at 4.24 kWh/kWp/day produces about 30.3 kWh per day, or roughly 11,070 kWh a year against the household's 10,800 kWh need. That is about 102% coverage.
Note how much the panel wattage matters to the count but not to the array. The same 7.08 kWp needs 18 panels at 400 W, 13 at 550 W, or 11 at 650 W. The kilowatt-peak figure is the real specification; the panel count is a consequence of which module you pick.
The roof area you actually need
Panel area is not roof area. A 550 W monocrystalline module is about 2,278 × 1,134 mm, or 2.58 m². Thirteen of them is 33.6 m² of glass. But you cannot tile a roof edge to edge: installers need walkways, fire-service setbacks at ridges and edges, clearance around vents and chimneys, and space between rows if the roof is flat and rows would shade each other.
Add 20 to 25% to the panel area for a simple pitched roof, and considerably more for a complex one with dormers and hips. Our example needs roughly 42 m² of usable, unshaded, correctly oriented roof — and "correctly oriented" is doing real work in that sentence, because a north-facing plane in the northern hemisphere is not usable area at all.
Sanity check before you go further: measure your usable roof plane first. If it is smaller than your calculated requirement, you have three choices — higher-efficiency panels, a smaller array that covers less of your bill, or reducing consumption. Discovering this at the quote stage wastes everyone's time.
Why 100% coverage is not automatically the goal
It is tempting to size an array to erase the bill entirely. Whether that is the best financial choice depends on one thing: the gap between what you pay for a unit you import and what you are paid for a unit you export.
If the two are equal — true net metering — then the grid acts as a free battery and sizing to 100% of annual consumption is sensible. If exported units earn a fraction of the import price, which is increasingly common, then every kilowatt-hour you send out is sold cheap and bought back expensive. In that case the array that pays back fastest is the one sized closer to your daytime load, not your total load, because self-consumed units are worth the full retail rate.
Work out your own tariff spread before committing to a size. Our guides on payback period and net metering rules both turn on this number, and it can change the optimal array by several kilowatts in either direction.
Size the array to the value of the electricity it displaces, not to the size of your bill. Those are the same number only when export and import tariffs match.
Common questions
How many panels for a 1,000 kWh per month bill?
At 5.3 peak sun hours and a 0.80 performance ratio: 33.3 kWh per day ÷ 4.24 = 7.9 kWp, which is 15 panels at 550 W and about 39 m² of panel area. Substitute your own sun-hour figure — at 3.0 peak sun hours the same bill needs roughly 14 kWp and 26 panels.
Does panel wattage change how many kilowatt-hours I get?
No. Two arrays of the same kilowatt-peak produce the same energy in the same location regardless of whether they are built from 400 W or 650 W modules. Higher-wattage panels only mean fewer units to mount and less roof used, which reduces labour and area, not energy.
Should I add extra panels for future needs?
Adding capacity later is usually more expensive per watt than building it now, because scaffolding, labour and possibly a larger inverter get paid for twice. If an electric vehicle or heat pump is a realistic plan within a few years, size for it now — but check your inverter's maximum DC input and your grid connection limit first, since both cap how far you can expand.
How accurate is this calculation?
Within about 10 to 15% for annual output, which is enough to size a system and sanity-check a quote. It cannot predict a specific month, and it does not model shading, which needs a site survey. Treat it as the number you use to judge whether an installer's proposal is reasonable.
Sources and further reading
Replace these with the specific datasets and datasheets you rely on. The irradiance source matters most here, because it drives the whole result.
- REPLACE — Global Solar Atlas (World Bank / Solargis), annual average daily global horizontal irradiance for your location.
- REPLACE — NASA POWER surface meteorology and solar energy dataset, as a cross-check on the above.
- REPLACE — Manufacturer datasheet for the module you cite, for exact wattage, dimensions and temperature coefficient.
- REPLACE — Your national or local rooftop installation standard, for setback and walkway requirements.
This guide is general information, not engineering or financial advice. See our full disclaimer.