What This Calculator Does
This solar panel calculator answers one simple question: how many panels do you need to cover the electricity you use in a year?
You type in your yearly use, the size of one panel, where you live and which way your roof faces. It tells you the number of panels, rounded up to a whole one.
How To Use This Calculator
- In Annual energy consumption, type your yearly use in kWh. Your last power bill shows it. Don't have one? Work it out first with the energy use calculator, which can send its number straight here.
- In Power rating per panel, type the watts on the panel's datasheet. New home panels in 2026 are about 430 to 470 W.
- In Your region, pick the country or area closest to you. This fills in Daily yield per kWp from PVGIS, the European Commission's solar tool. It starts at the Netherlands.
- In Roof direction and tilt, pick how your panels will face.
- Read the big number in the result card. That's how many panels you need. The small line under it shows the total size in kWp, what those panels make in a year, your daily need and what one panel makes per day.
The Formula, In Plain Words
First the calculator works out what one panel makes in a year: its watts divided by 1,000, times the daily yield, times the roof factor, times 365.
Then it divides your yearly use by that and rounds up. You can't buy 0.4 of a panel, so it always rounds up.
It uses exactly the same yield numbers as the yearly kWh calculator, so the two always agree. The daily yield is "effective sun hours": what 1 kWp of panels delivers per day after normal losses from heat, dirt, cables and the inverter.
A Worked Example
Say your home uses 4,000 kWh a year and you pick 435 W panels in the Netherlands, on a south roof.
PVGIS gives 2.81 kWh per kWp per day there. One panel makes 0.435 × 2.81 × 365 = about 446 kWh a year.
You need 4,000 ÷ 446 = 8.97, so the answer is 9 panels, or 3.9 kWp. Together they make about 4,015 kWh a year.
On an east or west roof, one panel makes 79% of that, about 352 kWh. Now you need 12 panels.
In southern Spain, at 4.58 kWh per kWp per day, 6 panels are enough. Same house, same panels: the place and the roof matter more than any other box.
What To Watch Out For
Sun hours come in two kinds. "Peak sun hours" count raw sunlight and skip the losses. "Effective sun hours" include them and are lower.
This calculator uses effective ones from PVGIS.
PVGIS assumes no shade from nearby objects. Shade from trees or chimneys can cost a lot, so add a panel or two if your roof is not perfect.
The count covers your use over a whole year, on average. An off-grid home has to get through winter too: in December a Dutch roof makes about a third of its average day.
That's why off-gridders often add extra panels or a backup. Read how many solar panels you need to go off-grid for that side of the story.
Roof space can be the real limit. Check that the panels fit before you buy them.
Hard Words
| Term | What it means |
|---|---|
| Annual energy consumption | All the electricity your home uses in one year, in kWh. |
| Panel rating (W) | How much power a panel makes in perfect test sunlight. |
| Peak sun hours | Hours of full-strength sunlight per day, counted before any losses. |
| Effective sun hours | Sun hours after normal losses from heat, dirt, cables and the inverter. Lower and more realistic. Here called daily yield per kWp. |
| Roof factor | How much a roof makes compared to the best case, south at about 35°. |
| Round up | Going to the next whole number, because you can only buy whole panels. |
Common Questions
How many solar panels do I need for 4,000 kWh a year?
With 435 W panels on a south roof in the Netherlands you need 9 panels, or 3.9 kWp. On an east or west roof you need 12. In southern Spain 6 are enough. Where you live and how your roof faces matter more than anything else.
How do I calculate how many solar panels I need?
Find the daily yield per kWp for your spot, for example in PVGIS. Multiply one panel's watts by that yield and by 365, then divide by 1,000 to get its yearly kWh. Divide your yearly use by that number and round up. The result is the number of panels.
What sun hours should I use?
Use effective sun hours, which include normal system losses. PVGIS gives about 2.5 to 3.2 for northwest Europe and about 3.4 to 4.6 for southern Europe. The calculator fills in a value per region, or you can look up your exact address in PVGIS and divide its yearly kWh per kWp by 365.
Will this many panels power my home in winter?
Not always. The calculator matches your use over a whole year, and summer makes far more than winter. A grid-connected home is fine with that. An off-grid home usually needs extra panels, a bigger battery, or a backup generator to get through the darkest weeks of the year.
Do bigger panels mean I need fewer?
Yes. For 4,000 kWh in the Netherlands you need 12 panels of 350 W but only 9 of 450 W. Bigger panels are also physically larger, though, so check your roof space. The total kWp on your roof is what really counts.