"How many solar panels do I need?" is the question everyone starts with. The good news: you can get a solid estimate yourself with a bit of arithmetic and three numbers. This guide shows you the method — it works anywhere in the world, you just plug in your local figures.
The three numbers you need
- Your monthly electricity use in kWh. Find this on your utility bill (look for "kWh used"). If it only shows cost, divide your bill by your price per kWh.
- Your local peak sun hours. This is the number of hours per day of full-strength sunlight your area averages — typically 3 to 6 depending on location. (Our solar map shows this for any spot using NASA data.)
- Your panel wattage. Modern residential panels are usually 400–550 watts each.
The formula
Work it in three steps:
Step 1 — Daily energy need:
Monthly kWh ÷ 30 = daily kWh need.
Step 2 — System size needed (kWp):
Daily kWh ÷ (peak sun hours × 0.8) = system size in kWp.
The 0.8 accounts for real-world losses — inverter inefficiency, heat, wiring, dust. No system is 100% efficient.
Step 3 — Number of panels:
(System size in kWp × 1,000) ÷ panel wattage = number of panels.
A worked example
Let's say you use 900 kWh per month, your area gets 4.5 peak sun hours, and you're using 450-watt panels:
- Daily need: 900 ÷ 30 = 30 kWh/day
- System size: 30 ÷ (4.5 × 0.8) = 30 ÷ 3.6 = 8.3 kWp
- Panels: (8.3 × 1,000) ÷ 450 = 18,444 ÷ 450 ≈ 18 panels
So this home needs roughly an 8.3 kWp system, about 18 panels, to cover its full annual usage.
How location changes everything
The same 900 kWh home needs very different systems depending on sunshine:
| Location type | Peak sun hours | System needed | Panels (450W) |
|---|---|---|---|
| Sunny (e.g. Arizona, tropics) | 6.0 | 6.3 kWp | ~14 |
| Moderate (e.g. mid-US, Spain) | 4.5 | 8.3 kWp | ~18 |
| Cloudy (e.g. UK, Pacific NW) | 3.0 | 12.5 kWp | ~28 |
This is why a solar quote from a sunny state won't match one from a cloudy region — same house, different sun.
Things that adjust your number
Roof direction and tilt
Panels facing the optimal direction (south in the Northern Hemisphere, north in the Southern) produce the most. East/west roofs lose roughly 10–20%, so you'd need a few more panels.
Shading
Trees, chimneys, or nearby buildings that shade your roof cut output and may mean more panels or optimisers.
Do you want to cover 100%?
You don't have to size for your entire bill. Many people install what fits their roof or budget and offset 60–80% of usage — often the sweet spot for payback, since the last bit of offset is the least cost-effective.
The shortcut
The maths above gives you a genuine ballpark. For a precise figure that factors in your exact location's sun data, roof direction, shading, and local electricity rates, a calculator does the heavy lifting in seconds — which is exactly what Sunlytic is built for.
Bottom line
To estimate your panel count: divide monthly kWh by 30, divide that by (sun hours × 0.8) to get system size, then divide by your panel wattage. Most homes land between 10 and 25 panels. Sunshine and roof direction shift the number more than anything else — so the same house genuinely needs different systems in different places.
See the numbers for your own home
Sunlytic's free calculator gives you a personalised system size, cost, and payback estimate in about two minutes — no sign-up required.
Calculate my solar savings →This guide is for general information only and does not constitute financial, engineering, or investment advice. Solar costs, tariffs, and government schemes change frequently and vary by household. Figures are indicative ranges drawn from publicly available sources at the time of writing. Always obtain written quotes from qualified, locally-licensed installers and verify current rates before making any decision.