How many solar panels do I need? A simple method that works anywhere
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The number of panels you need comes down to three numbers: how much energy the panels must produce each day, how much sun you get, and how big each panel is. Here is the method, step by step.
Step 1: how much energy must solar produce each day?
That depends on the job you give it.
- Recharging a backup battery: the energy you use while the grid is off. If your home uses 4 kWh a day and the power is off for 12 hours, that is about 2 kWh.
- Running the house on solar: everything you use in a day, 4 kWh in this example.
Add about 10% for the energy the inverter loses converting battery power. So 2 kWh becomes about 2.2 kWh, and 4 kWh becomes about 4.4 kWh.
Not sure what your home uses? List your appliances in the calculator, or read how to find your appliances’ watts.
Step 2: find your peak sun hours
Peak sun hours are the number of hours of full-strength sunshine your location gets per day, on average. They range from under 1 in a northern European winter to over 8 in a desert summer. Use the figure for your dullest month if the system has to keep up all year. Peak sun hours explains why.
Our locations pages show month-by-month figures for hundreds of cities, and the calculator fetches them for any town.
Step 3: allow for real-world losses
Panels rarely produce their label rating. Heat, dust, wiring, shade and the charge controller typically take about a quarter, so count on 75% of the rating.
Step 4: do the sum
panel watts needed = daily energy (Wh) ÷ (peak sun hours × 0.75)
number of panels = panel watts needed ÷ watts per panel, rounded up
Example: recharging a battery in a sunny city
- Energy to replace: 2,200 Wh
- Dullest month: 5 peak sun hours
- Panel watts needed: 2,200 ÷ (5 × 0.75) ≈ 590 W
- With 450 W panels: 590 ÷ 450 = 1.3, so 2 panels
Example: running a home through a cloudy season
- Energy to produce: 4,400 Wh
- Dullest month: 3 peak sun hours
- Panel watts needed: 4,400 ÷ (3 × 0.75) ≈ 1,960 W
- With 450 W panels: 1,960 ÷ 450 = 4.4, so 5 panels
Same home, same appliances: the sunnier location needs well under half the panels. That is why a quote for “a 3 kW solar kit” means little until you know where it is going.
Will they fit?
A typical 450 W panel is around 2 square metres. Five panels need roughly 10 square metres of unshaded roof or ground, plus space around them for fixing and airflow. Check the actual dimensions on the panel’s data sheet.
Facing the panels towards the equator (south in the northern hemisphere, north in the southern) and tilting them at roughly your latitude usually gives the best year-round output. Shade from trees, water tanks or neighbouring buildings can cut output sharply, so check the spot at different times of day.
Panels are only one part of the system
More panels won’t help if the battery is too small to store their energy, or if the charge controller can’t handle their current. Size the three together:
- the inverter for what you run at once;
- the battery for how long you run it (lithium or lead-acid);
- the panels and charge controller to refill the battery each day.
The calculator does all three for your appliances and town.