How Many Solar Panels Do I Need to Go Off-Grid?

Walk into a solar shop and they'll quote you "a 7 kilowatt system, around 25 panels". That sounds confident. It's also mostly wrong.

The right number of panels depends on you. Your electricity use. Your roof. Your weather. A house in sunny Spain needs half the panels of the same house in cloudy Scotland.

Here's how to figure out your real number, in five steps.

Step 1: How Much Power You Actually Use

Grab a power bill. Look for the kilowatt-hours (kWh) number. That's the unit electricity companies use to measure how much you used.

A typical European house uses about 10 kWh a day. An American house uses closer to 30 kWh. A small cabin off the grid might use 3 or 4.

You can be more accurate by walking around your home with a list. A fridge uses about 1 kWh a day. A TV uses 0.5. A laptop uses 0.2. A washing machine uses 1 per load.

Add it up. That's your daily target.

→ Use the yearly kWh calculator

→ How to calculate your solar energy needs

Step 2: How Much Sun You Get

Solar panels don't make power evenly throughout the day. They peak around noon and taper off at the edges. People simplify this by counting "peak sun hours".

One peak sun hour means one hour of full-strength sunlight. A long, weak winter day might only give you 2 peak sun hours. A bright summer day in Arizona gives you 7.

A few rough numbers by region:

The Netherlands: 3 to 4 peak hours a day, averaged across the year
Germany: 3 peak hours
Southern Spain: 5 peak hours
Texas: 5 to 6 peak hours
Arizona: 6 to 7 peak hours

→ How many sun hours does your area get?

Step 3: Do The Math

The formula is dead simple.

Daily power needed (kWh) ÷ peak sun hours = solar array size (kW)

So if you use 10 kWh a day in a region with 4 peak sun hours, you need a 2.5 kW solar array.

10 ÷ 4 = 2.5

A typical modern solar panel is rated at 400 watts (0.4 kW). So you'd need about 7 panels.

2.5 ÷ 0.4 = 6.25, round up to 7

That's your starting estimate. Easy.

→ Use the solar panel calculator

Step 4: Add A Safety Buffer

Real life is messy. Clouds happen. Dust covers the panels. The angle is never perfect. Panels lose a tiny bit of efficiency each year.

So most installers add 20 to 30% on top of the calculated number.

Our 7-panel example becomes 9 panels in the real world. Better to have a little extra than to run out of power.

If you're going fully off-grid (no backup from the city grid), bump the buffer higher. 50% is not crazy. A few cloudy days in a row will drain your batteries fast if you cut it too close.

Step 5: Match The Panels To Your Roof

Now you need to make sure the panels actually fit and make sense for your roof.

How much space do you have? A typical 400W panel takes about 2 square meters. 9 panels need 18 square meters of clear, sunny roof.

Which way does the roof face? In the northern hemisphere, south is best, east and west work, north doesn't. The opposite in the southern hemisphere.

How shaded is it? A tree, a chimney, or a neighbour's roof that throws shadow on part of your array will hurt your output.

If the space is tight, use monocrystalline panels. They make more power per square meter. If you have a big sunny field, polycrystalline is cheaper per watt. For curved or odd surfaces, thin-film is the answer.

A Real Example

Let's run through a real one. A family in Berlin uses 12 kWh a day. They live in a region with about 3 peak sun hours in winter.

12 ÷ 3 = 4 kW array

4 kW ÷ 0.4 kW per panel = 10 panels

Add a 30% buffer for cloudy weeks = 13 panels

That's the answer. 13 modern panels, total roof space of about 26 square meters, facing south.

In summer, they'll make way more than they need. In winter, they might just barely cover it. If they're fully off-grid, they need batteries to bridge the dark months.

→ What to think about before going off-grid with solar