How Many Solar Panels You Really Need

Ask three solar installers how many solar panels you need and you can get three different answers. Each one is guessing at your life.

The real answer depends on you. How much electricity you consume. How much sunlight your location gets. How big your roof is. The same house in sunny Seville needs only about half the solar panels it would need in cloudy Edinburgh.

The good news is that the math isn't complex. You can do it on the back of an envelope, and we'll walk through the whole process in five steps.

Step 1: Determine Your Daily Energy Use In kWh

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Grab an electricity bill and look for the kilowatt-hours (kWh). That's the unit electricity companies use to measure how much energy you used.

An average home in the EU uses about 3,600 kWh of electricity a year, which is about 10 kWh a day. An average American home uses about 30 kWh a day, three times as much.

You can get more exact by walking around your home with a list, including the small things you forget, like the router. For each device, multiply its wattage by the hours it runs each day, then divide by 1,000. A fridge switches itself on and off, so people often count it for a third of the day: a 150 W fridge for 8 hours is 1.2 kWh. A 50 W laptop for 5 hours is 0.25 kWh, and a 600 W washing machine for one hour is 0.6 kWh.

Add it all up and you have your daily energy consumption, the number the whole solar system is built around. If you expect your use to grow, say with an electric car later, take that into account now. Our step-by-step off-grid load calculation shows the whole process with a sample home that uses about 3.1 kWh a day, and the energy use calculator does the adding for you.

→ Use the yearly kWh calculator

→ How to calculate your solar energy needs

Step 2: How Much Sunlight Your Location Gets

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Solar panels don't make electricity evenly through the day, because sunlight conditions change from hour to hour. They peak at noon and fade at the edges, like a hill. To keep the math simple, people count "peak sun hours".

One peak sun hour means one hour of full-strength sunlight, the amount of solar energy a panel gets at noon on a clear day. A dark December day in Berlin gives a panel about 1 of those, while a June day in Phoenix gives it almost 6.

Here are some yearly averages from PVGIS, the free solar tool of the European Commission. They are for solar panels at the best angle, after normal losses from heat, dirt, cables and the inverter:

• The Netherlands: about 2.8 peak hours a day. Most real roofs get a bit less.
• Germany: about 3 peak hours
• Southern Spain: about 4.6 peak hours
• Texas: about 4 peak hours
• Arizona: about 5 peak hours

→ How many sun hours does your area get?

Step 3: The Formula For Calculating Solar Panels

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The formula is short enough to fit on a sticky note.

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

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

10 ÷ 4 = 2.5

Most solar panels for homes today are rated at about 400 watts (0.4 kW). Divide the array size by the panel wattage and you need about 7 panels.

2.5 ÷ 0.4 = 6.25, round up to 7

That's your first estimate for a solar energy system. A larger panel lowers the count, while a smaller one raises it. Want to skip the arithmetic? The solar panel calculator does it for you, with sun data for your region already filled in.

Step 4: Check The Darkest Month, Not The Average

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Here's the trap. The math in step 3 uses a yearly average. It tells you how many solar panels make the right amount of electricity over a whole year, but not whether your lights stay on in December.

Those are two very different questions.

In summer your solar panels will produce far more energy than you need. In the dark months the sun is low, the days are short, and they produce a much smaller amount. You can't use July's extra sunshine in January unless you have somewhere to keep it, and no home battery is big enough to store a whole summer.

If you're on the grid, that doesn't matter much, because the grid will cover the gap. If you're fully off-grid, it matters a lot, because your solar system is the only power plant you have. A fixed 20 or 30% on top won't fix it, because in northern Europe December output can be less than a third of the yearly average.

So consider an off-grid solar system in three layers:

1. Size the solar panels for your worst month. Look up your own address in PVGIS. It shows how much energy a panel makes in each month, not just the yearly total, so you can take the lowest one into account.
2. Add usable storage for several dark days. Some weeks bring days with almost no sun at all, and your battery has to carry you through. Count the usable part only, not the label. Our guide to choosing a solar battery shows how.
3. Plan a backup. A generator (or the grid, or wind) provides the solar energy the panels and storage can't. Off-grid in a cloudy country, this is a normal part of the system, not a failure.

A small extra margin for dust and aging is still smart. It just isn't a plan for the dark season.

→ Calculate your battery bank size

→ Check how long a generator can back you up

Step 5: Optimal Roof Characteristics

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Now check that the solar panels actually fit your roof and that the installation makes sense. A few roof details decide how much solar energy you can expect, and they are the main considerations before any installation.

How much space do you have? A common 400 W panel measures about 1.7 by 1.1 meters, so roughly 2 square meters. Nine solar panels need about 18 square meters of clear, sunny roof.

Which way does the roof face? In the northern hemisphere, a roof that points to the equator is best, east and west work, and north doesn't. In Berlin, PVGIS gives an east or west roof about 18 to 25 percent less energy than the best one, depending on the slope. Our guide to roof angle and orientation goes deeper.

How shaded is it? A tree, a chimney or a neighbour's roof that throws shadow on part of your array can reduce its performance a lot. However, the damage depends on the wiring: one shaded cell can switch off a whole group of cells inside the panel. Read more about how shade affects solar panels.

If space is tight, use monocrystalline panels. They make more power per square meter, and almost every new panel is mono anyway: in 2025 multicrystalline production had dropped to zero. Compare panels on panel efficiency (watts per square meter) and price per watt, since different brands offer different trade-offs. Polycrystalline is mostly found second-hand now, and for curved or odd surfaces, thin-film is the answer.

A Real Example, Worked Out

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Let's run through a real solar example, start to finish, the way you'd do it for your own location. Every number below is either an assumption (marked) or comes from PVGIS.

The setup

• A family in Berlin uses 12 kWh of electricity a day (assumption). That's 12 × 365 = 4,380 kWh a year.
• Panels of 400 W each (assumption), facing the equator at the best yearly angle, which PVGIS puts at 40 degrees for Berlin.
• PVGIS, with its standard 14% loss for heat, dirt, cables and the inverter, says 1 kW of panels in Berlin makes about 1,053 kWh a year. That's 1,053 ÷ 365 = 2.88 kWh per kW per day on average.
• But in December, the darkest month, the same 1 kW makes only 0.93 kWh a day.

Check 1: Does it add up over the year?

12 ÷ 2.88 = 4.17 kW of solar panels

4.17 ÷ 0.4 = 10.4, round up to 11 panels (4.4 kW, about 22 square meters)

→ Estimate solar panels with these numbers

Check 2: Does it work in December?

11 panels × 0.4 kW × 0.93 = about 4.1 kWh a day

They need 12. So in December they're short by about 7.9 kWh, every single day. The 11 panels that look perfect on paper give only about a third of a December day.

To cover December with solar panels alone, they'd need 12 ÷ 0.93 = 12.9 kW, which is 33 panels and about 66 square meters. Three times as many, and in June most of that power would go to waste. Tilting the panels steeper helps a little: at 60 degrees PVGIS gives 1.06 kWh per kW in December instead of 0.93, but the yearly total drops a bit. It doesn't close a gap this big.

Check 3: Storage for dark days

Say they want to ride through 3 days with almost no sun (assumption). That's 3 × 12 = 36 kWh they must be able to take out of storage.

A lithium battery should only be emptied to about 80% (assumption), and about 10% is lost in the inverter and cables (assumption). So the size on the label must be:

36 ÷ (0.8 × 0.9) = 50 kWh, or about 1,040 Ah at 48 V

→ Calculate your battery bank size with these numbers

Check 4: The backup, for the long gap

Storage handles a few dark days. It can't fix weeks of being 7.9 kWh short every day, because it would be empty after a few days and never fill up again. So in the dark months a generator has to make up roughly 8 kWh a day, on top of what the 11 panels give.

→ Check how long a generator can run for 8 kWh a day

What this family learns

11 solar panels match their year. They do not keep the lights on through a Berlin December by themselves. Off-grid, they need 11 panels or more, storage for a few dark days, and a backup for December and January. Living off-grid here is very doable. It just means planning for the darkest weeks, not for the average.

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

Sizing The Rest Of Your Solar System

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Solar panels are only half the story. To make the electricity useful, an off-grid solar installation also needs:

• An inverter to change DC to AC
• A charge controller if you're storing power
• Batteries to keep power for the night
• Wires, fuses, and a safety switch to tie it all together

→ All the electronic parts in a solar setup

→ The full story of AC vs DC power

→ Off-grid vs grid-tied: which is right for you?

→ How sunlight becomes electricity, the full story

Sources

• PVGIS, Photovoltaic Geographical Information System (European Commission JRC)
• ODYSSEE-MURE, Electricity consumption per dwelling
• U.S. EIA, How much electricity does an American home use?
• Virginia Cooperative Extension, Estimating Appliance and Home Electronic Energy Use
• Solarproof, Suntech Ultra V mini STP400S-C54/Umhm specifications
• Fraunhofer ISE, Photovoltaics Report
• Aurora Solar, How shading affects a solar array
• Victron Energy, Lithium iron phosphate batteries Smart datasheet