How many solar panels do you need?

How it’s worked out, step by step

1. What your home uses per day = the kWh per month on your bill ÷ 30.4 (the average days in a month: 365 ÷ 12).
2. What 1 kWp of panels gives per day where you live. One kWp (“kilowatt peak”) is 1,000 W of panels: for example, two 500 W panels. The figure comes from the World Bank’s Global Solar Atlas, which already deducts the normal losses of a system: the inverter, dirt, cables and panel heat.
   • Grid-tied, it uses the yearly average (kWh per year ÷ 365): in the weaker months the grid covers what’s missing.
   • Off-grid, it uses the worst month, because the panels have to be enough then too. And since much of the energy goes through the battery, it deducts what’s lost there: according to the U.S. national energy laboratory (NREL, now NLR), a home battery gives back 85% of what goes in. To be on the safe side, the calculator applies that to all the energy.
   • On a roof, 4% less. The Atlas assumes panels at the ideal tilt and well ventilated; in its own examples, a home rooftop system produces 3 to 4% less, because it runs hotter and gets dirtier.
3. The kWp you need = what your home uses per day ÷ what 1 kWp gives per day.
4. Number of panels = that kWp × 1,000 ÷ each panel’s watts, rounded up.

With what the calculator loads when you open it (300 kWh a month, Madrid, grid-tied, on a roof): the home uses 300 ÷ 30.4 = 9.86 kWh a day. In Madrid, 1 kWp gives 1,655 kWh a year, which is 1,655 ÷ 365 = 4.53 kWh a day, and on a roof 4.53 × 0.96 = 4.35. You need 9.86 ÷ 4.35 = 2.27 kWp, which with 450 W panels is 2,266 ÷ 450 = 5.04: 6 panels. Off-grid, with Madrid’s worst month (December, 3 kWh per kWp a day) and the battery: 3 × 0.96 × 0.85 = 2.45 kWh a day, so 9.86 ÷ 2.45 = 4.03 kWp, 9 panels.

The rest of the kit: how it’s worked out

Off-grid, the other parts appear below the panels. Grid-tied without batteries, you need neither batteries nor a charge controller: the U.S. Department of Energy explains these are needed when you want to store energy, and that the inverter must be one your power company accepts.

• System voltage: the lowest of 12, 24 or 48 V at which 100 amps or less flow from the panels to the batteries (the panels’ watts ÷ the voltage). It’s our own rule of thumb so the cables don’t need to be huge; the installer may choose differently.
• Batteries = what your home uses per day × the days without sun ÷ 0.85 (inverter losses) ÷ the share of the battery you use: 80% for lithium, as Australia’s CSIRO recommends so they last longer, and 50% for lead-acid, a cautious choice according to PVEducation. In amp-hours: the Wh ÷ the voltage. Which type to choose is explained in home battery types.
• Charge controller = the panels’ watts ÷ the voltage. We suggest an MPPT because it harvests more energy from the panels than a conventional PWM, as a test published in 2018 showed (explained here).
• Inverter: pure sine wave, because the U.S. Department of Energy notes electronics need steady power with a smooth waveform, and the same voltage as the batteries. Its size doesn’t depend on what you use over the day but on what you run at once: see our inverter article.

With what it loads when you open it, but off-grid: the 9 panels add up to 9 × 450 = 4,050 W, which at 48 V is 4,050 ÷ 48 = 85 A (at 24 V it would be 169). Lithium batteries for one day: 9.86 ÷ 0.85 ÷ 0.8 = 14.5 kWh, which at 48 V is 303 Ah. How it all connects, with or without a hybrid inverter, is explained in how a solar kit connects.

How to find your location’s figure in the Global Solar Atlas

1. Open globalsolaratlas.info and click on the map where your home is (or type the address in the search box).
2. In the panel that opens, find “Specific photovoltaic power output” (PVOUT). Its figure in kWh/kWp per year goes in the first field.
3. For the worst month, look at the monthly chart for that same figure: take the lowest bar and divide it by the days in that month (for example, 93 kWh in December ÷ 31 = 3 kWh a day).

What the calculator doesn’t account for

• Shade from trees, buildings or antennas. The Atlas only accounts for shading from mountains and hills. Shade on the panels can cut what they produce a lot.
• Which way your roof faces. The Atlas assumes panels facing the equator (south in the northern hemisphere, north in the southern hemisphere) at the ideal tilt.
• Ageing. The Atlas gives what new panels produce, without wear over time.

It’s an estimate to give you an idea before asking for quotes. Connecting panels to your home’s wiring, let alone to the grid, is a job for a qualified installer or electrician.