How Many Solar Panels Do I Need?

The number of solar panels you need depends on how much electricity you use each day and how much sun your location gets. The good news is that the math is simple. This guide gives you the formula, a quick chart, and the factors that change the answer.

The formula

Divide your daily energy use by the usable sun you get, then divide by the wattage of one panel:

Panels needed = Daily watt-hours / (Peak sun hours x (1 - losses)) / Panel watts

• Daily watt-hours: add up every appliance's watts x hours used per day. Your power bill also shows kWh per day or month (1 kWh = 1,000 Wh).
• Peak sun hours: the number of hours per day of full-strength sunlight at your location. Most places fall between 3 and 6.
• Losses: wiring, heat, dust, and inverter losses. A common estimate is 20% to 25%.
• Panel watts: printed on the panel label, often 200 W to 550 W.

Worked example

A small home uses 5 kWh (5,000 Wh) per day. It gets 4.5 peak sun hours, and we assume 22% losses. Panels are 400 W.

1. Usable sun: 4.5 x 0.78 = 3.51 hours.
2. Array size: 5,000 / 3.51 = about 1,425 W.
3. Panels: 1,425 / 400 = 3.56, so round up to 4 panels.

Quick chart

This chart assumes 5 peak sun hours, 22% losses, and shows the panel count for 400 W and 550 W panels:

Daily use	Array size	400 W panels	550 W panels
2 kWh	about 510 W	2	1
5 kWh	about 1,280 W	4	3
10 kWh	about 2,560 W	7	5
20 kWh	about 5,130 W	13	10

Try the solar panel calculator

Enter your daily or monthly energy use, your peak sun hours and your panel size. The calculator gives your panel count, array size, daily output and roof area, shows how the count changes in low-sun months, and lets you export the result as a PDF.

Solar Panel Calculator

How many panels do you need for your daily use?

Your numbers

My energy use is kWh per daykWh per month (bill)

Peak sun hours are usually between 3 and 6. Use your worst months if you need power all year.

Result

Checks

Panels needed at different sun hours

Panels = Daily Wh / (Peak sun hours x (1 - losses)) / Panel watts, rounded up. Losses cover wiring, heat, dust and inverter (about 20% to 25%). Shade and tilt are not included. Estimates only. Have your final design reviewed by a qualified electrician or installer.

What changes the answer

• Your location and season: winter has fewer sun hours. If you need power year-round, size for the worst months.
• Shade: even partial shade can cut a panel's output a lot. Place panels where they get clear sun from mid-morning to mid-afternoon.
• Panel tilt and direction: in the northern hemisphere, panels usually face south.
• Roof or ground space: a 400 W panel is roughly 2 m x 1.1 m, so check that your array fits.
• Battery or grid-tie: a battery system needs extra panels to charge the batteries and cover losses.
• Future needs: adding an AC, pump or electric vehicle later means more panels. Leave room to expand.

Common mistakes

• Using the panel's rated watts as if you get them all day. Panels reach full rating only in strong sun.
• Forgetting appliances that cycle on and off, like fridges, which use less than their label watts over a day.
• Ignoring losses. Without them you will under-size your system.

Next step

Once you know your panel count, you also need to decide how to wire them and size the battery and inverter. Our guides on solar battery bank sizing and MPPT vs PWM controllers cover those steps.

Try it yourself: Skip the math with our free solar system calculator. Enter your appliances and equipment and get instant results.

Disclaimer: This article is for general information and education only. Numbers are estimates and examples, not a certified design. Have your final solar design reviewed by a qualified electrician or installer and follow your local electrical codes.