How many solar panels do you need?

You don’t need a sales quote to get a solid first estimate of how many solar panels your home needs. Four numbers will do it: how much electricity you use, how much of it you want solar to cover, how much sun your location gets, and how big each panel is.

This guide walks through each one, then puts them together with a worked example. It uses the same method as our solar offset calculator, so you can follow along with your own numbers.

Step 1: Find your yearly electricity use

Your utility bill lists your usage in kilowatt-hours (kWh). That is the number to work from, not the dollar amount, because rates and fees change while your usage tells you how much energy solar has to replace.

Use a full 12 months if you can. Most homes use noticeably more power in some seasons than others (air conditioning in summer, heating or lighting in winter), so a single month can throw your estimate off. Many utilities show a 12-month usage history on the bill or in your online account.

• Have 12 months of bills? Add up the kWh.
• Only have one or two? Multiply your average monthly kWh by 12, and treat the result as rough.

Also think about what’s coming. If you plan to add an electric vehicle, a heat pump, or other big electric loads, your future usage will be higher than your past bills show.

Step 2: Decide how much of your bill to offset

You don’t have to cover 100% of your usage. Some people size for everything, some for half, and some for whatever fits on the roof or in the budget.

How much it makes sense to offset depends a lot on your utility’s rules for the extra power your panels send back to the grid. Under some rules, every kWh you export is worth about as much as one you buy. Under others, exports earn much less, which can make a smaller system the better deal. Check your utility’s net metering or export policy before you settle on a number.

Some utilities also cap system size, often based on your past usage. Look this up early so you don’t plan a system you can’t connect.

Step 3: Look up your peak sun hours

Peak sun hours are not the same as hours of daylight. One peak sun hour is the equivalent of one hour of full-strength sunlight. A location might get 12 hours of daylight but only 4 or 5 peak sun hours, because the morning, evening, and cloudy-day sun is weaker.

Peak sun hours vary a lot by location and season, and they make a big difference to the result, so use a number for your area rather than a national average. Free tools can give you one:

• PVWatts from the U.S. National Renewable Energy Laboratory estimates solar production for a specific address.
• Solar resource maps show average daily sun for your region.

Use the yearly average value, since we’re sizing against a year of usage.

Step 4: Allow for real-world losses

Solar panels never deliver their full label rating all year. Some energy is lost to:

• converting the panels’ power into the kind your home uses (the inverter)
• heat, since panels produce less when they’re hot
• wiring, dust, dirt, and snow
• partial shading

This guide and our calculator assume the system delivers about 80% of what the panels’ rating suggests. That’s a common planning assumption, not a guarantee. Shade, panel angle, and climate can push your real number higher or lower. A detailed estimate from PVWatts or an installer accounts for more of these factors.

Step 5: Calculate the system size

Now put it together. System size is measured in kilowatts (kW) of panel capacity:

System size (kW) = yearly kWh to offset ÷ 365 ÷ peak sun hours ÷ 0.8

In plain English: figure out how much energy you need per day, divide by how many full-sun hours you get, then add a margin for losses.

Step 6: Turn system size into a panel count

Each panel has a power rating in watts, listed on its datasheet. Divide your system size by the panel rating, and round up, since you can’t install part of a panel:

Number of panels = system size in watts ÷ panel rating in watts

Panel ratings differ between models, so use the rating of the panel you’re actually considering. If you don’t have one in mind yet, run the numbers for a couple of ratings to see the range.

Worked example

Here’s the whole method with example numbers, not figures for any particular home:

Input	Example value
Monthly use	900 kWh
Yearly use	900 × 12 = 10,800 kWh
Share to offset	100%
Peak sun hours	5 per day
Panel rating	400 watts

1. Daily energy needed: 10,800 ÷ 365 = about 29.6 kWh per day
2. Divide by sun hours: 29.6 ÷ 5 = about 5.9 kW
3. Allow for losses: 5.9 ÷ 0.8 = about 7.4 kW
4. Panel count: 7,400 watts ÷ 400 watts = 18.5, rounded up to 19 panels

Now see how much location matters. Change only the sun hours from 5 to 4, and the same home needs about 9.2 kW, or 24 panels. Offset 50% instead of 100% (back at 5 sun hours) and it drops to about 3.7 kW, or 10 panels.

You can try these same changes in the calculator, which also estimates cost and payback.

Step 7: Check that it fits

Before you get attached to a number, make sure it fits on your roof (or in your yard, for a ground mount):

• Panel size: each panel’s dimensions are on its datasheet. Multiply by your panel count for the total area.
• Usable roof: fire codes often require clear paths around the edges and ridge of the roof, and vents, chimneys, and skylights take up space. Your usable area is smaller than the whole roof.
• Direction and shade: in the Northern Hemisphere, south-facing roofs usually produce the most, while east and west faces produce less. Heavy shade from trees or nearby buildings can cut output a lot.

If the panels you need won’t fit, you can offset a smaller share, use higher-wattage panels, or add a ground mount.

What this estimate leaves out

This method gives you a solid starting point, not a final design. It doesn’t account for:

• panel output slowly declining over the years
• future changes to your usage or your utility’s rates and rules
• batteries, which change the math depending on how you use them
• the exact angle, direction, and shading of your roof
• local permit, code, and utility requirements

Next steps

1. Run your own numbers in the solar offset calculator.
2. Check your utility’s rules on net metering, export credits, and system size limits.
3. Get detailed estimates from more than one licensed installer, and compare their production numbers to your own. If a quote is far from your estimate, ask them to explain why.