How to Calculate the Right Size of Solar System for Your Home

Switching to solar energy is a smart move—for your wallet and the planet. But before you dive in, one crucial step is figuring out what size solar system your home actually needs. An oversized system means wasted money, while an undersized one won't cover your energy needs. So, how do you get it just right? Here's a step-by-step guide to help you calculate the perfect solar setup for your home.

Step 1: Review Your Energy Usage

Start by looking at your electricity bills from the past year. You're looking for your average monthly energy usage, measured in kilowatt-hours (kWh). Most homes fall between 500–1,500 kWh/month, but it varies depending on location, appliances, and household size.

💡 Pro Tip: Use at least 12 months of data to account for seasonal differences in usage.

Step 2: Determine Your Daily Usage

Once you have your average monthly usage, divide it by 30 to get your daily energy use.

Example: If your average monthly use is 900 kWh:

900 kWh ÷ 30 = 30 kWh per day

Step 3: Check How Much Sunlight Your Area Gets

This is known as your area's solar irradiance, or the number of "sun hours" per day. It varies by location:

• Arizona: 6–7 hours/day

• California: 5–6 hours/day

• New York: 4–5 hours/day

• Seattle: 3–4 hours/day

You can look this up using solar maps or tools like PVWatts (by NREL).

Step 4: Calculate the System Size

Now, divide your daily energy use by your average sun hours to get the system size in kilowatts (kW).

Example:

30 kWh/day ÷ 5 sun hours = 6 kW system

This means you'll need a 6 kW solar system to cover your energy needs.

Step 5: Account for System Losses

No system is 100% efficient—there are losses from shading, wiring, and inverter efficiency. To be safe, add a buffer of around 10–20%.

Adjusted size:

6 kW × 1.15 = 6.9 kW system

Step 6: Consider Your Roof Space

Most solar panels are 350–450 watts each and take up about 18–22 square feet.

To estimate how many panels you need:

6.9 kW ÷ 0.4 kW (400W panel) = ~17 panels17 panels × 20 sq ft = 340 sq ft of roof space

Make sure your roof can fit that amount in a sun-facing orientation (south-facing is ideal in the northern hemisphere).

Bonus Tips:

• Future-proof your system: Planning to buy an EV or switch to electric heating? You might want a slightly larger system.

• Battery storage: Adding a battery will require slightly more generation to offset nighttime use.

• Incentives: Check if your area offers tax credits or rebates for solar installation—it can affect your budget and sizing strategy.

Final Thoughts

Calculating the right solar system size might seem complicated, but it really comes down to knowing your energy use, your local sunshine levels, and a bit of basic math. Whether you DIY the numbers or work with a solar installer, this knowledge empowers you to make smart, cost-effective decisions about going solar.