Why System Sizing Matters
Getting the right size solar system is one of the most important steps in going solar. Too small, and you'll still rely heavily on the grid. Too large, and you'll overpay for capacity you never use. Proper sizing ensures you maximize savings while keeping upfront costs reasonable.
Step 1: Calculate Your Annual Energy Consumption
Start by reviewing your electricity bills. You're looking for your annual kilowatt-hour (kWh) usage. Most utility bills show monthly usage — add up 12 months to get your annual total.
- A typical U.S. home uses roughly 10,500 kWh per year, but this varies widely by location, home size, and habits.
- If you plan to add an electric vehicle or heat pump soon, factor in that future consumption now.
- Look for any seasonal spikes — summer air conditioning and winter heating can skew your numbers.
Step 2: Determine Your Peak Sun Hours
Solar panels only generate power during daylight, and their output depends on the intensity of sunlight — measured in peak sun hours (PSH). This is not the same as hours of daylight.
Peak sun hours vary significantly by location:
| Region | Avg. Peak Sun Hours / Day |
|---|---|
| Southwest (e.g., Arizona, California) | 5.5 – 7.0 |
| Southeast (e.g., Florida, Texas) | 4.5 – 5.5 |
| Midwest & Northeast | 3.5 – 4.5 |
| Pacific Northwest | 3.0 – 4.0 |
Use tools like the NREL PVWatts Calculator (free online) to get precise peak sun hours for your zip code.
Step 3: Apply the Sizing Formula
Once you have your annual kWh usage and local peak sun hours, apply this formula:
- Divide your annual kWh by 365 to get daily usage.
- Divide daily usage by your peak sun hours to get the raw kW needed.
- Divide by a system efficiency factor (typically 0.80 to account for heat loss, inverter inefficiency, etc.).
Example: 10,500 kWh/year ÷ 365 = 28.8 kWh/day. 28.8 ÷ 5.0 PSH = 5.76 kW. 5.76 ÷ 0.80 = 7.2 kW system.
Step 4: Factor in Roof Space and Panel Wattage
Modern residential solar panels typically range from 350W to 450W per panel. To find the number of panels needed:
- Divide your system size in watts by the panel wattage. A 7,200W system using 400W panels = 18 panels.
- Each panel requires roughly 17–20 square feet of roof space.
- Ensure your roof is structurally sound, free of significant shading, and ideally south-facing (in the Northern Hemisphere).
Step 5: Consider Battery Storage
If you want backup power or live in an area with time-of-use electricity pricing, adding a battery system may make sense. A battery doesn't change your panel count but does affect your total budget and independence from the grid.
Get Multiple Quotes
Once you have a rough system size in mind, get quotes from at least three certified solar installers. Make sure each quote specifies the same system size so you can compare apples to apples. Ask for their production estimates and verify them against your own calculations.
Key Takeaways
- Start with your actual electricity bills — don't guess.
- Use your local peak sun hours, not just hours of daylight.
- Account for system losses (typically 20%) in your calculations.
- Plan for future energy needs like EVs or heat pumps before finalizing size.