How Many Solar Panels Do I Need?

Most homeowners start their solar journey by asking one simple question: "How many panels do I need?" The answer isn't a single number—it depends on your electricity usage, where you live, and what you want to achieve. A home in Phoenix needs far fewer panels than one in Seattle, even if both use the same amount of electricity. Getting the sizing right saves you thousands. Too few panels and you'll still have a substantial electric bill. Too many and you're paying for capacity that sits idle. The sweet spot is a system that offsets 80-100% of your annual consumption—and finding that sweet spot is straightforward once you understand the math.

Step 1: Determine Your Monthly Energy Usage

Start with your electricity bill. Look for your monthly kilowatt-hour (kWh) consumption—the number the utility charges you for. The US average is about 886 kWh per month, but your actual usage varies wildly:

• Small apartment or condo: 300-500 kWh/month
• Average home (2,000 sq ft): 800-1,000 kWh/month
• Large home with pool/EV: 1,200-2,000 kWh/month
• Home with electric heating: 1,500-2,500 kWh/month

Pull your last 12 months of bills if possible. Solar should be sized to your annual total, not a single month—summer usage often dwarfs winter usage in northern states.

Step 2: Factor in Your Peak Sun Hours

Peak sun hours (PSH) measure how much useful sunlight your location receives. This isn't the same as "hours of daylight"—it's the equivalent number of hours at 1,000 W/m² of solar intensity. The National Solar Radiation Database provides these numbers by location:

Notice the range—Phoenix gets more than double the useful sun of Seattle. This is the single biggest factor in how many panels you need.

Step 3: Calculate System Size

The formula is straightforward: System Size (kW) = Annual kWh Usage ÷ (Peak Sun Hours × 365 × System Efficiency) System efficiency accounts for real-world losses: inverter conversion, wiring resistance, temperature effects, and panel degradation. Use 0.85 (15% loss) as a standard assumption per NREL PVWatts methodology. Let's work through an example. A home using 10,000 kWh/year in a location with 4.5 peak sun hours:

• Annual production needed: 10,000 kWh
• Daily production needed: 10,000 ÷ 365 = 27.4 kWh
• Daily production per kW of panels: 4.5 × 0.85 = 3.825 kWh
• System size needed: 27.4 ÷ 3.825 = 7.16 kW

Step 4: Convert kW to Panel Count

Modern residential panels come in standard sizes:

The industry has shifted toward 400-450W panels for residential installations. They offer the best balance of efficiency, physical size, and cost per watt.

Here's how the same home looks in two very different locations.

Example 1: Sacramento, California

• Monthly usage: 800 kWh (9,600 kWh/year)
• Peak sun hours: 5.5
• System size needed: 9,600 ÷ (5.5 × 365 × 0.85) = 5.54 kW
• Using 400W panels: 14 panels
• Roof space needed: ~260 sq ft

Sacramento's excellent solar resource means a smaller system covers the same usage. California SGIP and any verified federal incentive scenario can bring the net cost down to roughly $7,000-9,000.

Example 2: New York City

• Monthly usage: 800 kWh (9,600 kWh/year)
• Peak sun hours: 3.5
• System size needed: 9,600 ÷ (3.5 × 365 × 0.85) = 8.92 kW
• Using 400W panels: 23 panels
• Roof space needed: ~425 sq ft

The same home in New York needs 64% more panels. This doesn't mean solar isn't worthwhile there—New York's high electricity rates (average $0.28/kWh vs California's $0.25/kWh) and generous state incentives offset the lower production.

Calculated panel count means nothing if your roof can't hold them. Standard 400W panels measure about 18.5 square feet each. Factor in spacing, setbacks from roof edges, and clearances around vents and skylights. Rule of thumb: You need 1.5-2x the bare panel area to account for obstructions and installation requirements. A 20-panel system needs roughly 550-700 sq ft of usable south-facing roof. Homes with limited roof space have options:

• Higher-efficiency panels: Premium panels (SunPower, LG) pack more watts per square foot
• East-west configuration: If south-facing space is tight, splitting panels across east and west roofs captures morning and afternoon sun
• Ground-mount systems: Yards with unshaded areas can host panels on ground racks

Some homes hit a roof-space ceiling before reaching their energy goal. Common scenarios:

1. Tree shading: Large trees to the south block sunlight. You can trim branches, but sometimes the damage is structural—shading a single panel can reduce output by 25-40% across the entire string. Microinverters or DC optimizers mitigate this.
2. Complex rooflines: Dormers, valleys, and multiple roof planes complicate installation and reduce usable area.
3. HOA restrictions: Some homeowners associations limit panel visibility from the street. Federal law protects your right to install solar, but HOAs can dictate placement within reason.
4. Condo or townhome: Shared roof space means competing for area. Community solar or a balcony-mounted portable system may be alternatives.

You don't have to aim for 100% offset. Here's why 80% is often the better target:

• Net metering uncertainty: Utility policies change. In some states, excess generation credits are being reduced or eliminated. A 100% offset that generates large credits today may not pay off if policies shift.
• Usage changes: Families grow, kids move out, new appliances arrive. Sizing for 80% leaves room to adapt without overproducing.
• Cost efficiency: The last 20% of offset costs proportionally more because you're paying for capacity you'll only use during peak summer months.

Most solar installers recommend targeting 90-95% offset for maximum value, adjusting based on your risk tolerance and state policies.

1. Using one month's bill: A single winter month underestimates your annual needs. Use 12 months of data.
2. Ignoring future plans: Buying an EV or adding a heat pump? Size up now—adding panels later costs more than including them in the original installation.
3. Overcounting incentives: The federal incentive caveat (30% subject to current IRS guidance) reduces your net cost, but it doesn't change how many panels you need. Don't confuse affordability with sizing.
4. Trusting "average" numbers: Online averages are starting points. Your specific roof orientation, tilt, and shading conditions can shift the required count by 10-20%.

Ready to get your exact number? Our Solar Panel Sizing Calculator uses your real electricity usage and location to give you a precise panel count, system size, and cost estimate—instantly. No signup required. Just enter your monthly kWh, pick your state, and see what a properly sized solar system looks like for your home.