Solar System Size Calculator

Enter your annual kWh usage and location — find the exact kW system your home needs.

Input method

Enter usage as?

Your electricity usage

Annual kWh usage?

kWh/yr

Peak sun hours?

hrs/day

Bill offset target?

System efficiency?

Panel wattage?

Recommended system size

7.6 kW system

Panels needed19 panels

Annual production10,735 kWh

Your annual usage10,500 kWh

Est. system cost$19,000–$26,600

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How to Use This Calculator

Choose your input method

You can enter your electricity usage two ways: annual kWh (the most accurate) or monthly bill + rate. If you have last year's utility bills handy, add up all 12 months and enter the total kWh. If not, use your monthly bill and electricity rate — the calculator will estimate your annual usage automatically.

Enter your annual kWh usage

The US average household uses about 10,500 kWh per year. Your actual number will vary by home size, climate, and appliances. An all-electric home with EV charging can hit 20,000–30,000 kWh/year. Find your 12-month total on your utility's online portal, or on your December bill if it shows a year-end summary.

Set peak sun hours for your location

Peak sun hours (PSH) is the single biggest variable in system sizing. It represents the equivalent hours of full-strength sunlight your location receives per day, averaged over the year. Phoenix gets ~6.5, Denver ~5.0, Seattle ~3.5. Use our Peak Sun Hours Calculator to find your location's value, or check NREL's PVWatts tool.

Set your bill offset target

The bill offset controls how much of your electricity you want solar to cover. 100% means you produce as much as you consume annually — you'll still use the grid at night and buy back during the day. 110% means you produce extra to sell back via net metering. Start at 100% for most residential installations.

Review system efficiency

The system efficiency field (default 86%) accounts for all real-world losses: temperature derating, inverter conversion losses, wiring resistance, soiling (dust/bird droppings), and partial shading. Higher-quality installations with micro-inverters and no shading can achieve 90%+. Heavily shaded roofs may drop to 75-80%.

The Formula

The system size calculation works like this:

Annual kWh needed = Annual consumption × Offset target System kW = Annual kWh needed ÷ (365 days × Peak sun hours × System efficiency) Panels needed = (System kW × 1000) ÷ Panel wattage (W) Annual production = System kW × Peak sun hours × Efficiency × 365

The core insight is that peak sun hours times system efficiency gives you the effective daily production factor. A 1 kW system in Phoenix (6.5 PSH, 86% efficiency) produces 1 × 6.5 × 0.86 × 365 = 2,043 kWh/year. The same system in Seattle (3.5 PSH) produces only 1,100 kWh/year — nearly half as much.

This is why two identical-sized systems can have vastly different production numbers, and why installers always ask for your address before quoting a system size.

Example

The Martinez family — San Diego, CA

The Martinez family used 12,400 kWh last year and wants to cover 100% with solar. San Diego averages 5.8 peak sun hours per day. They're using 400W panels at 86% system efficiency.

Annual usage12,400 kWh

Offset target100%

Peak sun hours5.8 hrs/day

System efficiency86%

Panel wattage400W

Result

System size6.8 kW

Panels needed17 panels

Annual production12,430 kWh

Estimated cost (before ITC)$17,000–$23,800

After 30% federal tax credit$11,900–$16,660

At SDG&E's average rate of ~$0.38/kWh, the 6.8 kW system saves approximately $4,720/year. After the 30% ITC, the net cost of ~$14,280 implies a payback period of roughly 3 years — among the best in the country due to high rates and good sun.

FAQ

What size solar system do I need for a 2,000 sq ft house? ▼

Square footage alone doesn't determine system size — electricity consumption does. A 2,000 sq ft home with gas heat and cooking might use 7,000–9,000 kWh/year, requiring a 5–7 kW system. The same floor plan with electric heating, an EV, and a pool could use 20,000+ kWh/year and need a 15+ kW system. Enter your actual annual kWh from your utility bills for an accurate result.

Is a 10 kW solar system enough for an average US home? ▼

For most US homes, yes. The US average household uses ~10,500 kWh/year. At average US peak sun hours (4.5 hrs/day) and 86% efficiency, a 10 kW system produces about 14,200 kWh/year — more than enough. However, if you're in a low-sun region like the Pacific Northwest, a 10 kW system might only produce 9,500–11,000 kWh/year. Use your actual annual kWh and local peak sun hours for a precise calculation.

How does system efficiency affect the kW size I need? ▼

System efficiency directly scales the size needed. At 86% efficiency (our default), you lose 14% to heat, inverter conversion, wiring, and soiling. If you boost to 90% efficiency with micro-inverters and regular panel cleaning, your required system size shrinks by ~5%. If you have heavy shading and drop to 75% efficiency, you'll need a ~15% larger system to produce the same energy. Reduce shading before sizing — it's cheaper than adding panels.

Should I size my system for 100% or 110% of my usage? ▼

It depends on your net metering policy. If your utility offers 1:1 net metering (rare but still exists in some states), sizing to 110–120% makes sense — excess energy exported at the same rate you'd buy it. Most utilities now offer less favorable buyback rates (wholesale vs. retail), so oversizing loses money. In states with unfavorable net metering, size to exactly 100% or slightly under. If you're planning to add an EV or battery in the future, size for 110% now.

How much does a 10 kW solar system cost in 2026? ▼

In 2026, a 10 kW residential system typically costs $25,000–$35,000 before incentives ($2.50–$3.50/W installed). After the 30% federal tax credit, the effective cost drops to $17,500–$24,500. High-cost states (Hawaii, California) and complex roof installations run toward the top of that range. Simple installations on south-facing roofs in competitive markets can come in under $2.50/W. Get at least 3 quotes from certified installers before committing.