Solar Panel Requirement Calculator – Size Your Solar System

Sizing a solar system starts with one number: how much energy you use daily. Check your electric bill for monthly kWh, divide by 30, multiply by 1,000 to get watt-hours. That's your daily consumption.

Next, figure out your peak sun hours. This isn't just daylight hours—it's the equivalent hours of full 1,000 W/m² sunlight. Arizona gets 6-7 hours; Seattle gets 3-4 in winter. Your location matters.

Solar Panel Calculation Formula

Example: 30,000 Wh daily, 5 sun hours, 20% losses, 400W panels

Adjusted: 30,000 / 0.8 = 37,500 Wh

System: 37,500 / 5 = 7,500W

Panels: 7,500 / 400 = 19 panels

Not all the energy your panels produce makes it to your appliances. System losses eat 15-25% typically. Here's where it goes:

Default system loss assumption is 20%, which is realistic for most residential installations. Well-designed systems with microinverters and clean panels might see 15%. Older or poorly maintained systems can lose 25% or more.

Panel wattage determines how many you need. Higher wattage means fewer panels but each panel is larger and heavier. Here's what's available:

If you're going off-grid or want backup power, you need batteries. The calculator assumes a 12V system with 50% depth of discharge (DoD)—meaning you only use half the battery capacity to preserve lifespan.

The 1.5 multiplier accounts for inefficiency and ensures you don't drain batteries below 50%. For example, 30,000 Wh daily needs:

(30,000 / 12) × 1.5 = 3,750 Ah at 12V

That's a lot of batteries. In practice, off-grid systems use 24V or 48V to reduce amp-hour requirements. A 48V system would need about 940 Ah—much more manageable with modern lithium batteries.