Solar Panel Calculator

How to Calculate How Many Solar Panels You Need

The number of solar panels your home needs depends on three main factors: how much electricity you use, how much sunlight your roof receives, and the wattage of the panels you choose. Here's the basic formula:

1. Find your monthly kWh usage — Divide your monthly electric bill by your electricity rate. For example, $150 / $0.13/kWh = 1,154 kWh/month.
2. Calculate annual usage — Multiply by 12. That's roughly 13,846 kWh/year.
3. Determine system size — Divide annual kWh by (365 × peak sun hours × 0.80 system efficiency). For 4.7 sun hours: 13,846 / (365 × 4.7 × 0.80) = ~10.1 kW.
4. Count the panels — Divide system size (in watts) by individual panel wattage. 10,100W / 400W = ~26 panels.

Solar Energy Savings: What to Expect

Solar panels typically last 25-30 years with minimal degradation (about 0.5% per year). Over that lifespan, most homeowners save significantly more than the initial system cost — especially as electricity rates continue to rise at roughly 2-3% annually. The federal Investment Tax Credit (ITC) at 30% further accelerates your return on investment.

Tips to Maximize Solar Output

• Panel orientation: South-facing panels produce the most energy in the Northern Hemisphere. Southwest and southeast orientations are also effective, typically producing 85-95% of maximum output.
• Reduce shading: Even partial shading on one panel can significantly reduce output for the entire string. Trim nearby trees and consider microinverters if shading is unavoidable.
• Optimal tilt angle: Panels should be tilted at an angle roughly equal to your latitude (typically 25-40 degrees in the continental US) for maximum annual production.
• Keep panels clean: Dust, pollen, and bird droppings reduce efficiency. Most areas get enough rain for natural cleaning, but arid regions benefit from occasional washing.
• Monitor performance: Use your inverter's monitoring app to track daily production and catch issues early.

Understanding Net Metering

Net metering is a billing arrangement where your utility credits you for excess solar electricity you send back to the grid. During peak sunlight hours, your system may produce more than you consume — that surplus flows to the grid and your meter effectively runs backward. At night or on cloudy days, you draw from the grid using those banked credits. Net metering policies vary significantly by state: some offer full retail-rate credits, while others provide reduced wholesale rates. Check your utility's policy, as it directly affects your solar ROI.

Battery Storage Considerations

Home batteries like the Tesla Powerwall, Enphase IQ Battery, or LG RESU store excess solar energy for use when the sun isn't shining. A battery system makes the most sense if:

• Your utility doesn't offer net metering or has unfavorable net metering rates
• Your area has time-of-use (TOU) electricity pricing where nighttime rates are high
• You experience frequent power outages and want backup power
• You want to maximize self-consumption of solar energy

Battery systems typically add $8,000-$15,000 to your solar installation cost but are also eligible for the 30% federal tax credit. A typical 13.5 kWh battery can power essential home loads for 8-12 hours during an outage.