Solar Panel Wattage Calculator

Start by adding up the total watt-hours your appliances consume each day. Divide that figure by your peak sun hours and then account for system efficiency (typically 75–85%). The result is the total panel wattage required. Divide that by the wattage of a single panel to get the number of panels needed.

Peak sun hours represent the number of hours per day when sunlight is strong enough (around 1,000 W/m²) to produce rated power from your panels. They vary by location — southern US states average 5–6 hours while northern states may see 3–4. Fewer peak sun hours means you need more panel capacity to generate the same energy.

Energy is lost at multiple points in a solar system — through the inverter (converting DC to AC), wiring resistance, battery charging and discharging, and temperature effects on panels. A realistic efficiency factor of 75–85% is standard and ensures your system is sized to cover real-world losses, not just theoretical output.

Check the label on the appliance itself — most show volts and amps, and watts = volts × amps. You can also consult the owner's manual or look up the model online. A plug-in power meter (such as a Kill-A-Watt device) is the most accurate method for measuring actual consumption.

For off-grid systems, it's safest to size for your maximum daily load and the darkest month of the year (lowest sun hours). This ensures the system can meet your needs even in worst-case conditions. Grid-tied systems can be sized more conservatively since the grid provides backup.

Watts (W) measure the rate of power consumption at a given moment, while watt-hours (Wh) measure total energy consumed over time. A 100W appliance running for 5 hours consumes 500 Wh of energy. Solar panel sizing is based on watt-hours per day, not just wattage.

In the US, most locations average between 3.5 and 6 peak sun hours per day. The Southwest (Arizona, Nevada, California) enjoys the most sun at 5.5–7 hours. The Northeast and Pacific Northwest typically see 3.5–4.5 hours. Using your darkest winter month gives the most conservative and reliable system size.

Yes. The core calculation — total daily watt-hours divided by peak sun hours and efficiency — applies to both system types. For off-grid systems, you'll also need to factor in battery storage capacity. For grid-tied systems, the result tells you how large your array should be to offset your consumption.