Solar Panel Wattage Calculator
To figure out the wattage of your solar panel, the easiest thing to do is to look at the power rating on the box.
However, this value will change depending on weather, brightness, efficiency, and a number of other factors. So here’s what you need to do to figure out the real-world wattage of your solar panels.
All you need is a multimeter that can read both resistance and current.
Take the measurements of your solar panel system and write down the values under these variables:
R = resistance of the system (Ohms),
I = current of the system (Amps)
The wattage formula is:
P = IR^2
Why Are My Solar Power Producing Less Than Rated Power
Shading
Many solar panel systems fail to deliver the rated power output due to a combination of real-world conditions and technical inefficiencies. One common factor is shading. Even partial shading from trees, buildings, or debris can significantly reduce energy production. Panels are designed to perform best under full sun, and any obstruction can lead to a drop in performance.
Temperature
Temperature also plays a role. Solar panels typically operate less efficiently at higher temperatures, causing output to fall below rated levels in hot climates. Additionally, the angle and orientation of your panels matter. If panels are not optimally aligned to capture the maximum sunlight, the energy produced will be lower than expected.
Other Factors
Wiring losses, inverter inefficiencies, and mismatched panel configurations further contribute to reduced performance. Over time, degradation of panel materials can also lower output.
How to Decide Solar Capacity for Home
Measure your roof potential
Deciding on the right solar capacity for your home involves assessing your energy needs and understanding your roof's potential. Typically, you might expect about 75-80% of your roof to be available, after accounting for obstructions and maintenance access. You can refer to our Solar Panel Roof Area Calculator to calculate how many panels your roof can install. The calculator has considered these factors.
Determine your average power consumption in kWh
Start by reviewing your monthly electricity bills to determine your average energy consumption in kilowatt-hours (kWh). This information gives you a baseline for the size of the solar system required.
Assume you have a 400W panel, but due to inefficiencies the actual output is 25% lower than 400W, which equals 300W effective. With 4 hours of effective sunlight, one panel produces:
300W × 4 hours = 1,200 Wh or 1.2 kWh per day.
If your house uses 30 kWh per day, then you need:
30 kWh ÷ 1.2 kWh per panel ≈ 25 panels.
It's recommended to choose a system with at least a 25% higher output capacity than your average consumption to account for inefficiencies due to weather and other factors. Solar panel efficiency varies by type—monocrystalline panels are the most efficient. Weather and climate also impact efficiency, requiring potentially larger systems or battery backups for off-grid living.
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