Estimate how heat or cold changes solar panel output from its 25 C rated power.
Advanced options
Table of contents
How to use our Solar Panel Temperature Loss Calculator
- Choose Calculation type: use Known panel temperature if you have a module or cell temperature, or Estimate from weather if you only know air temperature and sunlight.
- Choose Temperature input unit, then enter Panel rated power and Power temperature coefficient from the panel datasheet.
- Enter either Known panel temperature or the weather inputs: Air temperature and Sunlight on panels.
- Open Advanced options if you want to change Number of panels or NOCT from datasheet; otherwise the defaults estimate one panel with a 45 C NOCT.
- Click Calculate and sanity-check the results: hot panels should usually show a positive Temperature power loss, while panels below 25 C can show a negative loss.
Definitions
Panel rated power: The STC power rating of one solar panel in watts, usually printed on the datasheet.
Power temperature coefficient: The percent power change for each 1 C change in panel temperature, usually listed for maximum power as % per C [3].
Known panel temperature: The module or cell temperature used directly in the calculation. It is not the same as air temperature.
Air temperature: The outside air temperature used only in the weather estimate mode.
Sunlight on panels: Irradiance, or the instant sunlight power hitting each square meter of panel surface, in W/m2.
NOCT from datasheet: Nominal operating cell temperature, a datasheet value used to estimate how much hotter the panel gets than the air.
Temperature power loss: The signed percent change from the 25 C rating condition. Positive means heat lowers power; negative means cold raises power.
Power kept after temperature: The multiplier applied to rated power after the temperature adjustment, such as 0.86 for 86 percent kept.
Common mistakes and quick fixes
Mistake: Entering air temperature in Known panel temperature .
Fix: Use Estimate from weather , then put the weather value in Air temperature .
Mistake: Typing a positive value in Power temperature coefficient when the datasheet lists a negative Pmax coefficient.
Fix: Keep the minus sign, such as -0.35, unless your datasheet truly gives a positive coefficient.
Mistake: Mixing Celsius and Fahrenheit after choosing Temperature input unit .
Fix: Enter Known panel temperature and Air temperature in the unit you selected.
Mistake: Using Sunlight on panels as a daily energy number instead of instant sunlight.
Fix: Enter irradiance in W/m2, such as 1000 for strong test-condition sunlight.
Mistake: Reading Estimated output after temperature as final home AC power.
Fix: Treat it as DC power after temperature only; it does not subtract inverter, wiring, dirt, shade, or angle losses.
Mistake: Treating negative Power lost to heat as an error.
Fix: A negative value means cold panel temperature raises power compared with the 25 C rating condition.
Limitations & Key Assumptions / Boundary Conditions
- The result is an instant DC power estimate after temperature only. It does not include inverter loss, wiring loss, soiling, shade, panel angle, clipping, aging, or battery limits.
- The weather estimate uses a simple NOCT formula. Real cell temperature can differ because of wind, roof height, mounting style, panel color, and backside airflow.
- Sunlight on panels must be irradiance in W/m2, not daily sun hours or daily energy in kWh.
- The temperature coefficient method is linear. If the derating factor is zero or negative, the calculator should stop because that input set is outside a useful range.
- NOCT from datasheet stays in Celsius even when Temperature input unit is set to Fahrenheit.
- Cold conditions can show negative Temperature power loss and negative Power lost to heat. That means a temperature gain compared with the 25 C rating, not a formatting mistake.
Methodology
Calculation path
The calculator first turns any Fahrenheit temperature entry into Celsius, because the power temperature coefficient is per 1 C. In direct mode, it uses the entered panel temperature. In weather estimate mode, it estimates cell temperature from air temperature, sunlight, and NOCT. Solar panels often run hotter than the surrounding air, so using air temperature as panel temperature can understate heat loss [1].
temperature_c = (temperature_f - 32) * 5 / 9
cell_temperature_c = panel_temperature_c
cell_temperature_c = ambient_temperature_c + (irradiance_w_m2 / 800) * (noct_c - 20)
Power adjustment
The 25 C reference is the cell temperature used for the rated panel power condition. The calculator finds how far the panel temperature is from 25 C, then applies the temperature coefficient.
delta_temperature_c = cell_temperature_c - 25
temperature_loss_percent = -temp_coefficient_pct_per_c * delta_temperature_c
derating_factor = 1 - (temperature_loss_percent / 100)
output_power_w = rated_power_w * panel_count * derating_factor
power_lost_w = rated_power_w * panel_count - output_power_w
Worked example
For one 400 W panel, a -0.35% per C coefficient, and a known panel temperature of 65 C, the temperature change is 65 - 25 = 40 C. The temperature power loss is -(-0.35) * 40 = 14%. The power kept after temperature is 1 - 14/100 = 0.86, so the estimated output after temperature is 400 * 1 * 0.86 = 344 W. Power lost to heat is 400 - 344 = 56 W.
Input checks
The calculator blocks blank required fields, nonpositive panel rated power, nonpositive panel count, negative sunlight, unusable NOCT values, impossible temperature conversions below absolute zero, nonfinite math, and any result where the derating factor is zero or negative. It can still calculate with a positive coefficient, but that result should be checked against the datasheet because most PV power coefficients are negative [3].