Rooftop PV
Rooftop PV calculator
Three steps: size the installed capacity, see the approximate annual yield, estimate the simple payback period. Each step's result is copied into the related field of the next step. You can also check how temperature affects module power.
Step 1: Sizing
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Each box stands for one module; at most 40 boxes are drawn. The sketch is not to scale.
Step 2: Annual yield
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Step 3: Simple payback
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Extra: Temperature effect
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How is it calculated?
The calculator uses four simple formulas. Results are approximate; replace the inputs with the data of your own project.
Sizing
n = floor(A × r / A_module), P = n × Wp / 1000
A: roof area (m²), r: usable area ratio (0–1), A_module: module area (m²), Wp: module power, n: module count, P: installed power (kWp).
Annual yield
E = P_kWp × Y × PR
Y: specific yield before losses (kWh/kWp/yr), PR: performance ratio (0–1), E: approximate annual energy (kWh).
Simple payback
Savings_n = E × (s × p_buy + (1 − s) × p_sell) × (1 − d)^(n−1)
s: self-consumption ratio, p_buy and p_sell: unit purchase and feed-in price, d: annual degradation. Payback is the first year in which cumulative savings reach the investment (undiscounted).
Temperature effect
ΔP% = γ × (T_cell − T_ref)
γ: power temperature coefficient (%/°C, datasheet), T_cell: cell temperature, T_ref: reference (STC) cell temperature.
Example
- A 120 m² roof with a usable area ratio of 0.7 and 450 Wp modules of 2 m² fits about 42 modules; installed power is approximately 18.9 kWp.
- With 18.9 kWp, a specific yield before losses of 1,600 kWh/kWp/yr and a performance ratio of 0.8, annual yield is approximately 24,192 kWh.
- For a module with a temperature coefficient of −0.35 %/°C, a cell temperature of 55 °C (reference 25 °C) lowers power by approximately 10.5% versus STC.
The example inputs are for illustration only; they are not verified defaults. Results are approximate and do not replace engineering advice.
Frequently asked questions
How many modules fit on my roof?
The calculator divides the usable area by the module area and rounds down. Modules are assumed to fill the area without packing loss; a real layout, structural, shading and regulatory limits can change the result.
Why is the annual yield approximate?
Specific yield and performance ratio depend on the site. Year-to-year weather variability and degradation are not included in this step. The specific yield for your site should come from an irradiation database.
Is the payback period exact?
No. The calculation is undiscounted; inflation, price escalation, O&M, insurance, inverter replacement and taxes are not included. The result is an estimate, not a guaranteed return.
How does temperature affect module power?
In the linear model the power change is the temperature coefficient times the difference between cell temperature and the reference temperature. Take the coefficient from your module’s datasheet.