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Solar calculator data sources and assumptions
This page explains the public references, calculation assumptions and limitations used by the Solar Panel Calculator. It is designed to help users understand the estimates before making a solar investment decision.
Last updated: 7 May 2026.
The calculator is an educational estimate, not a quotation, engineering design or financial recommendation. Actual solar production and payback depend on roof orientation, roof pitch, shading, inverter choice, installer pricing, grid rules, export tariffs, taxes, grants and local weather.
Primary data sources
- Solar resource and photovoltaic output: World Bank Global Solar Atlas and Global Solar Atlas PVOUT datasets are used as references for solar irradiation and photovoltaic power potential by country and region.
- Solar production modelling: NREL PVWatts is used as a reference model for grid-connected photovoltaic system energy production and real-world system losses.
- Solar installation and generation cost context: IRENA Renewable Power Generation Costs is used as a global reference for solar PV cost trends and renewable generation cost comparisons.
- Global PV market context: IEA PVPS Trends in Photovoltaic Applications is used as a reference for global solar PV deployment, module trends and market development.
- Panel degradation: NREL photovoltaic lifetime and degradation research is used as the basis for the calculator's default annual degradation assumption.
- Electricity prices: The calculator lets users enter their own electricity price because household tariffs vary by country, supplier, meter type, taxes and time period. Official references include Eurostat, the U.S. Energy Information Administration, Ofgem and local energy regulators.
Calculator assumptions
Performance ratio:
Default 80%. This allows for heat, inverter losses, wiring losses, dirt, mismatch and other real-world losses.
Default 80%. This allows for heat, inverter losses, wiring losses, dirt, mismatch and other real-world losses.
Solar panel degradation:
Default 0.5% per year. Users can adjust this in the calculator.
Default 0.5% per year. Users can adjust this in the calculator.
Lifetime model:
Default 25 years. This is used for long-term savings and ROI estimates.
Default 25 years. This is used for long-term savings and ROI estimates.
Panel wattage:
Default 430 W per panel. Users can change this to match a specific panel.
Default 430 W per panel. Users can change this to match a specific panel.
Self-consumption:
Default 65%. This estimates the share of solar electricity used directly by the home instead of exported.
Default 65%. This estimates the share of solar electricity used directly by the home instead of exported.
Export tariff:
User-entered. Export payments vary significantly by country, grid operator and tariff plan.
User-entered. Export payments vary significantly by country, grid operator and tariff plan.
Calculation method
- Annual solar production = solar system size in kW × average peak sun hours per day × 365 × performance ratio.
- Panel count = system size in kW × 1000 ÷ panel wattage.
- Self-used solar value = self-consumed solar kWh × electricity price.
- Export value = exported solar kWh × feed-in or export tariff.
- Annual benefit = self-used solar value + export value.
- Payback period = installed system cost ÷ annual benefit.
- 25-year ROI = ((25-year solar benefit - installed cost) ÷ installed cost) × 100.
The calculator intentionally uses simple, transparent inputs. This makes it easier to compare scenarios, but it does not replace a site-specific design from a qualified solar installer.
What the calculator does not know
- Exact roof angle, orientation or usable roof area.
- Partial shading from trees, walls, chimneys, nearby buildings or roof structures.
- Local grid connection limits and export restrictions.
- Installer-specific pricing, warranty terms or financing costs.
- Future electricity price changes, inflation or tariff reforms.
- Country-specific tax incentives, grants or permitting costs unless the user includes them manually in the installed cost.
Source links
- Global Solar Atlas: https://globalsolaratlas.info/
- World Bank PVOUT dataset: https://datacatalog.worldbank.org/search/dataset/0038641/world-photovoltaic-power-potential-pvout-gis-data-global-solar-atlas
- NREL PVWatts: https://pvwatts.nrel.gov/
- IRENA Renewable Power Generation Costs in 2024: https://www.irena.org/Publications/2025/Jun/Renewable-Power-Generation-Costs-in-2024
- IEA PVPS Trends in Photovoltaic Applications 2025: https://iea-pvps.org/trends_reports/trends-2025/
- NREL Photovoltaic Lifetime Project: https://www.nrel.gov/pv/lifetime
- Eurostat electricity price metadata: https://ec.europa.eu/eurostat/cache/metadata/EN/nrg_pc_204_sims_me.htm