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Solar calculator data sources and assumptions
This page explains the public references, calculation assumptions and limitations used by Solar Panel Cost Calculator. It is intended to make the model auditable and help users replace generic defaults with better local inputs.
Last reviewed: 15 September 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 production and real-world system losses.
- Renewable cost context: IRENA's Renewable Power Generation Costs in 2024 remains a useful international reference for utility-scale solar PV cost trends and technology comparisons. It is not used as a household installer-price database.
- Current global PV market context: IEA PVPS Snapshot of Global PV Markets 2026 is used as a current reference for global deployment and market context. It reports preliminary 2025 deployment data and is not used to set local residential prices.
- Panel degradation: NREL photovoltaic lifetime and degradation research is used as a reference for the calculator's adjustable degradation assumption.
- Electricity prices: Users enter their own tariff because household prices vary by supplier, meter type, taxes, time-of-use period and date. Useful official sources include Eurostat, the U.S. Energy Information Administration, Ofgem and local energy regulators.
Why the calculator does not hard-code local tariffs
Electricity import prices, export credits, net-metering rules and incentive schemes can change faster than solar hardware assumptions. A site-wide default can therefore become misleading. The calculator keeps these inputs editable and asks users to verify the figures against a recent bill, utility tariff, regulator or current installer documentation.
The same principle applies to installation cost. National averages can provide context, but a roof survey and local quote are more relevant because labour, permitting, scaffolding, electrical work, inverter choice and battery storage differ from property to property.
Calculator assumptions
Default 80%. This allows for heat, inverter losses, wiring losses, dirt, mismatch and other real-world losses.
Default 0.5% per year. Users can adjust this in the calculator.
Default 25 years. This is used for long-term savings and ROI estimates.
Default 430 W per panel. Users can change this to match a specific module.
Default 65%. This estimates the share of solar electricity used directly by the home instead of exported.
User-entered. Export payments vary significantly by country, utility 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, rounded up.
- Self-used solar value = self-consumed solar kWh × electricity import price.
- Export value = exported solar kWh × feed-in or export tariff.
- Annual benefit = self-used solar value + export value.
- Payback period = installed system cost ÷ first-year annual benefit.
- 25-year ROI = ((modelled 25-year solar benefit - installed cost) ÷ installed cost) × 100, with degradation applied.
The model intentionally uses simple, transparent inputs. This makes it easier to compare scenarios, but it does not replace a site-specific production model, roof survey or installer design.
What the calculator does not know automatically
- Exact roof angle, azimuth or usable roof area.
- Partial shading from trees, walls, chimneys or nearby buildings.
- Hourly household demand unless the user models self-consumption manually.
- 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.
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 Snapshot of Global PV Markets 2026: https://iea-pvps.org/snapshot-reports/snapshot-2026/
- 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