How we calculate

1. Output: PVGIS for every municipality

For the centre of each of 2,077 municipalities and 365 counties we downloaded from the PVGIS 5.3 API (PVGIS-SARAH3) the yearly yield of a system tilted at 35° facing south, with 14% system losses. The result ranges from 925 kWh per kWp (Piechowice) to 1,114 kWh (Jawor), averaging 1,060 kWh. We spread the yield over the months using the regional profile, and account for another roof direction or tilt with PVGIS factors computed for Warsaw. Source: PVGIS 5.3, European Commission Joint Research Centre (SARAH3 data, 2005–2023).

2. Hour-by-hour self-consumption

The Renewable Energy Act requires import and export to be netted in each hour (Art. 4(2b)). For each month we therefore build an average day: output spread over 24 hours using the PVGIS hourly profile, consumption using the chosen profile (out during the day: 30% of daily use between 8 a.m. and 6 p.m.; evenly: 10/24; home office: 55%). Self-consumption is the sum of the smaller of the two values in each hour. The profiles are model assumptions, not statistics, which is why the user chooses them.

3. Home battery

The day’s surplus charges the battery up to its usable capacity, but only up to what the house will use in the evening, at night and in the morning, and discharge allows for a 90% round-trip efficiency. The model does not know a specific inverter’s behaviour or any grid-charging strategy.

4. Deposit, refund and saving

Energy exported in each month is valued at RCEm over the last 12 published months, and for RCE settlement we add the gain from counting negative hourly prices as zero (Art. 4b(2)). The value is multiplied by 1.23 (Art. 4c(4)). The deposit pays only the energy part of the bill for imported energy (Art. 4c(2)–(3)). The unused rest comes back as a refund up to 20 % (RCEm) or 30 % (RCE) of the value of exported energy, and the rest is lost (Art. 4(11a)). The saving is the difference in the variable bill without and with the system. Sources: Polish Renewable Energy Sources Act, consolidated text Dz.U. 2026 item 68 (Art. 4, 4b, 4c), PSE: RCEm, monthly market electricity price, PSE report API: RCE for every settlement period.

5. Power prices

The 2026 G11 tariff approved by the President of URE: energy PLN 0.4970/kWh net, variable distribution PLN 0.3485/kWh net from the Energa-Operator tariff as an example, the quality, renewables and cogeneration charges, and 23 % VAT. In total about PLN 1.09 per kWh gross. We leave out fixed charges, which a solar system does not change. Every price field can be overwritten with the figure from your own bill. Sources: URE, 17 Dec 2025: 2026 electricity sale and distribution tariffs, Energa-Operator 2026 tariff (URE bulletin).

6. Payback

Payback years are price minus grant, divided by the yearly saving. We assume neither rising power prices nor panel degradation, because they pull in opposite directions and their size is unknown. Instead of an average system price, for which there is no official source, we show a break-even price: how much the system may cost to pay back within a given number of years.

7. Heating cost

Heat demand is floor area times the EU index in kWh/m² a year, the same index Czyste Powietrze uses to class houses. Heat pump: heat divided by SCOP; the default SCOP of 3.1 is the minimum seasonal efficiency from EU Regulation 813/2013 times a factor of 2.5. Coal: 24 MJ/kg, 75 % boiler efficiency, Statistics Poland average of PLN 1,602 per tonne in 2025. Gas: myORLEN tariff PLN 197.29/MWh net, PSG W-3.6 distribution and fixed charge, 86 % boiler efficiency. Heat pump with solar: the pump’s power is spread over October to April in proportion to the number of days and added to the house’s use. Sources: Commission Regulation (EU) No 813/2013, ecodesign of space heaters (gas boilers, heat pumps), Statistics Poland, Local Data Bank: average hard coal price per tonne (2025), URE: myORLEN gas tariff extended to 31 Dec 2026 (PLN 197.29/MWh), PSG tariff no. 14 for 2026, distribution rates (URE bulletin).

8. Grants

Czyste Powietrze income thresholds and amounts are taken from NFOŚiGW documents after the changes of 20 July 2026; the highest level is granted only to houses above 140 kWh/m². Home battery grant (PME part 2): 30 % of costs, at most PLN 800 per kWh and PLN 16,000, from 10 kWh, at a price of up to PLN 3,000 per kWh. Sources: Czyste Powietrze: what and how much (maximum grant amounts), Czyste Powietrze: who qualifies (income thresholds after the 20 July 2026 changes), PME priority programme, part 2 (Modernisation Fund), October 2026.

9. Data updates

Every month the update script downloads the new RCEm from the PSE table and RCE prices from the PSE reports API, recomputes the negative-price gain and stores the window of the last 12 months. Last download: 21 October 2026; latest RCEm: September 2026. Tariffs, thresholds and grant amounts are checked whenever the rules change and at least once a year. Before every publication automated tests check the energy balance, the deposit, the refund caps and the grants.

10. What the model does not know

It does not know your roof’s shading, the state of your wiring, connection capacity limits or inverter cut-outs at high grid voltage. It does not know your contract with the seller if your price differs from the tariff, nor future prices. It calculates for the centre of the municipality, not your plot. The result is an estimate with an open method, not an offer, an expert opinion or an official decision. Report comments and errors via the contact page.

Method version: 22 October 2026.