Solar panels for water heating: how much they really cover
Hot water is a steady load all year round. Work it out for your house and add it to the use in the calculator below.
Yearly saving on your bills
PLN 2,321
4,200 kWh produced · 28 % used on site
| Used directly and from the battery | 1,184 kWh |
| Exported to the grid | 3,016 kWh |
| Deposit (×1.23) | PLN 1,027 |
| Deposit spent on energy | PLN 988 |
| Overpayment refunded | PLN 39 |
| Bill before / after | PLN 3,061 / PLN 779 |
| Pays back in 10 years up to | PLN 23,212 |
Hourly model of an average day in each month; surplus priced at the last 12 months of RCEm (PSE) plus the gain from negative RCE counted as zero. An estimate, not an offer; fixed charges excluded.
Solar PV for water heating works best when the element or heat pump water heater runs in the hours when the panels produce. The Polish building energy performance methodology assumes 1.4 litres of hot water per square metre per day for a single-family house, heated from 10 to 55 °C. For a 120 m² house that is about 2,890 kWh of heat a year, or 7.9 kWh a day. An electric element uses the same amount of power, while a heat pump water heater with a COP of 3 uses three times less. One kilowatt of panels gives on average 4.3 kWh per day in July across Poland and only 0.8 kWh in December. In summer a system of a few kilowatts therefore covers the water with plenty to spare, while in winter most of that power comes from the grid. Under net-billing, energy used for water at the moment it is produced is worth the full tariff price, and energy exported only the market price, so heating water at midday is one of the cheapest ways to raise self-consumption.
Checked by Radif Partners · Editorial policy · Method
Summer and winter: how much water 1 kWp heats
Hot water demand is almost the same all year, while daily panel output in July is about 5 times that of December. The table shows the size needed to cover, with an element, a 120 m² house’s water on an average day of each month.
| Month | 1 kWp per day | Water per day | Size to cover |
|---|---|---|---|
| January | 1.0 kWh | 7.9 kWh | 7.8 kWp |
| April | 4.0 kWh | 7.9 kWh | 2.0 kWp |
| July | 4.3 kWh | 7.9 kWh | 1.8 kWp |
| October | 2.3 kWh | 7.9 kWh | 3.4 kWp |
| December | 0.8 kWh | 7.9 kWh | 9.9 kWp |
How much power for hot water, and how many panels?
Power for hot water per year
2,890 kWh
| Cost from the grid, G11 2026 | PLN 3,160 |
| PV size giving that much a year | 2.7 kWp |
| Need per day / 1 kWp in July / in December | 7.9 / 4.3 / 0.8 kWh |
Heat: 1.4 l/(m²·day), 10 → 55 °C, formula (61) of the energy performance regulation. Tank losses not included.
What heating water in the daytime saves
Take a 120 m² house with a 2,500 kWh bill before water and a 4 kWp system. When the element runs evenly round the clock, the house saves PLN 2,986 a year on power. When the same element heats in the daytime (modelled as the “a lot in daytime” profile for the whole house), the saving rises to PLN 3,360, because more output stays in the house instead of going to the grid. A heat pump water heater with a COP of 3 in the same house gives PLN 2,777 of solar saving and also cuts the power used for water by two thirds. These figures come from the calculator’s hourly model and 2026 G11 prices.
You will see the result for your house in the calculator at the top: add the mini-calculator’s result to the yearly use on your bill and choose “a lot in daytime” if the element runs on a schedule. If a heat pump heats the water together with the house, use the heat pump and solar page.