Heating Power Calculator — How Many kW to Heat a House

Rule-of-thumb indices are: well-insulated houses (new, WT2021) about 40–60 W/m², standard buildings about 70–90 W/m², and old, poorly insulated ones about 100–130 W/m². The calculator uses 50, 80 and 120 W/m². For a 100 m² house with standard insulation that is about 8 kW.

The standard reflects approximate energy demand. "Low" (50 W/m²) means well-insulated buildings or those meeting WT2021. "Medium" (80 W/m²) is a typical house from 2000–2015. "High" (120 W/m²) means old buildings from before the 1990s, uninsulated or with large heat losses.

WT2021 is the Polish technical requirement in force since 2021, setting the maximum building energy demand and minimum insulation parameters of the envelope. Houses built to WT2021 have low heat losses and fall into the "low" demand class (about 50 W/m² or less).

Yes. We heat volume, not just floor area. Taller rooms have a larger air volume and bigger walls through which heat escapes. The calculator scales power proportionally to height relative to a 2.7 m baseline — at 3.0 m the power rises by about 11%.

An oversized boiler runs in constant on/off cycling (short-cycling), which lowers efficiency, increases fuel use and accelerates wear. That is why power is chosen close to the real demand, adding only a small margin — about 10% in this calculator.

Heat pumps must be sized especially precisely, because oversizing worsens efficiency (COP/SCOP) and causes compressor short-cycling. Power is chosen so the pump covers most of the season, with a heating element assisting in extreme cold. Here an OZC calculation is especially recommended over the rule of thumb.

Yes, if the same boiler or pump also prepares domestic hot water. A typical addition is about 0.2–0.3 kW per person, or sizing by tank capacity. The calculator covers heating only — when sizing a unit with DHW, add an extra power margin.

It is the heating power increased by about a 10% margin and rounded up to a full 0.5 kW, matching the typical resolution of boilers and heat pumps on the market. The margin covers start-up losses and extreme cold. It does not replace an OZC calculation for DHW and buffering.

OZC is the calculated heat demand — a detailed analysis of the building heat loss done by a designer based on envelope insulation, ventilation and climate zone. It is the basis for professional heat source sizing, especially for heat pumps. The W/m² rule of thumb is only for a first estimate.

No. The calculator only gives a preliminary, indicative estimate based on a simplified rule of thumb (W/m²). It does not account for the actual envelope insulation, ventilation or climate zone. For sizing a boiler or heat pump, a proper OZC calculation by a designer or installer is recommended.