usefulweb.appDependable little tools for people and AI agents. Ledgerly Heat Pump Estimate
Theme: Auto

Theme

Apps Part of Ledgerly

Ledgerly Heat Pump Estimate

Would a heat pump be cheaper than my gas or oil boiler? Heat load, COP and running costs.

More options

Condensing gas boilers about 90–95 %, older boilers 75–85 %.

Above this outdoor temperature the house needs no heating.

Underfloor heating 30–35 °C, large radiators 45–55 °C, small old radiators 60–70 °C.

Good current units reach about 40–50 % of the ideal value; 40 % is a cautious default that also covers defrosting.

Example prices when empty: gas 0.11 per kWh, oil 1.00 per litre, LPG 0.80 per litre, pellets 0.35 per kg (per kWh: about a tenth). Enter your own price.

Default: German mix 2025 incl. upstream emissions (Umweltbundesamt).

☁ Sign in to save to your Google Drive

Try an example: · ·

More about this tool

How it works

Turns your past gas, oil, LPG or pellet use into heat demand, spreads it over the year with the outdoor temperatures (DWD climate normals or your own), and estimates the heat pump’s electricity from a temperature-dependent COP and heating curve. Compares running costs and CO₂ with your boiler, and shows the heat load on the coldest day and the payback of the investment. Part of Ledgerly’s Solar & Heat.

  • Heat = fuel use × energy content × boiler efficiency (typical energy contents: natural gas 10 kWh/m³; heating oil 10 kWh/litre; liquid gas (lpg) 6.57 kWh/litre; wood pellets 4.8 kWh/kg). The hot-water share is spread evenly over the year.
  • Space heat is spread over the year by degree-hours below the heating limit, with each month’s daily means assumed normally distributed around the monthly mean (σ = 4 K, five bands); this also gives the heat-loss coefficient and the heat load on the coldest day.
  • COP = quality factor × T_flow ÷ (T_flow − T_source) in kelvin, limited to 1–7; air source uses the outdoor temperature, ground source 2 °C. The flow temperature follows a straight heating curve from your design value on the coldest day to 25 °C at the heating limit.
  • Running costs compare fuel × price + fixed boiler costs with heat-pump electricity × price + fixed heat-pump costs; CO₂ uses GEG Anlage 9 for fuels and the Umweltbundesamt’s 2025 German grid factor by default.

An estimate for comparing options, not a heat-load calculation to EN 12831 or installer advice: size the heat pump with a qualified installer, and check local subsidies and tariffs yourself.

Sources

Part of Ledgerly

  • Ledgerly Household finance & life-admin desk

    Budgets, bills, subscriptions, loans, contracts and deadlines on one private desk.

    Open Ledgerly

Related tools

How to use the Heat Pump Estimate

Turn your past gas or oil bills into heat demand and estimate a heat pump's electricity, efficiency, running costs, CO₂ and payback compared with your boiler.

What it is for

The best predictor of a heat pump's running cost is the fuel your house burns today. This estimate turns your yearly gas, oil, LPG or pellet use into heat, spreads it over the year with the outdoor temperatures of the nearest German DWD station (or your own monthly means), and calculates the heat pump's electricity with an efficiency (COP) that depends on the outdoor temperature and the flow temperature your radiators need.

It compares running costs and CO₂ with your current heating and shows the heat load on the coldest day. It is not a room-by-room heat-load calculation to EN 12831 and does not size the unit — that is the installer's job.

Worked example

A house near Hanover (latitude 52.37, longitude 9.73) burns 2,200 litres of heating oil a year in a boiler with 82 % efficiency. Oil costs 1.05 EUR per litre, heat-pump electricity 0.30 EUR/kWh. The radiators need 55 °C on the coldest day. The heat pump costs 32,000 EUR, with 9,600 EUR subsidy.

The result: 22,000 kWh of oil gave about 18,040 kWh of heat. An air-source heat pump would need about 5,112 kWh of electricity, a seasonal COP of 3.53, and cost 1,734 EUR a year instead of 2,660 EUR — 926 EUR less. The heat load on the coldest day is about 7.9 kW, CO₂ falls from 6,820 kg to 2,076 kg a year, and the net price of 22,400 EUR pays back after about 24 years.

Lower the flow temperature to 45 °C (for example after replacing a few radiators) and the seasonal COP rises to 3.88, electricity drops to 4,654 kWh and the yearly saving grows to 1,064 EUR — payback after about 21 years.

How to read the result

  • Running cost per year for both systems — fuel or electricity times price, plus the fixed costs you entered.
  • Seasonal COP — heat delivered divided by electricity used over the whole year. Above 3 is good for a radiator system.
  • Heat load on the coldest day — a first idea of the heat pump size; the installer should confirm it.
  • CO₂ per year — fuel with the factors of Germany's GEG, electricity with the Umweltbundesamt's grid mix; both can be changed.
  • The Month by month table shows space heat, hot water, electricity and the COP per month: winter months have the lowest COP.

Tips and common mistakes

  • Use the fuel from several years of bills, averaged; one mild winter can mislead.
  • Enter your real fuel price. Without one the tool uses an example price and tells you so.
  • The flow temperature matters most. Test 35, 45 and 55 °C to see how much better radiators or insulation would help.
  • Ground-source heat pumps keep a better COP in cold weather, but cost more to install.
  • If the boiler also heats hot water, keep the hot-water share at about 10–20 %.

FAQ

How is the COP calculated?

As a share of the ideal Carnot value: quality factor × flow temperature ÷ (flow − source temperature), in kelvin. The default quality factor of 40 % is cautious and also covers defrosting losses of air-source units.

Where do the temperatures come from?

From the nearest of about 300 DWD climate stations (1991–2020 averages) within 50 km. Outside Germany, choose your own monthly mean temperatures.

Does it include a backup heater?

No. It assumes the heat pump covers the full heat load. If yours is smaller, the coldest days will need some direct electric heating.

Can I enter gas in cubic metres?

Yes: choose m³ as the unit; about 10 kWh per m³ is assumed. Your bill usually states the exact conversion — enter kWh directly if it does.

Guide last updated:

For scripts and AI agents

  • JSON API: POST https://ledgerlyheatpumpestimate.usefulweb.app/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://ledgerlyheatpumpestimate.usefulweb.app/mcp · tool name ledgerly_heat_pump_estimate
Example request
curl -s -X POST https://ledgerlyheatpumpestimate.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"latitude":52.52,"longitude":13.405,"fuel":"gas","fuelUse":20000,"fuelUnit":"kwh","flowTempC":50,"fuelPrice":0.11}'
Feedback · ideas & bug reports welcome