Heat pump sizing calculator for commercial buildings
This heat pump sizing calculator turns a year of gas use into an indicative heat pump size in kW for a commercial building; it is a screening step, not a design.
How the gas-bill method works
The tool above uses the consumption shortcut: it starts from what the building already burns and works back to the heat it delivers. Three steps, all plain arithmetic.
- Annual fuel energy. Add up twelve months of gas in kWh from your invoices or meter data. A bill in cubic metres or units needs converting using the conversion factor and calorific value printed on the bill.
- Useful heat. Multiply by the seasonal efficiency of the existing boiler. A boiler that burns 100 kWh of gas and delivers 85 kWh to the building has a seasonal efficiency of 85 per cent. The tool lets you set this, because ageing plant and poor controls lower it.
- Indicative kW. Divide the useful heat by the equivalent full-load hours, which is the number of hours at full output that would deliver the same annual heat. Annual heat in kWh divided by hours gives kW.
Equivalent full-load hours is the soft spot in the method. It describes how peaky the building’s demand is. A building used all day through a cold season runs more hours near its peak than one with short, sharp morning peaks, and we have not verified a standard figure that fits every building type, so the tool treats it as an input for you to set. If you have no better figure, look at the range below and see how far the result moves.
Worked example - arithmetic only, not a real project
Worked example - arithmetic only, not a real project. Assumptions, all stated inputs: annual gas use 80,000 kWh; existing boiler seasonal efficiency 85 per cent.
- Useful heat = 80,000 × 0.85 = 68,000 kWh.
- At 2,000 equivalent full-load hours: 68,000 ÷ 2,000 = 34 kW.
- At 1,800 hours: 68,000 ÷ 1,800 = about 37.8 kW.
- At 2,500 hours: 68,000 ÷ 2,500 = 27.2 kW.
The same heat gives 27 kW or 38 kW depending on an assumption that nobody has measured. Doubling the annual gas use to 160,000 kWh with the same 85 per cent gives 136,000 kWh of heat and, at 2,000 hours, 68 kW. A larger site with 300,000 kWh gives 255,000 kWh of heat and 127.5 kW at 2,000 hours, or about 141.7 kW at 1,800 hours and 102 kW at 2,500 hours.
The range, not the single number, is what the tool is for: it tells you whether you are looking at tens of kW or over a hundred, which is a different kind of project.
Why this is screening only, not a design
A fuel-use shortcut cannot see several things that decide a real specification:
- Heat-loss calculation. Peak demand on the coldest design day comes from the building’s fabric, ventilation and set points. A building with a historic over-sized boiler may use little gas but still have a large nominal load, or the reverse.
- Peak day, not average. The unit has to meet the coldest days within its operating limits, and a mean-demand figure does not show that.
- Hot water. Gas used for hot water, kitchens or process is mixed into the bill. A high-temperature hot water duty needs different design and affects the seasonal figure; see commercial hot water heat pumps.
- Defrost and capacity at low air temperature. Air source units lose some output on cold, humid days and need to run defrost cycles, so the rated capacity is not the capacity at the design temperature.
- Diversity and controls. Zones rarely peak together, and how a system is controlled changes how much capacity is needed.
- Gas use that is not space heating. A bill that includes cooking or process loads overstates heating.
- Existing boiler efficiency. If it is set wrongly the result moves in proportion.
The sizing guide explains the full method, including degree days and the risks of over-sizing. The result of this tool is indicative only.
What to do with the result
Use it to place yourself in a size band and decide what to ask next. The Boiler Upgrade Scheme bands from Ofgem installer guidance (V5.1) are:
- Up to 45 kWth for a single heat pump.
- Up to 70 kWth for a multi-heat-pump (cascade) system serving one property under one voucher.
- Up to 300 kWth for a shared ground loop, with each heat pump no more than 45 kWth.
The scheme covers small non-domestic buildings in England and Wales where the applicant owns the building, and does not fund a hybrid system. Above these bands the scheme does not help; see the Boiler Upgrade Scheme guide. Against the example, 34 kW sits inside the 45 kWth band while 127.5 kW is above 70 kWth, so a single-property scheme would not apply.
Where the answer is several tens of kW or more, you are likely to be looking at several modules in a cascade rather than one large unit, which also gives some resilience and better part-load matching. Keep in mind the grant limit counts kWth of heat pump capacity, so the nominal size you are quoted and the scheme size need to be read from the same basis. For running cost against your existing boiler, move to the running-cost calculator and use your own tariffs.
Data to gather before you rely on a number
- Twelve months of gas (or oil or LPG) consumption in kWh, preferably half-hourly or monthly, so seasonality shows.
- The age, type and rated output of the existing boiler, and any known efficiency issues.
- What the fuel is used for: space heating, hot water, kitchens, process.
- Flow and return temperatures and the emitter type: radiators, fan coils, underfloor; see the flow temperature guide.
- Occupancy pattern: hours per day, days per week, seasonal closures.
- Any heat-loss survey or building-services report you already hold.
- Electrical supply capacity and the distribution network operator’s details.
If you want to compare options after you have a range, you can send your details; we reply by email. Check any installer’s MCS and other credentials on the public registers before you sign.
Screening estimate, not a design. This turns twelve months of fuel use into an indicative heat load. A real design needs a heat-loss calculation, hot water demand, defrost and diversity allowances, and a survey. Nothing here is a quote or an engineering recommendation.