Heat Pump vs Boiler Break-Even Calculator

The Heat Pump Math

A heat pump usually costs more to install than a gas or oil boiler and less to run, because it moves heat rather than burning fuel to make it. That turns the choice into a payback question: how long the annual running saving takes to cover the extra upfront cost. This calculator does that one division and reports the year count, along with the upfront gap and the annual saving it is dividing.

The physics behind the running-cost gap is a coefficient of performance. A heat pump delivers several units of heat per unit of electricity because it is transferring energy rather than creating it, where a condensing boiler delivers slightly less heat than the energy content of the fuel it burns. OpenStax College Physics sets out how that coefficient is defined and why it exceeds one.

What Drives Each Input

All four figures are entered rather than assumed, which matters because every one of them varies by country, by property and by year. Installation quotes turn on the size of the unit, the state of the existing pipework and radiators, and whether a hot water cylinder has to be added. Running costs turn on the local electricity-to-gas price ratio, the insulation of the building, and the temperature the system is asked to reach.

Insulation is the variable that moves the running-cost gap most, because a poorly insulated building forces the system to work at a higher flow temperature, which lowers the coefficient of performance and narrows the saving. Local quotes and recent bills are the figures worth entering; national averages describe a building that is not the one being heated. The IEA's The Future of Heat Pumps covers how widely the economics differ between markets.

Worked Example for Retrofit

Take a heat pump at 10,000 against a boiler at 3,500, running at 900 and 1,500 a year respectively. The upfront gap is 6,500 and the annual saving 600, so the break-even lands at 10.8 years and the twenty-year net saving at 5,500.

That is a payback inside the equipment's expected life with a modest surplus behind it, which is a different result from a decisive one. Change the annual saving to 550 and the break-even stretches to 11.8 years; change it to 950 and it drops to 6.8. The answer is far more sensitive to the running-cost gap than to the install quote, because the gap is the divisor.

Where the heat pump costs no more upfront, there is nothing to pay back and the tool reports zero years rather than dividing into a negative. That case arises when a boiler replacement is expensive or a grant closes the gap entirely.

What the Calculator Does Not Model

Grants and subsidies are the largest omission, and they are also the least stable: schemes open, close and change value by country and by year, so the tool takes none of them as given. Where one applies, subtracting its value from the heat pump cost before entering it produces the post-subsidy picture.

Beyond that: insulation or radiator work that a retrofit requires, a hot water cylinder where the outgoing system had none, maintenance differences between the two systems, and any divergence between electricity and gas prices across the period. The calculation holds both running costs flat for twenty years, which no energy market has ever done. It is a clean comparison of two fixed cost streams rather than a forecast.

Where the Comparison Is Commonly Set Up Wrong

The most frequent is comparing a new heat pump against the running cost of a boiler that is already installed and working. If that boiler is near the end of its life, the honest comparison is a new heat pump against a new boiler, which is what the two cost inputs here are for; entering a boiler cost of zero answers a different question.

The others are entering running costs from a building whose insulation has since changed, and reading the twenty-year figure as a forecast rather than as the same annual saving multiplied out. Both produce a number that is arithmetically correct and describes a situation that does not exist.