Free tool
Air Conditioning Running Cost Calculator
Air conditioning is often assumed to be expensive to run, and heat-pump heating is often assumed to be cheap. Both depend entirely on the equipment, what you pay for electricity and how much the system is actually used. This calculator does the arithmetic in the open, so you can see where the figure comes from and change any assumption you disagree with. It starts from the one thing that catches most people out: a system's kW figure is the cooling or heating it delivers, not the electricity it uses.
What this tool can and cannot tell you
This is an estimate, not a prediction of your bill. It assumes the system runs at the output you enter for every hour you enter, which a correctly sized system rarely does — in practice it automatically increases and reduces its output to match the room, and usually draws less. The efficiency ratings it uses (SEER for cooling, SCOP for heating) are averages across a whole season, measured under a standard test, so they describe how a system performs over a season rather than at any given moment. The weather, the insulation, the thermostat setting, how often doors are open and the condition of the equipment all move the real figure.
Your estimate will appear here
Fill in the form and the estimate, along with every assumption behind it, appears here. Nothing is sent anywhere.
Two different numbers, both in kW
What the system delivers
The capacity on a specification — 2.5 kW, 3.5 kW, 5 kW — is the cooling or heating the system puts into the room. It is the figure a system is chosen by, and it is the one most people mean when they say “a 3.5 kW unit”.
What it costs you to run
A 2.5 kW system does not use 2.5 kW of electricity. It moves heat rather than creating it, so it typically delivers several times more cooling or heating than the electricity it draws. That smaller figure — the electrical input — is what your bill is based on, and it is what this calculator works out.
How many times more is the system’s efficiency rating: SEER for cooling and SCOP for heating. A higher number normally means the same cooling or heating for less electricity. Neither is a prediction of your bill — they are a fair way to compare two systems.
How this is worked out
There is no hidden model here. A few lines of arithmetic, and you can check every one of them.
Two different things are measured in kW here, and keeping them apart is the whole calculation. The OUTPUT is the heating or cooling the system delivers. The ELECTRICAL INPUT is the electricity it draws to do that. The input is normally much the smaller of the two, because the system moves heat rather than creating it.
If you know the electrical input and enter it, that figure is used exactly as given and no efficiency rating is applied to it.
Otherwise the electricity drawn is worked out as the rated output divided by the SEER (for cooling) or SCOP (for heating) figure. In symbols: electricity drawn (kW) = output (kW) ÷ the seasonal efficiency rating.
Cost per hour is the electricity drawn multiplied by your unit rate, converted from pence to pounds. In symbols: cost per hour = electricity drawn (kW) × unit rate (p/kWh) ÷ 100.
Electricity per year is that figure multiplied by the hours a year you entered, plus any standby power applied to the remaining hours of the year — the hours the system is switched on but not running.
A seasonal efficiency figure is an average across a whole season, measured under a standard test. It describes a season, not a moment, so the hourly figure above is an average rather than a reading you would see on a meter.
Why your real bill will differ
The system will not run flat out
A modern system automatically increases and reduces its output to match the room — the trade word is modulating. Once the room is at temperature it settles to a fraction of its full output and stays there. This calculator has no way to know how much of the time yours would do that, so it assumes the output you entered for every hour you entered. That makes the figure a ceiling for those hours rather than a prediction.
A seasonal rating is an average
SEER and SCOP describe a whole season, measured under a standard test. In heating mode the system works as a heat pump, moving heat in from the outside air, and it has to work harder — and draw more — on the coldest days than on mild ones. Dividing by the seasonal figure gives you the average across all of that, not the worst day.
The building matters more than the equipment
Insulation, glazing, how often doors are opened and what the thermostat is set to change how many hours the system actually has to run. Two identical units in two different rooms can differ by a wide margin on the same tariff.
We do not put a saving on it
This page will not tell you what you would save against gas, oil or electric heaters. That depends on what you have now, its condition, how you use it and what you pay for the fuel — and a comparison built on our guesses about all four would not be worth reading.
Related services
What this tool is a first step towards.
Want this checked properly?
A survey measures the things this page had to ask you to estimate. It is free, there is no obligation, and you get an answer based on your building rather than on a general rule.
Before you rely on this
- This is indicative, not a specification. It is a guide to help you plan and ask better questions. It is not a design, not a quotation and not a substitute for a survey.
- A survey is what decides. An engineer measures the things this page can only ask you to estimate, and confirms where equipment can physically go, how it will be run and what the electrical supply allows.
- Installation and refrigerant work are regulated. Air-conditioning systems contain refrigerant under pressure and are connected to a fixed electrical supply. Installing, moving, opening, recharging or decommissioning one is work for a qualified, F-Gas registered engineer — never a DIY job.
- We do not publish a saving. Nothing on this page claims what you would save against any other way of heating or cooling a room. What you would save depends on what you have now, how you use it and what you pay for energy, and we will not put a number on that without knowing all three.