Oil-Filled Radiator Energy Use Calculator

How the calculation works

How Much Does an Oil-Filled Radiator Use?

An oil-filled radiator is one of the most common electric heaters in UK homes without central heating, and in rooms that need one extra source of heat. Sealed fins hold a charge of oil, which the electric elements warm. The oil then heats the fins, and the fins warm the air around them by natural convection. There is no fan, so the unit runs quietly and does not move dust around the room the way a fan heater does.

The calculator estimates how much electricity the heater uses per hour, per day and over a heating season. It takes the rated power, the thermostat cycle and the hours of use as inputs, and reports the result in kilowatt-hours (kWh). If you also enter the unit rate from your electricity bill, it converts the energy into a cost.

Why the thermostat cycle matters

The rating on an oil-filled radiator gives the maximum power the elements draw when they are switched on. It does not give the average power. Modern units have a built-in thermostat. When the room reaches the set temperature, the elements switch off, and they switch on again when the room cools. Over an hour, the heater therefore spends only part of its time drawing full power.

The thermostat cycle is that share of time, written as a percentage. The energy used per hour of operation is:

E_h = P × c

Here P is the rated power in kilowatts and c is the cycle in decimal form, so a 65% cycle is 0.65. A 1.5 kW heater at a 65% cycle uses 1.5 × 0.65 = 0.975 kWh per hour. Six hours a day gives 5.85 kWh per day, and thirty days gives 175.5 kWh per month. The calculator then applies the same logic to a season: E_day = E_h × H for the daily figure, and E_season = E_day × M × 30 for the season, where H is the hours per day and M is the number of heating months.

A simpler method multiplies the rated power by the hours of use. For the same example, that gives 9 kWh per day, which overstates the real consumption because the thermostat switches the elements off for part of the time.

What sets the cycle

The cycle depends mainly on two things: how well the room is insulated, and the gap between the indoor set temperature and the outdoor temperature. A well-insulated room in a milder area needs little additional heat, so its cycle is low. A room with draughty windows, an uninsulated external wall or a large exposed surface needs the elements to run far more often, so its cycle is high. Heat losses are greatest on cold, windy nights, so the cycle in a poorly insulated room can approach the maximum on the coldest days.

Because the cycle depends so strongly on the building, a generic table gives only a rough starting point. The calculator does not assume a single value. You set the cycle to suit the room, and the most reliable figure is one measured on your own heater, as described further down.

Edge cases that change the result

Several conditions move the real figure away from the estimate. Direct sunlight through a window warms the room without the heater, so the cycle drops during sunny hours. An open door to a cold hall or stairwell lets heat escape, which raises the cycle. Some heaters sense the temperature close to the unit itself, so a heater placed very near a curtain, a wall recess or a sofa may switch off before the rest of the room is warm.

Timing also matters. If the heater runs on a timer, the hours to enter are the hours it is actually switched on, not the hours the room is occupied. If the heater is switched on only for an evening, but the room is used all day, the daily figure must reflect the evening use alone.

How to measure the real cycle

A measurement gives a better figure than any table. Plug the heater into a plug-in energy monitor and wait until the room reaches the temperature you normally want. Then record the energy used over one full hour. Divide that energy by the rated power in kilowatts. If a 1.5 kW heater records 0.9 kWh in one hour, its cycle is 0.9 ÷ 1.5 = 0.6, or 60%.

Repeat the test on a cold day and on a milder day, because the cycle changes with the outdoor temperature. Use the cold-day result for the coldest part of the season and the milder-day result for the rest of it. Keep the room's doors and windows in the same state as usual during each test, otherwise the result describes a different situation.

Reducing consumption without losing comfort

Because the cycle has the greatest effect on the result, anything that keeps the room warm for longer lowers it. Keep the room's door closed so that heat stays in. Keep the heater clear of curtains and furniture, and away from draughts from windows and doors. Lowering the comfort temperature by a degree or two reduces the time the elements run. A timer, or a programmable plug, can switch the heater on shortly before the room is used instead of leaving it on for hours.

The effect is easy to quantify. The 1.5 kW heater above, used for 6 hours a day over 5 months (150 days), consumes 877.5 kWh at a 65% cycle. At a 55% cycle, the same use consumes 742.5 kWh, which is 135 kWh less across the season. The saving comes only from less time with the elements on, so check the result on your meter after making a change.

Some UK electricity tariffs have a cheaper overnight rate. Running the heater at those times does not reduce the kWh used, but it can lower the average price of each kWh. Compare the charges on your bill with the tariff details from your energy supplier. Ofgem, the energy regulator, publishes information on the different types of tariff available to households.

Common mistakes

The most frequent mistake is to multiply the rated power by the hours of use, which assumes full power for the whole time. For a 2 kW heater used for 8 hours with a 65% cycle, the calculated figure is 2 × 8 × 0.65 = 10.4 kWh. The simple method gives 16 kWh, so the simple method overstates the use by more than half.

The second mistake is to calculate one day and assume that figure holds for the whole season. Outdoor temperatures fall through the winter, so the cycle, and therefore the consumption, rises. A cycle set during a mild October will understate the energy used in January.

The third mistake is to forget what the calculator leaves out. It estimates only the energy used by the heater. Standing charges, and any other fixed charges on the bill, are not included, so the total on your bill will be higher than the estimated cost.

The fourth mistake is to treat the rated power as the average. The rating is the maximum the heater can draw, and it is the right figure to check the load on a socket. It is the wrong figure for estimating energy use over a season.

Local norms and electrical safety

Portable room heaters sold in the UK are expected to meet the safety requirements of BS EN 60335-2-30, the British adoption of the international standard for room heaters. Standard UK plugs and sockets follow BS 1363. A 13 A socket supplying 230 V can carry about 3 kW, so a 2 kW or 3 kW heater uses most of that capacity. Do not share the socket with other high-power appliances, and do not use an extension lead rated below the heater's power.

Fixed electrical work, such as a new circuit for a high-power heater, falls under the wiring regulations in BS 7671 and, in England and Wales, under Part P of the Building Regulations. Such work should be carried out by a registered electrician.

When to use this calculator

Use this calculator to plan the running energy of an oil-filled radiator, to check the size of an electricity bill, to compare settings or to decide how to use a heater in a single room. It is a planning tool. The result is an estimate, and actual use varies with the room, the weather, the settings and the way the heater is used.

When not to use it

Do not use this calculator to decide whether a whole home needs a new heating system, to size heating for a building, or to compare a heater with a heat pump or another whole-house system. Those decisions need a room-by-room heat loss calculation, carried out to BS EN 12831 by a qualified heating engineer. The calculator does not model heat loss through walls, roof, floor or windows, so it cannot say how much heat a room needs.

Comparing with other heaters

At the same electrical power, an oil-filled radiator and a fan heater turn almost all of the electricity into heat, so they use the same energy per hour. The difference lies in how long each one runs. An oil-filled radiator holds heat in its body after the elements switch off, which can keep a room warm for longer and lower the overall cycle in a room used for long periods. A fan heater warms the air quickly, which suits short, occasional use.

A heat pump works differently. It moves heat from the outside air rather than generating it from electrical resistance, so for the same heat output it draws less electricity. Its performance depends on the system and its installation, and this calculator does not model it. For whole-home comparisons, ask a qualified heating engineer.