Price a unit of heat, then the space you heat
Paying for hundreds of litres at once makes a heater the easier option when money is tight, but it doesn’t make electricity the cheaper fuel. Compare the cost of a unit of heat first, then how much heat you need and where.
A 2 kW heater running at full power for one hour uses 2 kilowatt-hours (kWh); for four hours, 8 kWh. Neither figure tells you how warm the room will get.
Sources: SEAI: energy conversion factors · Electric Ireland: published electricity-rate example
Worked example: €830 of oil against a four-hour heater session
Take an illustrative delivered quote of €830 for 500 litres of kerosene, or €1.66 a litre. It is a worked-example price, not today’s lowest price, a national average or a quote for your address.
SEAI’s figure gives about 9.82 kWh in a litre of kerosene. At an assumed 85% boiler efficiency, that is about 8.35 kWh of heat leaving the boiler, so €1.66 buys heat at about 19.9 cent per kWh.
Electricity at 38.04 cent per kWh, Electric Ireland’s VAT-inclusive standard-rate example for July 2026, puts a 2 kW heater running for four hours at 8 kWh and €3.04. Seven evenings cost €21.30. Your own tariff may be lower or higher.
The same 8 kWh from the boiler would cost about €1.59 in oil on these assumptions. That is heat leaving the boiler, not heat delivered to the room you want warm, so it isn’t the price of warming your room.
Sources: SEAI: energy conversion factors · Electric Ireland: published electricity-rate example
Cheaper heat does not automatically mean a cheaper evening
Two hours at a desk in one room is a different heating job from an evening with people in the kitchen, sitting room and bedrooms. For the first, a plug-in heater may cost less overall if the alternative is warming rooms nobody is using. For the second, the oil system may make more sense, especially when its heat is cheaper per unit and the rooms need heating anyway. Fuel prices alone can’t settle which situation you are in.
Heating zones and thermostatic radiator valves can steer central heating to the rooms you use, though not every radiator is designed to be shut off.
It also matters whether the boiler would be running anyway. Firing up the whole oil system for one room is a different comparison from heating one more room in a house that is already warm.
Sources: SEAI: heating controls
Use the rate for the hours you will actually run it
Use the unit rate you pay after any discount, including VAT. Dividing the whole bill by your usage mixes standing charges into the heater’s cost.
On a day, night or peak-rate plan, use the rate in force while the heater is on; a cheap overnight rate doesn’t price an evening on the sofa. If a session crosses two rates, cost each part separately. On a dynamic tariff, use the full unit rate, not just the wholesale-price component.
For oil, enter the delivered total and the litres in that quote. 500 litres isn’t necessarily half the price of 1,000. For oil already in the tank, use what you paid for it, not today’s price.
Four hours switched on is not always four hours heating
A heater draws its rated power only while the element is on. Its thermostat may cut the element once the room is warm and restart it as the temperature falls, so actual heating time depends on the room, the weather and your setting.
The calculator assumes 100% heating time. Under “Change the heater schedule” you can enter a lower share: 50% means the element runs for the equivalent of two of the four hours. That is a scenario you choose, not a prediction that the thermostat will halve your bill.
An “eco” label says nothing about running cost. Only the electricity used and the rate you pay do.
Sources: Centre for Sustainable Energy: electric room heaters
An oil-filled plug-in radiator still runs on electricity
The oil inside an oil-filled electric radiator stores and spreads heat; it doesn’t burn. In this comparison it sits on the electric side with fan, ceramic and convector heaters, all of which turn electricity into heat in the room.
An oil-filled radiator stays warm after it switches off, but that warmth came from electricity already used. Noise, warm-up time, controls and where you feel the heat differ between types; the running cost per kWh doesn’t.
Sources: Centre for Sustainable Energy: electric room heaters
Heat pumps need a separate comparison
A heat pump uses electricity to move heat into the home rather than turning electricity into heat, so it can deliver more than one kWh of heat per kWh of electricity.
Its power rating doesn’t belong in this plug-in-heater calculator. A fair heat-pump comparison needs its actual or carefully estimated performance, the home’s heat demand and your tariff.
Sources: SEAI: how heat pumps work
How the calculation works, and what it leaves out
Oil cost per kWh of boiler output = delivered price ÷ litres ÷ (9.8222… × assumed efficiency). An 85% efficiency is 0.85 in the calculation. The software keeps the full 35.36 ÷ 3.6 conversion and rounds only the displayed answers.
Electric-heater cost = power in kW × hours × heating-time share × electricity price per kWh, where a 100% share is 1. The weekly figure repeats the daily session on the days you enter; it isn’t a monthly or winter forecast.
The 85% boiler efficiency is illustrative, on the same net-energy basis as SEAI’s kerosene figure, and isn’t measured for your boiler. Efficiencies quoted on a gross-energy basis aren’t directly comparable, and a combustion-efficiency reading from a service isn’t necessarily what the boiler achieves over a season.
The oil figure stops at heat leaving the boiler. It excludes pipework and distribution losses and the electricity used by the boiler and pump. Some pipe heat may warm the house, though not necessarily the room you meant to heat, and boiler cycling and controls can change the result.
Equipment, installation and servicing are excluded, as are standing charges and levies that don’t change when you run a heater. Hot water, building heat loss, a target room temperature and safe, comfortable heating aren’t modelled.
These are running-cost calculations from stated inputs, not a prediction of your household bill, and not advice to leave the rest of the home cold or damp while you heat one room.
Sources: SEAI: energy conversion factors · SEAI: heating controls · Electric Ireland: published electricity-rate example
Portable heater safety
Fire service advice is to plug a portable heater straight into a wall socket rather than an extension lead or multi-way adaptor, and to switch it off when you leave the room or go to sleep, even if it has a timer or thermostat.
Sources: Northern Ireland Fire & Rescue Service: portable-heater safety
Which to use tonight
For a short spell in one room, price the heater session and check whether your oil controls can heat that room alone. For several occupied rooms, compare fuel costs, remembering that several plug-in heaters are a different purchase from one.
Being able to afford a refill and getting good value from it are separate problems. The calculator shows the running-cost trade-off; it can’t make a large upfront bill easier to find.