What size electric boiler do I need?

How to size an electric boiler

Just like  gas boilers, an electric boiler is sized based on the energy output (kilowatts) rather than physical size. The higher the kilowatt (kW) rating, the more taps and radiators it can supply, and you can use this rating to determine the correct size for your home.

Electric boilers tend to have a much lower power output than that of gas or oil boilers. This makes them suitable for small homes and flats, but not large homes or commercial buildings. Whereas a gas boiler usually has an output of up to 35 kW, an electric boiler doesn’t often come in variations over 15 kW.

As a basic rule, you should add 1.5 kW for every radiator in your home. For example, if you have six radiators, you should get a 9 kW electric boiler. This rule may vary depending on how high your ceilings are, whether you have single or double glazing and how effective your insulation is.

A boiler that is too small to cope with your heating needs won’t  heat your home up efficiently  and you could run out of hot water quickly and regularly. An electric boiler that is too powerful will use much more electricity than is needed and you may see an increase in your energy bills. If you’re unsure, you can find an online calculator that can work out the right size for you.

As previously stated, the boiler size doesn’t correspond directly to the physical dimensions of the boiler, however, electric boilers do tend to be smaller in size than their gas or oil counterparts. This is another reason why they can be a good choice for small flats and homes that have limited space. You could have the boiler installed in a kitchen cupboard, in the bathroom or even in your bedroom. Electric boilers have fewer moving parts than gas models, so they run more quietly and won’t disturb you if they’re installed in or near your bedroom. You may also have more options when it comes to finding a location. Unlike gas boilers, an electric boiler doesn’t need to be connected to the mains gas supply and also doesn’t need a flue.  

How efficient are electric boilers?

Efficiency is rated using an Energy related Products (ErP) score. This rating labels each type of boiler from the highest level of efficiency (A+++) to the lowest (G).

Despite electric boilers being very efficient (around 99 per cent), they generally have an ErP score of D. This is because the process of producing electricity involves burning fossil fuels, which creates twice as much carbon as burning natural gas in a gas boiler. Electricity is also much more expensive than gas, at around 14 pence per kilowatt hour compared to three pence for gas. It is for these reasons that electric boilers have a lower ErP score, however, they lose almost no energy during operation.

It would be much more beneficial to run an electric boiler using solar panels. By generating the electricity in this way, you would likely see a decrease in your electricity bills.

It’s also important that you check the efficiency rating of your  hot water tank. There’s no use in having a boiler that’s very energy efficient if most of this energy is being lost via an inefficient water tank. The cylinder stores the hot water that the boiler has heated up to be used for later. The tank should be made from a good quality material and must be well insulated to prevent the heat from escaping too quickly.

If you’re worried about the rate at which your tank is losing heat, you could purchase an additional insulation jacket for it or use a blanket to cover the tank and any hot water pipes that run into it. A well-insulated tank should be able to keep the water warm for at least 24 hours.