Heating energy costs

Posted on 11th November 2019

Having had to do some emergency plumbing on my ETA Pellet boiler before breakfast, I got to thinking about energy costs for heating. We are off the mains gas grid, and previously the house was heating by an oil boiler, but the previous owner installed the pellet boiler. Government subsidies can affect costs, but for reasons I don't fully understand, we are not on the RHI. So I pay full fuel costs, and maintenance, but obviously didn't pay installation costs.

I used Energy comparison a very useful table of data; but I think electricity costs are inflated. Our electricity is supplied by Good Energy. As is typical of electricity costs in the UK, we pay a standing charge (currently £100 a year) and then a charge per kWh used (15.0465p). I will take the attitude that I need an electricity supply regardless, and so extra usage (e.g. to heat the house) should be costed at only the per unit charge. However, this makes comparing tariffs difficult.

(We also have a small wind turbine. This reduces our electricity usage: excess electricity is sent back into the grid. If we had electrically based heating, then intermittently, we would save on electricity costs. We do not have a battery system: this might make sense to us, but from a "global" perspective, it is more efficient to send unwanted electricity back into the grid, and let the grid network balance supply and demand.)

For comparison, we use about 2400 kWh a year (certainly more, but reduced by the wind turbine; at a previously smaller house with gas hob, we used 2700 kWh a year). I have previously used Shell Energy. Their current tariffs are:

Tariff Name Standing charge (year) Unit rate
Fixed Jan 2021 £89 16.048p
Flexible £90.7 17.173p
Fixed June 2021 £129.7 16.140p
Fixed Oct 2022 £136.6 16.242p

I checked, and I get the same tariffs I use a different (much large) annual usage. For Good Energy, current tariffs are:

Tariff Name Standing charge (year) Unit rate
Fixed £120 17.4p
Flexible £115 18.3p
EV charging £167.5 15.8p

The EV charging (meant for electric cars) is interesting, as it clearly trades standing charge for unit charge, but otherwise, the values don't vary that much.

How much heating do we use? This is hard to estimate, but if I trust the claimed calorific value of the wood pellets we use, then over a year we used 18 mWh. At 90% efficiency, this is 16 mWh. At my old house, smaller and perhaps better insulated, we used 10-12 mWh of gas a year (which included a gas hob, and excludes efficiently loses; but this shows the figures are reasonable).

So, here's my table:

Fuel Effective cost per kWh of heat Cost per year (16mWh)
Electric heaters 15.8p £2528
Wood pellets1 4.54p £726
GSHP2 4.51p £722
ASHP3 5.85p £936

Next steps would be to estimate ongoing maintenance costs. For example, electric heaters have essentially zero ongoing costs.


  1. £275 for 965kg at 6.9 mWh per tonne @ 90% efficiency = 4.5419p a kWh. These pellets are much cheaper by kWh than others; I am told this is because they are from a small supplier which is not RHI accredited (doesn't affect me, as I cannot file for RHI payments). I wonder if I really believe the calorific value claim?
  2. Ground Source Heat Pump. Claim is 350% efficiency.
  3. Air Source Heat Pump. Claim is 270% efficiency.

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