How much energy does it take to heat your home – Can I afford to heat my home (and eat cake)?

Sandy Hickey

by Sandy Hickey

17/10/2022, in Main Blog | South Warwickshire Blog

I love a gadget that can give me useful information, so I was very excited to see a smart meter was already installed in my newly acquired house.

It took quite a lot of button-pushing to figure out the right interrogation technique. I suggest sitting down with a cup of tea and a big slice of cake.
Here’s one I was measuring earlier! As you can see, I really do like measuring things. But joking aside, to understand something, it is important to use an appropriate unit of measurement and to understand what you are measuring.

If someone tells me their gas bill was £6000, does it help me know how much my house will cost to heat? No! They might be the homemade cake supplier for all the coffee shops in South Warwickshire, where I live, and have their gas oven at full capacity every day of the year.

If they tell me their gas bill was £75.01 for the last 12 months, that might seem good until I do a calculation using my own daily standing charge of 20.551p x 365 days = £75.01. No gas actually used!

If I paid £450 for a fuel bill in 2019 and £650 in 2020, this information doesn’t tell me if the standing charge went up, the price per kWh went up or my kids came home from university due to a pandemic and took a lot of long showers!

You get the point, if you want to measure energy and compare it, you need to use an appropriate unit of measure and understand what you are measuring.

As I’m already thinking about tea and cake, let’s start with a kettle. If I run my 3kW kettle for two minutes to make a cup of tea, I would use 0.1 kWh of energy. 

Whether it is gas or electricity, our smart meters measure how much energy we get from them in kWh.

So, let’s look at the thing in my house that uses the most energy – my boiler. As you can see, it is situated in my loft. I know it’s a combi-boiler as there is no cold-water feeder tank or hot water storage tank in the house.

I’ve just found the manual on-line and read lots of technical information. The most interesting facts for me are that it is 89.3% efficient and it can be adjusted to provide heat from 6kW to 24kW.

If I can improve the thermal performance of the house substantially, the boiler settings can be modified to ensure it always runs at optimum efficiently.

I’m going to assume it is correctly adjusted to suit the property’s current heating requirements, and at the next yearly service I’m going to ask lots of questions about the set up. By then I will also know more about how much energy is lost through my building’s external surfaces. I am already planning to investigate what insulation there is in the house at the moment and how to improve it.

Whenever the boiler fires up, for the space heating or hot water from the taps, the smart meter records how much gas it has used.

So, let’s get back to my smart meter. Here’s my simple explanation of how I use it at the moment.

I’m really interested in the amount of energy I use for different things. If I know that, I can compare that directly with what other homes are using. The fact that each energy supplier has a different price structure won’t skew the calculation, if I look at kilowatt-hours used, not pounds sterling.  

The photo on the right shows 3 of my measuring gadgets.

I took the photo, coming up to lunch time on a mild middle October day. I’ve gallantly gone without porridge or any hot water to get a reading just from the space heating. 

On the left is my thermostatic controller, heating the house to 18 degC.

In the middle is one of my room thermometers, saying it is 17.4 degC inside. I suspect this is more accurate than the thermostat, but more importantly it is showing the outside temperature (15.2 degC).

The temperature difference is really important, particularly in a poorly insulated house. As my boiler fires heat into the rooms, some of it is escaping through the external surfaces. The greater the temperature difference, the more heat is sucked out. When you choose to reduce the temperature in your house, you aren’t calling for less heat directly, you are making the temperature difference between the inside and outside smaller, so less heat is being lost to the outside world.

On the right is the smart meter. It is 12.25pm and the heating has been running since 8.30am when the temperature inside was 16.5 degC and outside 13.5 degC . I can see that to heat my house up by 1.5 degC from 16.5 degC to 18 degC with an approximate temperature difference of 2.5 degC, I have used 9.66 kWh.

I can do several things with this information. For example, by looking at the same time slot, start and end temperature and the comparative size of each house, I can compare my house with my neighbour’s house. My house is just under 100 sqm so it used 0.1 kWh/sqm to keep it warm. If they had used 10 kWh too, but their house was 200sqm, it would take 0.5 kWh/sqm. to heat it the same amount as mine. This tells me that they have a more insulated house, and possibly a less draughty house., because they are using half the amount of heat I am. 

But today I am interested in cost. My unit rate is 10.236p/kWh so it has cost me 99p to heat my house this morning. This morning it was definitely cheaper to work from home, rather than go to a coffee shop. It looks like I will have to wait until it is a lot colder out before it’s worth my turning the heating off and heading out for a ‘free’ slice of that cake!