Lowering bills, cutting carbon
26 Jan 2023
With household energy bills at eye watering levels, you may be looking for ways to cut your electricity and gas use. A quick internet search will reveal hundreds of energy saving tips – including from Big Clean Switch – providing cost saving estimates for individual changes you can make. But how are energy savings calculated?
Here are a few examples:
If you’ve ever wondered, “This is all fine, but how are these energy savings calculated, and how do I know if they apply to me?” then this article is for you.
Our goal is to help the most people save the most energy.
For this reason, whenever we have to make an assumption about how things are working before you’ve made any changes, we assume the worst. Otherwise, we might be making homes that do represent a worst case feel that the benefit from making a change will be smaller than would really be the case.
The flip side to this is obviously that for homes that don’t represent a ‘worst case’, our savings figures may overestimate potential benefits. Wherever possible, we try to be transparent about this by framing the savings as ‘up to’ or ‘as much as’. But it’s also why we write articles like this one – to help you take a more informed view of savings figures and how they might relate to you personally.
Ultimately, we think it’s a lesser evil for someone to make a change that will save them some money, but possibly less than our headline savings figure, than for someone to decide not to make a change that could have saved them more money because we’ve been too cautious about the potential benefits.
Most of the advice you’ll hear about reducing shower costs focuses on two changes: reducing the amount of time you spend in the shower, and reducing the amount of water you use when showering by lowering the ‘flow rate’. But to provide a savings estimate for these simple-sounding changes, we have to make a series of assumptions about:
Below we’ve explained in detail how we arrived at a savings figure of up to £104.52 per person per year as a result of reducing the ‘flow rate’ to 6 litres per minute. If you’d like to have a play with these assumptions, we’ve also created a calculator that does the hard work for you – find it at the bottom of this article.
The most obvious impact this choice has is on energy costs: the amount of energy required to heat a litre of water is the same whether you’re using an electric element or an oil or gas boiler, but a unit of electricity costs around three times as much as a unit of gas.
Only around a third of UK households have electric showers, with the remainder using variations on ‘mixer’ showers (which mix cold water with hot water fed by a boiler or tank). And with only around 5% of UK homes using an oil boiler (and some of these using electric showers rather than showers fed by the boiler), we’ve opted to calculate savings for a shower fed by a gas-powered boiler.
The decisions don’t end there, though. Gas boilers come in several shapes and sizes – most notably split between those that heat water that is then stored in a tank, and those that heat water on demand. ‘On demand’ boilers (known as ‘combi boilers’) make up around 60% of all UK household boilers, so our calculations are based on a gas combi boiler.
And, because we want to present people with something approaching a worst case scenario, our calculations assume an old, ‘G-rated’ combi boiler operating at just 68% efficiency (which means that 32% of the energy in the gas used by the boiler is wasted).
If you’re wondering what difference that single assumption about the efficiency of the boiler makes, if we opt for an ‘A-rated’ combi boiler running at 92% efficiency, our £104.52 savings figure from reducing the flow rate to 6 litres a minute falls to £77.25 – still a change worth making.
It may sound obvious, but the amount of energy required to heat water for a shower depends both on the starting temperature of the water and on the temperature you heat it to. It’s surprisingly hard to find quality data on average mains water temperatures for the UK, but this chart published by Severn Trent is helpful. It suggests a normal range of between 9 and 19 degrees, with a fairly dependable and equal variation from winter to summer, so in the absence of a hard figure, and bearing in mind that Severn Trent is broadly Midlands-based, we’ve opted for a mid-point of 14˚C. (If you know of a better data source than this, let us know).
Reducing the average mains temperature from 14˚C to 12˚C increases the amount of energy used, and so the annual cost of showering. As a result, the saving from a reduction in the flow rate to 6 litres a minute would increase too – from £104.52 a year to £113.23 a year.
There is a little more certainty over shower temperatures, which tend to be close to human body temperature. In fact, many mixer showers have a default temperature setting of 38˚C, and you have to depress a button to raise the temperature above this level. We have therefore assumed the mains water temperature is increased to 38˚C in our calculations.
The longer you spend in the shower, the more hot water you use. The more hot water you use, the more energy is needed. According to this report by the Energy Saving Trust [PDF], people in the UK spend 7.5 minutes in the shower on average. We’ve used this figure, although it’s worth noting that the charity WaterWise puts the average shower at ‘around 10 minutes long’).
According to this YouGov poll, the majority of people in Great Britain (49%) say they shower once a day, so our figures are based on taking a single shower every day of the year.
The more water your shower uses, the more you have to heat, and the higher your energy bills will be. If you’re using a combi boiler, then the flow rate will be determined by the boiler itself. Flow rates can reach as high as 20 litres per minute, but this is rare, and 15 litres per minute is still fairly high. We’ve therefore used 15 litres per minute in our calculations, and dropped this to 6 litres per minute to calculate potential savings from the installation of a flow reducer (these cost less than £10 and usually come with a range of settings to allow you to choose the flow rate that gives you the maximum savings without negatively impacting on your showering experience).
The cost of electricity and gas varies depending upon which tariff you’re on, and which region of the country you live in. We have used the limits set by the UK government’s ‘Energy Price Guarantee’, which caps the price suppliers can charge on most tariffs at a set level. We’ve then taken the average of these across all regions to arrive at a cost of 10.33 pence per kilowatt hour. This is inclusive of VAT (which is 5% for household energy bills).
Now we know how much water needs to be heated and what temperature it need to be heated to, and we know the cost of the energy, we need to calculate how much energy is required to achieve that change in temperature. The amount of energy required to raise the temperature of any material by 1˚C is known as its ‘specific heat capacity’, and is measured in joules per kilogram, written as J/(kg ˚C).
The specific heat capacity of water changes depending on the current temperature of the water. We have used a figure of 4,186 joules per kilogram of water, which is the specific heat capacity of water at 15˚C, close to our assumed average mains water temperature. If we had used the specific heat capacity of the water once it had reached a higher temperature – 26˚C, say – this would have reduced our savings figure by about £0.18 a year per person. So, the assumptions we make about specific heat capacity have relatively little impact on our savings estimates compared with the assumptions we make about say, the efficiency of the boiler.
Converting litres of water into kilograms is easy – one litre of water weighs one kilogram.
And a joule is simply an alternative unit of measurement for energy. Because our household bills are quoted in kilowatt hours, once we’ve worked out how many joules of energy are required to heat the shower water to the desired temperature, we need to convert that figure into kilowatt hours to estimate the cost of the energy. This is pretty straightforward, too – one kilowatt hour is equal to 3,600,000 joules (which means one joule is equal to 0.00000027778 kilowatt hours).
All these assumptions allow us to calculate how much showering would cost over a whole year for one person based on a 15 litres per minute flow rate.
First we work out how much water is heated for showering per person per year:
Flow rate (litres per minute)
Minutes in the shower per day
Shower days per year
365.25 (the 0.25 allows for leap years)
41,090.625 litres of water per year.
Next, we work out how much the temperature of the water needs to be increased by:
Shower water temperature
Average mains water temperature
We can then work out how much energy is required (in joules) to increase the temperature of 41,090.625 litres of water by 24˚C:
Heat required to increase water by 1˚C (its specific heat capacity)
4,186 J/(KG ˚C)
Amount of water heated (remember one litre equals one kilogram)
Increase in water temperature required
Now, we convert that figure into kilowatt hours:
Energy required in joules
Kilowatt hours per joule of energy
1,146.711548619 kilowatt hours of energy
Now we need to account for the energy wasted by the boiler, which we’re assuming operates at 68% (or 0.68) efficiency:
Kilowatt hours of energy required to heat water
Efficiency of the boiler
Kilowatt hours of gas required to heat water
Finally, we multiply the amount of gas by the unit rate to arrive at an annual cost estimate:
Gas required per person per year
Average unit cost of gas including VAT
Estimated annual shower cost per person per year
By repeating the calculation using a flow rate of 6 litres per minute, we can estimate the saving achieved by reducing the flow rate.
Estimating savings from a reduction in tumble dryer use is a little more straightforward, but still requires us to make a number of assumptions about:
There are three broad categories of tumble dryers – heat pump dryers (the most energy efficient), condenser dryers (pretty inefficient), and vented dryers (the least energy efficient). As we’re trying to work out the maximum savings achievable for a household in a ‘worst case’ scenario, our calculations are based on a vented dryer.
Estimating electricity consumption is tricky because this will vary depending on how full a tumble dryer is, what materials are being dried, and how wet they are when they’re put into the dryer. Thankfully, the European Union has, as part of its appliance labelling regulations, developed a standard process for estimating the ‘weighted energy consumption’ [PDF] of a tumble dryer based on a mix of different loads (four in every seven are assumed to be full loads, while three in every seven are assumed to be ‘partial’ loads).
We searched the EU’s product registry for its least efficient vented dryer (based on weighted energy consumption), which at the time of running the calculations was a Hotpoint TVFS 83C GG.9 UK with a weighted energy consumption of 3.65857142857143 kilowatt hours per cycle.
It’s worth noting that the weighted energy consumption is based on the dryer always being run on a cottons programme. Most households will probably use a mix of programmes depending on the laundry they’re drying, but there is no publicly available, comparable data on other cycles. Assuming the household always uses the cotton cycle is also in keeping with our ‘worst case’ methodology.
We couldn’t find a huge amount of data on typical tumble dryer use, but the European Commission did run some research in the UK, France and Poland in 2018. This found average summer use of 2.3 cycles per week per household and 3.6 cycles in winter. We’ve therefore used these figures in the absence of UK-specific numbers.
As with showering, we’ve used the energy price limits set by the UK government’s ‘Energy Price Guarantee’. We’ve then taken the average of these across all regions to arrive at a cost of 34.04 pence per kilowatt hour, inclusive of VAT.
It is possible that a household may opt for an alternative approach to drying clothes that also impacts on energy use. A dehumidifier, for example, can speed up the drying process if laundry is hung on a clothes rack. Equally, hanging clothes on radiators would reduce the usable heat output from the radiators (because heat energy would be used to convert the water in the clothes into water vapour), which could impact on heating bills.
However, if our goal is to highlight the maximum potential savings of someone in a worst case scenario adopting a best case change, then it makes sense to assume the household switches from tumble drying to air drying without use of either a dehumidifier or radiators.
We can then use these figures to estimate total tumble dryer costs…
First, we’ve calculated the total number of cycles per year by taking the summer and winter cycles per week and multiplying by the number of weeks, first for summer:
Number of cycles per week in summer
2.3 cycles per week
Number of weeks in summer
Number of cycles in summer
And then the same for winter…
Number of cycles per week in winter
3.6 cycles per week
Number of weeks in winter
Number of cycles in winter
Adding the two seasons together gave us:
Total cycles over 52 weeks
Because this only gave us the number of cycles for 364 days (52 x 7 day weeks), we’ve then reweighted the total for the missing 1.25 days:
Total cycles over 364 days
0.421428571428571 cycles per day
Number of days in a year
153.926785714285714 cycles per year
We then multiplied the number of cycles by the weighted energy consumption of our least efficient vented tumble dryer:
Cycles per year
Weighted energy consumption of vented tumble dryer
3.65857142857143 kilowatt hours per cycle
563.152140306122669 kilowatt hours per year
Finally, we multiplied the number of kilowatt hours by our unit rate to estimate the total annual electricity costs:
Kilowatt hours per year
Electricity unit cost
£0.3404 per kilowatt hour
£191.70 per year
This then easily allows us to estimate savings from halving tumble dryer use, by dividing this total cost by two.
Estimating savings from ensuring a laptop is put into sleep mode rather than left on involved assumptions about:
Laptops have three broad states when not in active use:
‘Sleep’ modes use less power than ‘long idle’ modes, and much less power than ‘short idle’ modes. In keeping with our worst case approach, we wanted to estimate the difference in annual electricity costs between leaving a laptop in ‘short idle’ mode, and either manually or automatically putting a laptop into sleep mode when it is not in use.
Short idle laptop power consumption varies enormously. Laptops with larger displays and more processing power use much more energy. To find a plausible worst case scenario, we searched for the laptop that had the worst energy rating on the EnergyStar database. This had a ‘short idle’ power consumption of 24.4 watts, and a sleep mode power consumption of 1.1 watts.
According to research carried out by Centrica in April 2022, UK homes leave computers switched on (but not in use) for an average of 20.9 hours a day.
As with showering and tumble dryer use, we’ve used the energy price limits set by the UK government’s ‘Energy Price Guarantee’, taking the average across all regions to arrive at a cost of 34.04 pence per kilowatt hour, inclusive of VAT.
Here’s how we used these assumptions to estimate the savings from ensuring a laptop goes into sleep mode when not in use…
First, we calculated how many kilowatt hours of energy our worst performing laptop would use if left in short idle mode for 20.9 hours:
Wattage in short idle mode
Number of hours left on
509.96 watt hours
1,000 (to convert to kilowatt hours)
0.50996 kilowatt hours
Next, we multiplied this by the number of days in the year to calculate the total annual power consumption:
Daily power consumption
0.50996 kilowatt hours
186.26289 kilowatt hours
Finally, we multiplied this annual energy use by our average unit rate for electricity to estimate a total annual cost:
Annual energy consumption
186.26289 kilowatt hours
Electricity unit cost
£0.3404 per kilowatt hour
£63.40 per year
By rerunning this calculation for full sleep mode (with an equivalent annual cost of £2.86), we were able to estimate a savings figure of £60.55 a year from ensuring the laptop went into sleep mode rather than remaining in short idle mode.
The savings figures we’ve explored above are great for giving people a sense of what might be achieved by making particular changes. But it’s also true that they won’t apply to some people. Households with a less efficient boiler, a more efficient tumble dryer, or a better performing laptop wouldn’t see the same savings.
That’s why it’s important, alongside particular savings figures, to highlight some overarching principles to help people identify the measures that can be more impactful for them. For us, the chart below, which breaks down a typical household’s energy spend according to what the energy is used for, is always a good starting point.
It’s pretty clear that space heating and water heating are the big energy munchers. Appliances too, but that is actually a very fragmented category made up of mostly smaller wins (and some stuff, like fridge freezers and broadband routers, that you can’t do much about).
So as well as looking at potential savings for individual measures you can take, always remember this simply hierarchy:
1️⃣🔥 Focus on how you heat your home first. Use the most efficient heating system you can afford, reduce heat loss as much as you can, and then heat less space to as low a temperature as is comfortable and safe.
2️⃣ 💧Then look at water heating. Whether you’re taking showers, filling the kettle or running the washing machine, heat less water to as low a temperature as is comfortable and safe.
3️⃣ 📺 Finally look at your appliances. Maximise use of standby, use the most eco standby setting where a device has more than one (games consoles, set top boxes) and don’t worry about your mobile phone charger.