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Energy-efficient hot water

Picture of Ben Gallizzi, Senior Content Editor at Uswitch
Written by Ben Gallizzi, Senior Content Editor, Energy and Electric Vehicles

26 October 2021

Hot water accounts for around 10% of the average energy bill. It's not something most of us would want to do without, so use this guide to find energy-efficient ways of getting your hot water, and tips to help you use less of it.
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Cups of hot water

How can I heat water in my home more cheaply?

Traditionally, most homes heated their water using their existing central heating system, storing and heating a finite amount of water in a hot water cylinder.

An alternative to using the central heating system – and one found in most homes as a backup should the central heating go down – is the immersion heater. These are electric water heaters contained inside an insulated hot water cylinder. The electric elements heats the water inside the cylinder in the same way a kettle does. These can be switched on and off as demand requires it.

In recent years, this kind of arrangement has been supplanted by combi boilers, which heat hot water on demand. Today’s combi boilers have been designed as much with efficiency in mind as they have convenience, making them a better choice for most households.

Alongside combi boilers are electric showers that heat the water as it passes through the shower unit rather than relying on the house’s hot water supply. These cost around £500 but are again more efficient than traditional heating methods.

As we’ll discuss below, if you can afford the upfront cost, it’s also possible to heat your water using solar panels. While the initial cost is high, given that over 10% of your energy bill goes on heating your water, you can soon recoup the costs.

All the above looks at the cost of heating the water, but how much water you consume also plays a part – not only through making greater demands on your water-heating system, but also through your water bills. If you’ve not done so already, you could save money by switching to a water meter to help track usage and identify potential savings.

How much of my energy consumption is used for heating my water?

It's difficult to be exact about the energy that is used for heating water because it's hard to measure, and difficult to separate out from the energy your boiler uses for heating.

However, in a typical home the cost of heating water is likely to exceed 10% of the energy bill or approximately a quarter of the fuel used by the boiler. This may seem high, but remember you need hot water all year round, whereas heating is seasonal.

The true cost of hot water should also take account of the cost of the water supply itself, which in some cases can be as high as the cost of heating it.

Some people can save by switching to a water meter – as a rough guide, if you have more bedrooms than you have people living in your home a meter is worth looking into.

What types of shower are there?

We're often told to take a shower rather than have a bath to save energy and water. But what types of shower are there, and are they all really better than taking a bath?

Showers fall into three broad types:

Electric showers

This popular type uses your house’s electricity supply to heat the water instantly as it passes through the mains unit. Typical water consumption is 62 litres of hot water for an eight-minute shower.

Power showers

Another popular type, this draws hot water from your hot water cylinder, which it mixes with your cold-water supply, so relies on a central heating system or immersion heater. Some types can consume up to 136 litres for each shower.

Mixer showers

This type of shower is less popular, and like the power shower draws hot and cold water from your existing systems. Lower-end mixers are connected directly to your taps with no electric unit involved. They’re best suited to homes with high water pressure but consume less water than both power showers and baths (which consume 80 litres).

While gas costs less than electricity, electric showers are still far more efficient than relying on your hot water system.

What type of shower is the most cost-effective?

Given the cost of gas is only about 40% that of electricity, in theory, a mixer or power shower might be more cost efficient than an electric shower. However, electric showers usually use less water and therefore require less energy, so a lot depends on how long you are showering for.

In addition, electric showers are efficient due to the fact that the water is heated at the point of use, whereas with power and mixer showers heat is lost through the cistern and pipes before it even reaches the shower.

Power showers also use so much water that the traditional advice of taking a shower rather than a bath to save energy and water may not apply. Water costs will also be higher for power and mixer showers if you are on a water meter.

What can I do to lower my water heating costs?

Here are some straightforward ways to lower your water heating costs:

  • Choose carefully how you heat water.

  • Be sensible about when you heat hot water.

  • Use less hot water.

  • Heat your hot water to a lower temperature.

  • Don't let hot water cool down before you have used it.

Top tips to save on your water heating that are FREE

  • Turn down the thermostat on your cistern to 60-65°C.

  • If you have an old central heating system, the temperature of the hot water will be determined by the internal circulatory system, as set on the boiler, so turn that down.

  • Use a kettle for the times you need very hot water.

  • Check the timings and other controls on your boiler for your water heating, which on an older boiler will often be integrated with the timings of your central heating.

    You may be able to reduce the length of time that the water heating is turned on, especially in the summer.

  • Check to see if your power shower pump has flow controls. If it does, you should be able to reduce the flow.

  • With any type of shower, check to see if you can adjust the shower head spray pattern - you may find that you can reduce the flow of water.

  • If you wash up by hand, don't rinse the dishes under running hot or cold water. Instead, use a separate bowl.

  • If you have an older boiler it's a good idea to use an immersion heater in the summer.

Top tips to save on your water heating, for just a small investment

  • If you have an old cistern (tank) that isn't insulated, then fit a tank jacket.

    This should pay for itself in just a few months and you can add to the insulation effect by storing bedding, pillows and towels around the cistern.

  • Where they're accessible, insulate the 'downpipes' - these are the pipes that lead from the cistern to the taps.

  • If you have a mixer or power shower, fit a shower head flow regulator or a shower head that aerates the flow. Both reduce the flow of water, cutting water and energy use.

  • If your hot water is solely or often heated using an immersion heater and you don't already have a timer on it, it's a good idea to fit one.

    This is particularly useful if you have an Economy 7 supply, as it means you will be able to heat your water using overnight cheap electricity.

  • Fix dripping taps. If you don't feel confident putting new washers in your taps, you could ask your plumber to do it.

  • Buy a shower timer to check how long you're really spending in the shower.

  • Make sure your shower is in good condition, including the shower head, the hose and the wall mounting.

    Replace parts if necessary and de-scale your shower head regularly if you live in a hard water area, otherwise scale build up will affect the water flow.

Top tips to save on your water heating, moderate to high investment

  • If your hot water cistern is very old, consider replacing it with a modern insulated one.

    Water can stay warm for up to two days and they perform much better even compared to an old-fashioned tank with a jacket on it.

  • If you're thinking of installing a solar hot water system in the future, it may be worth installing the larger-sized tank with the required two heat inputs - one for the central heating and one for the solar panel.

  • If your boiler is over fifteen years old, it could be time to replace it. A modern combi-boiler, which provides instant hot water, is generally considered to be more energy efficient than a 'stored' hot water system.

  • You could install solar water heating (SWH), also known as solar hot water (SHW).

    This is an expensive measure, but the introduction of the Boiler Upgrade Scheme by the government could provide you with a grant to bring the installation costs down.

How does solar water heating work and should I install it?

Solar water heating works by circulating a liquid through a panel on an approximately south facing roof, or occasionally a wall or some kind of ground-mounted system. There are two types of panel -- a 'flat panel' and the more expensive, but more efficient, 'evacuated tube'.

The liquid is warmed by sunlight — even in cloudy weather — and this liquid is circulated through the hot water cistern, transferring its warmth to the water there. This type of cistern works by harnessing what heat is available from the sun before ‘topping up’ using your existing boiler. It should qualify for the Boiler Upgrade Scheme, which will help you bring down the initial expense of installing it.

Use your existing solar panels

If you already have a solar panel system to provide your home with free electricity, then a solar hot water system isn’t a practical add-on. If you have an existing hot water cylinder with electric immersion heater, however, then you can divert any excess energy your panels produce into heating your hot water using what’s known as a solar immersion heater.

It’s not the most efficient use of your electricity, and if you’re currently earning money from your panels by exporting that excess back to the grid, you may need to work out whether the lost income is outweighed by the reduced costs of heating your water (it’s estimated a solar immersion heater can provide the equivalent of eight-to-nine months of free hot water each year). Units can be bought for as little as £230, but you’ll need to employ the services of a qualified electrician to fit them. Expect to recoup the outlay in 2-5 years depending on how much you pay.

When’s the best time to replace my hot water system?

If your hot water cylinder is over ten years old, now might be a good time to replace it with a new, well-insulated model. The best can keep water warm for up to two days.

Most boilers are designed to last around fifteen years – if yours needs replacing, now is a good time to consider replacing it with an instant-on combi boiler.

Hot water: FAQs

How long should I heat my water for?

If you have a hot water cylinder, then hot water stays hot for hours if not days if it isn’t used. Invest in a timer to heat the water only when it’s most frequently used – typically in the morning and evening when people are most likely to take baths or showers, and you need to wash up.

Could I leave my hot water on permanently?

This is wasteful, given that the water will stay hot for hours if it’s not being used.

How long does water stay hot in a tank?

This depends on the type and age of your cylinder or cistern, and how well-insulated it is. Older models won’t hold heat for more than a day at best, while newer models can hold it for up to 48 hours.

Do I need a combi boiler when I only need hot water?

A combi boiler is used both to heat water and your home. They’re energy-efficient because although they’re on all the time, they only produce hot water when it’s needed, thus helping to reduce your bills. They’re also perfect for smaller homes, as you no longer need to find space for a separate hot water tank.