Can a Heat Pump Water Heater Work with an Off-Peak Electricity Tariff?

Can a Heat Pump Water Heater Work with an Off-Peak Electricity Tariff? The short answer is yes – a heat pump water heater can work really well with an off-peak electricity tariff. You can cut down the cost of producing hot water without having to cut back on the amount you use. The cylinder basically stores the energy as hot water, ready to be used later when prices are higher – when you need it.

How does off-peak hot water heating work?

A heat pump water heater uses electricity to power the compressor, fan and controls but extracts even more heat from the surrounding air. Because it’s not just using electrical resistance to produce heat, it can supply loads more useful heat than the amount of electricity it uses.

For example, with a Coefficient of Performance (COP) of 3, every 1 kWh of electricity used could give you around 3 kWh of useful heat. And if that electricity is bought during a cheaper off-peak tariff period, you get the best of both worlds – the efficiency of the heat pump plus the cheaper electricity.

Our product range starts with the 100 litre wall mounted air source heat pump hot water heater, intended for applications with lower stored hot water requirements.

For larger domestic applications, choose between the 200 litre air source heat pump hot water system and 300 litre air source heat pump hot water system.

For larger residential and commercial applications, our high-capacity range includes the 400 litre air source heat pump hot water system and 500 litre air source heat pump hot water system.

Can the cylinder get heated overnight?

Absolutely it can! This is especially useful when you have a tariff that offers cheaper electricity for a few hours overnight. The heat pump can be programmed to heat the cylinder during that cheaper period so you’ve got a full tank of hot water in the morning. A good, well-insulated hot water cylinder will then store that heat until you need it next.

There will be some heat loss overnight but if you’ve got a modern well-insulated cylinder, it’ll retain most of its stored heat. Just make sure the cylinder’s got enough capacity to cover your household’s hot water needs.

What about time-of-use electricity tariffs?

New time-of-use tariffs mean you get different electricity prices at different times of the day. So instead of just having one fixed cheap period overnight, you’ve got even more opportunities to run your heat pump water heater without breaking the bank. For instance, the controls can just heat the water overnight when it’s cheap and then do another heating period in the afternoon if the electricity price is low again.

Some electricity tariffs can change from day to day so more advanced controls or a home energy management system can be really useful if you want the heat pump to automatically respond to changing prices.

Does the heat pump need special controls?

You don’t necessarily need anything fancy. Many heat pump water heaters already have built-in timers and scheduling controls. As long as the off-peak period is fairly predictable, you can probably just programme the normal hot water heating schedule around it.

If you want to get more advanced, you might be able to use external control inputs, smart controls or integrate it with a home energy management system. Just make sure the controls you choose are compatible with the heat pump manufacturer’s requirements – you don’t want to have to keep turning the thing on and off all the time.

Is it better than heating with an immersion heater off-peak?

A cheap tariff will definitely cut down the cost of running an immersion heater, but the heat pump still has a big efficiency advantage. For example, an immersion heater might need 6 kWh of electricity to get 6 kWh of heat, while a heat pump with a COP of 3 would only need about 2 kWh to deliver the same amount of heat.

If electricity prices are the same off-peak, the heat pump will use less electricity overall.

Do you need a larger hot water cylinder?

Possibly – if you want to do most of your hot water heating during a limited low-cost period, the cylinder needs to store enough hot water to last until the next heating period. A household with a lot of showers or baths may benefit from a bigger hot water storage capacity than one where the heat pump’s allowed to recover the cylinder throughout the day.

You need to balance storage size against space, cost and heat loss though. Don’t just go for the biggest cylinder you can find; calculate your household hot water demand first.

What if you also have solar PV?

Off-peak tariffs and solar PV really work well together. The heat pump can heat the cylinder overnight when it’s cheap and then use any surplus solar electricity during the day to top it up. The controls should be set up so that if the heat pump’s got plenty of solar to work with, it doesn’t heat up the cylinder again.

This basically turns the hot water cylinder into a useful way of storing thermal energy – storing energy when electricity is cheap or readily available and using it later as hot water.

Is an off-peak tariff worth using with a heat pump water heater? It Can Definitely Make a Difference

A heat pump water heater is already a pretty efficient choice compared to the standard direct electric water heater, but by scheduling the usage during those off-peak hours when the electricity is cheaper, you can cut down on your running costs even further.

The outcome will largely depend on a few key factors – specifically how cheap the off-peak electricity is, what hours that is available to you, the size of your water cylinder, how much heat is just lost, the efficiency of the heat pump (its COP), how much hot water you need and what kind of controls you have on the heat pump.

Rather than just letting the heat pump run each time the cylinder temperature drops, you can really get the most out of it by planning out a smart heating schedule that lets you take advantage of the cheaper electricity but still ensures you’ve got enough hot water when your household actually needs it.