How I use off peak energy tariffs to make my heat pump running costs cheaper

With air source heat pumps fast becoming the go to technology to replace many boilers, especially in new homes where legislation is making it hard not to fit one, understanding how to make your investment more cost-effective to run is a key requirement for many homeowners.

I love heat pumps. They are amazing bits of technology but, as with all tech, they have very specific operating conditions and design parameters that mean it's not simply a case of switching to an energy saving tariff and hoping for the best.

In theory, the way heat pumps work make them less than ideal for night time based off peak energy saving tariffs. But, with the right knowledge there are ways of making them more suitable and here's how.

Pros and cons of using off peak tariffs with heat pumps

Heat pumps come in very many different shapes and sizes and also have a variety of different refrigerants that are optimised for different environmental conditions, and working temperatures. They also run on electricity, whereas boilers run on oil or gas.

Why does this matter? Gas can punch heat into a property much faster than a heat pump. Heat pumps operate on a a ‘low and slow’ basis, but boilers operate ‘hot and a lot’. This makes a very big difference when it comes to setting up your heating system.

Electricity at the moment is not cheap, so if you've also made the switch to an electric vehicle, you may have switched to a tariff that is cheaper for a set period at night time, but slightly more expensive during the day.

While this is beneficial for electric vehicles because they can charge at around 25 miles for every hour they are plugged in, giving you up to 150 miles of charge during the cheaper tariff period, because heat pumps operate on a ‘low and slow’ basis, they ideally want to run all day – not just at night.

If a heat pump only runs during off peak times, then it is effectively trying to heat the water as hot as it can during this time (a period when it is less likely to be needed), which will ultimately result in lower efficiency.

And, because it is running hard, it will also be at its noisiest at exactly the wrong time. Although heat pumps being noisy is perhaps one of the most common heat pump myths. A well designed air source heat pump positioned in the right place shouldn't really bother anyone.

While the lower tariff may justify the lower efficiency, the heat pump will still need to run during the day at the higher tariff which can add extra cost if it is not carefully controlled.

But, there are ways of making the most out of your air source heat pump and lower tariffs and here's how.

How to optimise off peak tariffs and heat pumps for central heating

As I have an electric vehicle, I've opted for an off peak tariff. Currently the tariff is around 9p per kWh from 12.30am - 5.30am in the morning. The rest of the day it is around 22p per kWh. When the heat pump runs at night, the efficiency may drop to around 2.25 which means a kW of heat costs around 2p.

Through the day, the efficiency becomes higher, but so is the cost, meaning that each kW of heat in the day is around 7p which is 3.5 times more expensive.

In order to optimise on the lower costs per unit, during the off peak time I set the schedule on the heating to lift the temperature in my home by three degrees, but let it hold at the lower normal temperature during the day. Thermal mass (the ability of a building to hold heat) does the rest in maintaining the temperature it's achieved overnight.

Solar panels can further reduce costs

Another way of reducing the amount of higher rate electricity I use is via the solar photovoltaic panels I have on the roof. These do supply some energy to the heat pump in the day, although the lack of winter sunshine can mean this is limited.

With a solar battery (something I haven't yet installed), this saving would be even bigger as any electricity stored would also help power the heat pump during the peak periods.

How I manage off peak tariffs, heat pumps and my hot water supply

How I make my hot water cost effective

Hot water preparation in my home is handled differently. When I first renovated my house, I had a micro heat pump. This is a very small heat pump that is built into the top of a water cylinder. It has a different refrigerant that is better at producing hot water, but it needs higher ambient temperatures.

Stale air from the home is drawn through the heat pump and the heat is extracted from the stale air and transferred to the cylinder. The unit only uses 600W to run and produces 1.8kW of heat.

Unfortunately mine did not last very long, five years to be exact, so while the idea is great, the cost (both financial and carbon) was not justified. Perhaps a better quality one would have produced a better result.

I have since replaced the micro heat pump with a standard heat pump cylinder that is connected to the outside fan unit. I have however set the schedule to run during off peak times only, and I use the immersion to lift the temperature to over 60°C each night.

Excellent insulation on the cylinder and insulated pipes means the cylinder retains this heat and it only needs to be topped up once a day during the off-peak tariff period.

Still in the early stages of deciding how to heat your self-build project? Find out more about other methods of heating your home in our guides to ground source heat pumps and just how much it costs to install a heat pump.