Economy 7 and Solar PV

Solar Consultant Jon Cowdrill explains the Economy 7 electricity tariff

Economy 7: the basics

Economy 7 is a special electricity tariff for people that use electricity to heat their homes and hot water. They get two tariffs, one at a cheaper night rate and a more expensive day rate. The night rate normally stretches from about midnight to 6 or 7 in the morning. This allows you to top up electric storage heaters and a water cylinder over night at the cheaper rate. The storage heaters then slowly release the heat across the day keeping your house warm and if you have a well-insulated hot water cylinder, the water will stay hot all day ready for whenever you need it. The night rate tends to be 2-3 times cheaper than the day rate. I checked Good Energy and EDFs Economy 7 prices:

Good Energy EDF
£65 a year standing charge £69 a year standing charge
15.87p per kWh day rate 17.28p per kWh day rate
8.15p per kWh night rate 6.31p per kWh night rate

It is also a fairly green way of heating your house. Electricity generated over night is less carbon intensive than daytime electricity. This is due to the low night time demand for electricity coupled with the fact that some low carbon generators like wind and nuclear carry on generating through the night, regardless of how much energy is actually required by the national grid. Electric storage heaters are a low tech way of storing excess night time energy for daytime use. If you have no access to the gas grid, economy 7 is a reasonable way of managing your energy.

There are down sides, for example you don’t really know what the temperature will be the following day. This makes it hard to judge how much energy to divert into the storage heater the night before. A lot of storage heaters don’t even have controls to allow you to vary the input even if you did know. Lastly, some properties may start to get cold in the evening and expensive top ups may be required.

What if you have Solar PV as well as economy 7?

It’s good news, solar generates electricity in the day time when electricity is most expensive leading to greater savings on your bill. Your daytime electricity use will be offset by the solar generation to varying degrees depending on your normal day time electricity use.

Should I change my storage heaters to come on in the day time?

The simple answer is no. Solar PV is not the correct technology for heating your home. This is because solar PV will generate most from spring through to autumn, precisely when you need the least space heating. If you are on Economy 7 you should continue to use the cheaper night time rate for your storage heaters. Also heating is very energy intensive compared to say, running your washing machines, lighting, fridges and gadgets (even electric cars)! A typical storage heater burns about 2 – 4 kW so if you’ve got a few of them, there is no way the solar PV would cover all that consumption. Solar will make a saving but not enough to compensate you for using the more expensive day time electricity rate, unless you have an Immersun device (see below).

What about water heating?

This is a little more complicated, as you’ll need hot water in the summer months when you re generating a lot of energy as well as the winter. Depending on the rating of your immersion, solar PV could contribute most, if not all of the demand on a sunny day. However in general solar PV will not be enough to power an immersion coil. On a cloudy day for example your solar PV system would be kicking out between 0.5 – 1.5 kW where as your immersion would be gobbling up 3  kW. So in conclusion, unless you are paying very close attention to your systems generation, it’s not worth altering the way you heat your water. Fortunately there are bits of kit available which will do all this monitoring for you, and adjust the amount fed to the immersion. These devices really do make sense financially if you are using economy 7. Even on a cloudy day solar PV will generate a reasonable amount of power and will keep it up for some time, meaning that there is quite a bit of energy to play with.

What about my other electricity consumption?

Yes, it makes sense to use other appliances when you are generating the most, unless you were planning on using the cheaper night time energy for it. I’m guessing you won’t be waiting until midnight to run your washing machine, charge your gadgets, watch TV or make yourself a cup of tea. On a cloudy day, you may decide to run appliances consecutively rather than at the same time in order to use the most solar energy. But after the fun and novelty of having a Solar PV system has worn off (that’s if it ever can!) you probably won’t want to be concerned with timing your electricity usage to perfectly balance your generation. We can now install smart plugs to charge or turn devices on when you have surplus solar PV generation.

Lastly, things to watch out for

Generally, studies have shown that having a form of renewable energy not only displaces energy that is consumed from the national grid, it also helps to promote energy efficiency by making people more aware of the energy they generate and consume. Potentially though, there is a danger that the opposite could happen and people could think, ‘It’s sunny and I’m generating free energy, I can be less concerned about how I use energy in my home’. If you are using energy intensive appliances (particularly ones which are for space and water heating) an almost imperceptible change in behaviour could lead to increases in consumption over all. We find that generally people are quite interested in the amount of energy they generate and are constantly looking at their inverter to see how their system is doing. If the inverter is tucked away in the loft, it makes sense to get a display you can access more easily or get online monitoring. Whether you have solar or PV or not it also makes sense to monitor your consumption and know what appliances are the energy hungry trouble makers.

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