Changing to Economy 7 meter for battery use

ejoyner

We are installing an air-source heat pump, battery and solar panels in stages, the first stage being the heat pump and removing our gas boiler/connection.

We would like to install the battery at the same time, and change to Economy 7 to take advantage of the cheaper rates (to charge the battery for use each day). When we have the solar installed we will use that to power the battery.

Has anyone done this, and can anyone advise as to which companies will allow us to do that, please?

ispookie666

Most big companies should offer Economy 7.  I changed my old E7 meter to a smart meter and have kept the Economy 7 tariff.  I had my batteries and solar installed as a single project but due to supply issues there was a gap of 5 months before the batteries were installed.  You will need to notify DNO(likely your installer) for any home battery storage.  I would probably stick with the same company and get them to do the battery and solar installation as a single project staggered to save on VAT 20%.  

I'm planning to charge my 16kw battery using night tariff and use during the day when the solar panels are not generating much.  Even factoring in round trip loss load shifting will be economical.  

Swipe

Will your battery have enough capacity to power your heat pump during the day and evening?

QrizB

Welcome to the forum.

Economy 7 is a strange choice for use with a heat pump, unless you have a truly huge battery. It could easily cost you more than standard-rate electricity would.

mmmmikey

Hi - building on the above post, you need to be aware that the heat pump itself uses a fair bit of electricity to run. You need to establish (a) how much the heat pump uses, and (b) when it uses that electricity. From that, you can work out the size of battery needed and see if that is a viable option for powering the heat pump. If it is, then great, but I would be surprised if that was the case. If the battery isn't big enough to run the heat pump and the heat pump runs in the day then you may find the extra cost of running the heat pump at higher rates during the day will outweigh the saving fom economy 7. Hope this makes sense

ejoyner

Thanks everyone, helpful responses.

@ispookie666 I'm not sure I understand your point about VAT?

@QrizB and @mmmmikey good point and I'd wondered that too. So you think a battery charged overnight would not be sufficient for a day of heat pump and other household power use? I wonder how I could work that out...

ispookie666

@ejoyner standalone battery installs or installs at a later date will attract VAT @20% but when installed at the same time of Solar will be @ 0%.  I do not know if this would be applicable if batteries are installed at the same time as ASHP.  

You would be normally using majority of your heating in the 3 months, for 12000KWh per year of heat requirement, this would come to just over 100KWh of heating per day.  Even with the best COP you are looking at 25KWh battery storage just for the heat pump.  
Might be easier to get a Nissan leaf and plug it into your house 

ejoyner

Thanks @ispookie666 that's extremely helpful. 20% is quite a chunk!

I hadn't quite twigged that a Tesla battery or similar is nowhere near enough to get you through a day, it would basically only be enough for an evening or so.

It's also surprisingly difficult to work out how many kWh an ASHP would actually use in winter months.

QrizB

ejoyner said:

It's also surprisingly difficult to work out how many kWh an ASHP would actually use in winter months.

Look at your gas bill and see how many kWh of gas you use per month in the winter. Divide that by the number of days in the billing period, and then by three, and that's a rough guide for how much electricity your heat pump will need.

And then remember last winter (2021-22) was a very mild one for most people, so you might want to check the gas bill for the year before (2020-21).

Looking at my records, my household (2 adults 2 teens, 3-bed semi, central south of England) used ~1800kWh in January 2021. That's an average of 60kWh/day, so an ASHP would need something like 20kWh/day of electricity on average (some days more). If you wanted to feed it largely from a battery you're looking at eg. two Tesla Powerwalls or seven Pylontech US3000 units.

(My total solar generation in January 2021 was less than 70kWh and didn't touch the sides of our domestic loads, without thinking about heating on top.)

ejoyner

Thanks @QrizB that's helpful. That all tallies with my calculations which is reassuring. Presumably you have a high bill over the winter months, but then pay essentially nothing for the rest of the year?

QrizB

ejoyner said:

Thanks @QrizB that's helpful. That all tallies with my calculations which is reassuring. Presumably you have a high bill over the winter months, but then pay essentially nothing for the rest of the year?

Yes, my winter months are expensive (particularly December and January, where the sun is at its lowest) but it drops off quickly in the spring.

If you're interested there's a thread here where various PVers keep track of these things:

https://forums.moneysavingexpert.com/discussion/6260671/how-low-can-you-go-daily-weekly-monthly-whatever-electricity-grid-import/p1