Economy 7 of peak home battery.

[Caroline1979]

Hello, apologies if this isn’t the correct place to ask, however….. 

 

I have economy 7 and was wondering if there is such a thing as being able to buy a battery to charge overnight and then use the stored energy during the day? 
 

I assume there is as solar panels store energy in a battery which is then used during the day but thought you guys can give me a better answer than just my assumptions. 
 

My economy 7 tariff is roughly four times less than my on peak charges so to me it makes sense to utilise the cheaper rate if I can. 
 

I'm also assuming it’s not illegal and if it can be done then an electrician can do it for me? 
 

thanks in advance for your help 

[SteamyTea]

Just now, Caroline1979 said:

I have economy 7 and was wondering if there is such a thing as being able to buy a battery to charge overnight and then use the stored energy during the day? 

Yes it can be done.  Would involve a battery, charger and an inverter, and something to stop the battery power going back out the building.

None of it is going to be cheap.

 

Do you know how much electricity you use during the two time periods (can read that off the meter).

You may find that just a bit of careful management can shift some of the load to the cheaper period.

 

3 minutes ago, Caroline1979 said:

My economy 7 tariff is roughly four times less than my on peak charges

Wish mine was, hardly half the price down here.  Who are you with, Octopus?

You have to be careful that they don't pull a very good tariff off the market after you have invested.

[Caroline1979]

1 hour ago, SteamyTea said:

Yes it can be done.  Would involve a battery, charger and an inverter, and something to stop the battery power going back out the building.

None of it is going to be cheap.

 

Do you know how much electricity you use during the two time periods (can read that off the meter).

You may find that just a bit of careful management can shift some of the load to the cheaper period.

 

Wish mine was, hardly half the price down here.  Who are you with, Octopus?

You have to be careful that they don't pull a very good tariff off the market after you have invested.

Thank you, glad it can be done and it’s not just a stupid idea of mine! I’ve looked online for storage batteries but they all say for solar panels! 
 

The majority of my usage is during the day, my son works from home and I’ve got a lizard that needs 12 hours of heat and light ( will soon be moving some of his hours to get the night rate) I do my washing during the night now.
 

I’m with e.on - they’ve always been the best for me but now you’ve got me thinking what happens if they remove the economy 7 tariff! It’s a minefield  

[SteamyTea]

41 minutes ago, Caroline1979 said:

do my washing during the night now

I always do mine at night, my washer has a delay timer on it. I lune dry. Never use the tumble dryer.

Not a huge saving, maybe 30p a wash.

Don't think a lizard is going to take much, unless it is over lit (assuming IR heater here). Lizards were like vermin where I grew up. 

Working from home need not increase usage much. I am at home during the day, I work the late shift. Laptop is permanent on, kettle on a fair bit, usually cook lunch. Use about 1.3 kWh/day. 

I lit the the E7 window to between 3 and 4 hours a night. No need to charge the DHW and storage heaters up before they are needed (storage heaters close a flap when charging, then release it when not charging, so some savings to be made there).

My ratio of day to night is approximately 1:4.

My biggest saving is having almost nothing drawing power when it is not needed. Easier to do than many think.

Edited November 30, 2022 by SteamyTea

[Caroline1979]

1 hour ago, SteamyTea said:

I always do mine at night, my washer has a delay timer on it. I lune dry. Never use the tumble dryer.

Not a huge saving, maybe 30p a wash.

Don't think a lizard is going to take much, unless it is over lit (assuming IR heater here). Lizards were like vermin where I grew up. 

Working from home need not increase usage much. I am at home during the day, I work the late shift. Laptop is permanent on, kettle on a fair bit, usually cook lunch. Use about 1.3 kWh/day. 

I lit the the E7 window to between 3 and 4 hours a night. No need to charge the DHW and storage heaters up before they are needed (storage heaters close a flap when charging, then release it when not charging, so some savings to be made there).

My ratio of day to night is approximately 1:4.

My biggest saving is having almost nothing drawing power when it is not needed. Easier to do than many think.

I’ve just looked at my daily usage and my electric is around 7kw/h a day, I’m very conscious of turning plugs off when not using, use air fryer or slow cooker more than the oven - I’m jealous of your usage! 
 

[SteamyTea]

23 minutes ago, Caroline1979 said:

7kw/h a day

Two things.

Is that a total of 7 kWh a day, or just the 17 of day rate. You can work the long term average out from the meter, it will show t, for total, 1 for day usage and 2 for night usage.

 

Second thing is it is kWh, not kw/h, that is dividing power, which is energy per unit time, by time and will give misleading numbers.

The unit of energy is actually actually the joule (J, named after the man Joule).

Power is the watt (W, named after the man Watt).

Power, multiplied by time, is energy.

There are 3,600,000 J in a kWh.

k is 1000

W is J/s, where s is time in seconds

h is hour (3,600 seconds).

Multiply them all together and you get:

1000 x 1 (W) x 3,600 (hour)

1 kW x 1 h 

1 kWh

 

 

 

 

[Caroline1979]

55 minutes ago, SteamyTea said:

Two things.

Is that a total of 7 kWh a day, or just the 17 of day rate. You can work the long term average out from the meter, it will show t, for total, 1 for day usage and 2 for night usage.

 

Second thing is it is kWh, not kw/h, that is dividing power, which is energy per unit time, by time and will give misleading numbers.

The unit of energy is actually actually the joule (J, named after the man Joule).

Power is the watt (W, named after the man Watt).

Power, multiplied by time, is energy.

There are 3,600,000 J in a kWh.

k is 1000

W is J/s, where s is time in seconds

h is hour (3,600 seconds).

Multiply them all together and you get:

1000 x 1 (W) x 3,600 (hour)

1 kW x 1 h 

1 kWh

 

 

 

 

I looked on my latest bill and the estimated usage is day rate 2088.6 kWh and night rate 280.9 kWh adds them both up and divided by 365 so sorry it should be 7kWh (roughly) 

[SteamyTea]

So roughly 75% on the night rate, that is actually pretty good.

7 kWh a day is pretty good as well.

So say you use 2 kWh during the day.  That is about £1 to £1.30 a day.

A battery system would have to be around 5 kWh to give it a decent longevity, and be able to reliably power a decent size inverter, around 6 kW so it could cope with kettle, oven and a cooking ring (hopefully you have an induction hob as they are just better all round).

Now I would think you would be looking at around £8,000 to have something like that installed, and it should last 10 years, so £800/year, plus the running costs i.e. the night rate electricity.

 

If your night rate is around 15p/kWh, and you can get 80% efficiency on the charge/discharge cycle, that is 54p/day, £200/year.

Not looking good.

But you will save on the high daytime rate, that is between £365 and £475 a year, a real saving of between £165 and £275.

Not looking good at all as it cost approximately £800 a year, a you are in the red somewhere between £525 and £635.

 

You could probably save more by showering for 1 minute less each day, or 1 inch (25.4mm) less bathwater.

[Dave C]

This is exactly what I do. But I paid <£5K for my 7kWh battery system, and pay a lot less than 15p on the night rates, so I make my savings out to be £700-750/year ... which is perfectly acceptable given the <£500/year cost of the system.

 

Personally I didn't think a larger inverter was worthwhile - it turns out that in practice a 3.6kW inverter covers the vast majority of my usage, which is fine, the goal was never "must run house entirely on batteries absolutely 100% of the time" ... and keeping to 3.6kW makes the inverter cheaper and sidesteps the more complicated process to get permission to install it.

 

Battery prices have gone up since I bought my system at the start of the year, mind. Certainly true that you want to work out how much it'll cost you and how much you'll save before making a decision. And yes, it's always possible the night rates you'll get in future years won't be as generous as they are now - do you know what you'd do if that happened?

 

56 minutes ago, SteamyTea said:

So roughly 75% on the night rate, that is actually pretty good.

 

Isn't 2088 kWh day rate and 280 kWh night rate nearly 90% on the day rate?

[SteamyTea]

11 minutes ago, Dave C said:

Isn't 2088 kWh day rate and 280 kWh night rate nearly 90% on the day rate

Got to add the two together to get the total, then dived into the day or night.

 

Edited November 30, 2022 by SteamyTea

[Caroline1979]

2 hours ago, SteamyTea said:

So roughly 75% on the night rate, that is actually pretty good.

7 kWh a day is pretty good as well.

So say you use 2 kWh during the day.  That is about £1 to £1.30 a day.

A battery system would have to be around 5 kWh to give it a decent longevity, and be able to reliably power a decent size inverter, around 6 kW so it could cope with kettle, oven and a cooking ring (hopefully you have an induction hob as they are just better all round).

Now I would think you would be looking at around £8,000 to have something like that installed, and it should last 10 years, so £800/year, plus the running costs i.e. the night rate electricity.

 

If your night rate is around 15p/kWh, and you can get 80% efficiency on the charge/discharge cycle, that is 54p/day, £200/year.

Not looking good.

But you will save on the high daytime rate, that is between £365 and £475 a year, a real saving of between £165 and £275.

Not looking good at all as it cost approximately £800 a year, a you are in the red somewhere between £525 and £635.

 

You could probably save more by showering for 1 minute less each day, or 1 inch (25.4mm) less bathwater.

Thank you so much for the detail! Based on these maths it doesn’t really seem worth investing, but based on Dave c’s response it does - it’s all very confusing! 

[Caroline1979]

1 hour ago, Dave C said:

This is exactly what I do. But I paid <£5K for my 7kWh battery system, and pay a lot less than 15p on the night rates, so I make my savings out to be £700-750/year ... which is perfectly acceptable given the <£500/year cost of the system.

 

Personally I didn't think a larger inverter was worthwhile - it turns out that in practice a 3.6kW inverter covers the vast majority of my usage, which is fine, the goal was never "must run house entirely on batteries absolutely 100% of the time" ... and keeping to 3.6kW makes the inverter cheaper and sidesteps the more complicated process to get permission to install it.

 

Battery prices have gone up since I bought my system at the start of the year, mind. Certainly true that you want to work out how much it'll cost you and how much you'll save before making a decision. And yes, it's always possible the night rates you'll get in future years won't be as generous as they are now - do you know what you'd do if that happened?

 

 

Isn't 2088 kWh day rate and 280 kWh night rate nearly 90% on the day rate?

Thank you for the advice, I think I need a good run at doing the maths to see if it’s worth it based on your savings, I understand everyone’s is different, it would be worth investing and pay for itself within a couple of years. 
 

If they phased out economy 7 then it wouldn’t be worth it and I genuinely have no idea what I’d do apart from eat cold food and live in the dark - the son and lizard would have to go 😂

[ed g]

A bit late to this topic. I have an ASHP and have just got EDF to apply their E7 tariff to my account. (I have a new Smets2 meter so it was just a case of applying the tariff, no wiring or meter changes needed). I'm in the Eastern region so the difference is big: 7p /kWh at night (11pm to 7am, so actually 8 hours), 54.5p /kWh in the day. I'm lucky enough to have wet underfloor heating and a well insulated house, so it seems I can heat at night and last nicely to the following evening, combined with programming DHW and D/W, W/M and dryer running to nighttime I'm at around 75% off peak.

 

Preamble over, I do still have 5-7kwh per day daytime usage as a minimum that I want to install a battery to benefit from the low rate. I'm looking at a Fox ESS option that comes with 2.6kwh scalable battery units. 3 of these would have usable capacity of 6.9kwh. Including their battery management system and an AC (NOT a hybrid/PV) inverter this looks to be £4,300k for the kit incl VAT. I know if I add *some* PV it would be zero-vat rated, (£3,600) but not sure the minimum approach, as the eco and cost benefit comes from using, say, wasted nighttime off shore wind or nuclear, and reduce the need for peak hour gas-based electricity.

 

6.9kwh per day is 2,518 kWh per year. Each kWh has a saving 47.5p/kwh, or £1,196 a year. Accepting energy and capacity losses I'd hope for £1,100 to be achieved. So a 3-5 year payback depending on finding an electrician to install and the VAT answer).

 

This is entirely hypothetical at the moment - I'm not an electrician, and am yet to approach one, so interested on people's thoughts of the feasibility.

 

I would also be interested to hear of any other options versus this:

- 3x Fox ESS HV2600, with brackets plus

- BMS plus

- AC1-5.0 A/C inverter.

The 5kW inverter feels generous, but the saving for a 3.0kW option seems small for the added limitation on running appliances (e.g. oven plus microwave plus kettle plus)

Edited December 22, 2022 by ed g
Typo *well not *week

[ProDave]

25 minutes ago, ed g said:

A bit late to this topic. I have an ASHP and have just got EDF to apply their E7 tariff to my account. (I have a new Smets2 meter so it was just a case of applying the tariff, no wiring or meter changes needed). I'm in the Eastern region so the difference is big: 7p /kWh at night (11pm to 7am, so actually 8 hours), 54.5p /kWh in the day. 

 

Preamble over, I do still have 5-7kwh per day daytime usage as a minimum that I want to install a battery to benefit from the low rate.

 

6.9kwh per day is 2,518 kWh per year. Each kWh has a saving 47.5p/kwh, or £1,196 a year. Accepting energy and capacity losses I'd hope for £1,100 to be achieved. So a 3-5 year payback depending on finding an electrician to install and the VAT answer).

 

So you have chosen a tariff with very cheap off peak and very expensive peak to get cheaper heating?  but then face the challenge of the extortionate day rate.

 

You plan to solve that by installing a battery system to shift all your day usage to off peak.

 

It is no good calculating your "saving" by basing it on your now very high day rate.  You really want to cost your electricity on your old rate, probably about 30p compared to shifting all if it to the 7p off peak rate, so a saving of about 23p per kWh not 47.5

 

 

[ed g]

Thanks, ProDave. I've just moved to Economy 7 as the maths made a lot of sense even with the "extortionate" day rate. Before, I was paying 35p/kWh for every kWH under the EPG; right now, I pay 7p/kWh for 75% of my usage and 54.5p/kWh for the rest, averaging about 18p/kWh. Now admittedly that's skewed in my favour as so much of my usage is heating at this time of year (which I can do at night), but crunching the detailed stats for the whole year, I expect 63% of my usage to be off peak and my weighted average cost to be around 25p/kWh saving me around £800 in the year (from £2,800 for 8,000 kWh to £2,000, of which £350 is for 5,000kwh night units and £1,650 for the 3,000kwh day units): 

 

The thing about EDF's particularly skewed day/night rates is that it then makes it worthwhile trying to use a battery to shift a large predictable, and regular, chunk of the 3,000kwh at day rate to the night rates... 6.9kwh per day would be around 2,500 of them. Under my plan, I'd then have 7,500kwh @7p and 500kwh @54.5p total £800 annually for 8,000kwh, with weighted averaged cost of 10p/kWh... Does that stack up to you?

Edited December 22, 2022 by ed g
Typos

[Nickfromwales]

Is everyone factoring in the ( replacement ) cost over useful lifespan of the battery system?

[Alshamal]

I had my 9.6 kWh Lux 3600 AC Battery System fitted 13/10/22 I’m on Octopus Go with an EV the system comes with 10 years guarantee extendable to 20 years for an extra £450.00 not sure I would benefit from this as all my future is behind me at 75👨‍🦳 But so far so good saving approximately £2.46p per day!

[ed g]

Hi Alshamal, thanks for the reply. I've got a bunch of questions of you don't mind...

 

Does the Lux 3600 provide the easy option to charge itself at night and then to be the primary power source until empty at user selected times in the day? Does it also offer the option to automatically step in if there's a power cut?

 

Is the Lux just the inverter, and were there options for the battery modules?

 

Did you consider any other options for the battery? Or for the inverter?

 

What was the approximate cost? Was that including installation?

 

THANK YOU!

[ed g]

On 30/11/2022 at 20:08, Dave C said:

This is exactly what I do. But I paid <£5K for my 7kWh battery system, and pay a lot less than 15p on the night rates, so I make my savings out to be £700-750/year ... which is perfectly acceptable given the <£500/year cost of the system.

 

Personally I didn't think a larger inverter was worthwhile - it turns out that in practice a 3.6kW inverter covers the vast majority of my usage, which is fine, the goal was never "must run house entirely on batteries absolutely 100% of the time" ... and keeping to 3.6kW makes the inverter cheaper and sidesteps the more complicated process to get permission to install it.

 

Battery prices have gone up since I bought my system at the start of the year, mind. Certainly true that you want to work out how much it'll cost you and how much you'll save before making a decision. And yes, it's always possible the night rates you'll get in future years won't be as generous as they are now - do you know what you'd do if that happened?

 

 

Isn't 2088 kWh day rate and 280 kWh night rate nearly 90% on the day rate?

Hi Dave C, what was the model of inverter and battery you had installed? Is your system hybrid or just for making the most of an E7 tariff? Thanks

[billt]

9 hours ago, Nickfromwales said:

Is everyone factoring in the ( replacement ) cost over useful lifespan of the battery system?

 

No. Mine's a hobby project so return on investment isn't a factor. The initial system might have paid for itself in 35 years. I've in the process of testing another 32 cells to increase the capacity to 68kWh, but the repayment time for the whole system has dropped to about 11 years with the increase in electricty prices.

 

In theory the batteries should last a long time. Typical cycle life is claimed to be 3,500 to 6,000, or 10-16 years if cycled fully every day. If the system is reasonably sized they shouldn't be cycled fully very often so the cycle life should be longer. Aging is a different and unknown factor.

[Nickfromwales]

48 minutes ago, billt said:

 

No. Mine's a hobby project so return on investment isn't a factor. The initial system might have paid for itself in 35 years. I've in the process of testing another 32 cells to increase the capacity to 68kWh, but the repayment time for the whole system has dropped to about 11 years with the increase in electricty prices.

 

In theory the batteries should last a long time. Typical cycle life is claimed to be 3,500 to 6,000, or 10-16 years if cycled fully every day. If the system is reasonably sized they shouldn't be cycled fully very often so the cycle life should be longer. Aging is a different and unknown factor.

Indeed, and close to my thoughts tbh. With those numbers I'd happily throw the money at it, not just to have self-sustainability but to also shove 2 fat fingers up, directly into the face of the grid.

I'm thinking longer term of complementing this type of capacity ( 48kWh of battery for the get-go ) with a water-cooled self-start genny which I can bury underground, running on LPG. The waste heat from running the genny ( inevitable in winter vs pulling from grid ) would be directed to a big TS and then used as a pre-heat to space heating. I'd retain the gas combi, but convert to LPG for the times where I need high grade heat. No matter who turns what off, or when, I will be warm and well lit. After a couple of years of measuring reliance, I would then look to yanking the meters and going fully off-grid.

[Dave C]

5 hours ago, ed g said:

Hi Dave C, what was the model of inverter and battery you had installed? Is your system hybrid or just for making the most of an E7 tariff? Thanks

 

Also a Lux, with 7.2kWh of Pylontech batteries. No solar - just making use of Octopus Go off-peak cheap rates (although if off peak rates become less favourable in the future, installing solar is a possible next step).

 

5 hours ago, ed g said:

Does the Lux 3600 provide the easy option to charge itself at night and then to be the primary power source until empty at user selected times in the day? Does it also offer the option to automatically step in if there's a power cut?

 

Since I also have a Lux...

-Yes, you can set multiple charging timeslots, although I only use one of them (00:30-04:30 Octopus Go). Well, except on the one day per month that Octopus gives me an extra hour of free electricity then I use the second timeslot for that day.

-It then defaults to being primary power source until empty (or until a certain charge level, so you can choose how much power to always hold back for a possible power cut)

-The default offering is that you get a double plug socket hanging off the inverter that's continually powered including during a power cut - it doesn't feed the whole house. When I had mine installed they did offer the whole house switchover at additional cost, but I didn't bother to go for that (and I can't see it's on offer on the website now).

 

Cost wise, prices have increased in the 8 months since I had mine installed so you're best just checking the website! https://homeenergygroup.co.uk/lux-ac-battery-storage-greenlinx/

I was pleasantly surprised that the cost on their site was indeed exactly what I paid, no hidden extras. (Shouldn't really be surprising,

 

12 hours ago, Nickfromwales said:

Is everyone factoring in the ( replacement ) cost over useful lifespan of the battery system?

 

Yes, in my case - since I have the same system as Alshamal, my calculations were very similar The system and batteries have a 10 year warranty, and the expected cycle lifetime of the battery means they should still be usable at ~12 years. Since the entire system will have broken even at 7 years (hopefully!) that means a good few years of banking savings before the batteries start degrading to the point of needing replacement, or the inverter fails and incurs costs. In principle...

[Kelvin]

I’ve come to the same conclusion on the economics of battery storage but we are going ahead with it anyway mostly for the same reasons as above. We’ll have a private water supply and treatment plant so everything is powered. Ultimately I’d also like to be completely off-grid just on principle but it’s not so easy to achieve. 

[George]

British Gas reckon what they'd actually be charging (without the price cap) for electricity is 62p/kWh. 

[SteamyTea]

What Centrica are try to do down here.

 

https://www.centricabusinesssolutions.ie/knowledge-centre/ideas/matt-hastings-bbc-r4-battery-storage

 

(slight disclaimer, I had to spend 3 years working with Matt Hasting)