What sized battery?

[Gaz Bancroft]

I am currently preparing a specification to enable me to tender for a new build house. 

 

I am planning to have 14no. solar panels on a south facing roof and an air source heat pump to power ground floor and first floor underfloor heating. The house is 144sq.m and I have been advised that an air source heat pump of 9kW would be appropriate. Will be three people in the house. We do not currently have an electric car but will be likely to get one if I'm getting free power for it through the solar panels! 

 

My query is... what sized battery should I be specifying? 

 

My aim is not to be entirely self-sufficient but would like to find a balance where I can reduce energy requirements from the grid (and therefore reduce bills) without over specifying and paying for battery capacity I don't need. 

 

I am currently tempted to specify a 5kW storage capacity to begin with hoping that will suffice to power the house most of the time and then maybe add an additional 5kW if get an electric car.

 

Thank you  

 

 

[SteamyTea]

A lot more information would be needed.

First things first though, run your proposed PV system though PVGIS and see what kind of output you are likely to get, but remember that it is long term averages, not real time, fine grained data.

 

My view is that battery systems will actually cost you money, so better off seeing who will buy your excess PV generation.

[JohnMo]

1 hour ago, Gaz Bancroft said:

. The house is 144sq.m and I have been advised that an air source heat pump of 9kW

If that's the case you need to insulate and do more air tightness work. I'm at nearer to 200m2 and using a 6kW ashp and it's really is too big.

 

I would say a usable capacity of battery of around 15kWh. Then in winter you can fully charge on cheap rate and get most your days energy on cheap rate. I have 6.6kW of PV and most days in winter get very little electric from it.

[PNAmble]

2 hours ago, Gaz Bancroft said:

The house is 144sq.m and I have been advised that an air source heat pump of 9kW would be appropriate

 

Have you done heat loss calcs. I'm 207m2 and I'll be installing a 5kW ashp.    I'm currently looking at a 9.5kWh battery, with about 5.5kW PV,  I may in the future add an additional battery, not looking to export anything as I also have a EV.

[SteamyTea]

47 minutes ago, JohnMo said:

I would say a usable capacity of battery of around 15kWh. Then in winter you can fully charge on cheap rate and get most your days energy on cheap rate

So spend £7500 on storage to halve the cost of your electricity, maybe.

Now I know there are some fantastic deals on ToU power, but are they going to outlast the battery i.e. 5000 cycles.

 

As it is winter, and I am at home a lot at the moment, my usage and usage split has gone the wrong way.

I use about 11 kWh/day, with 2 of those outside the E7 window.

Now I could fit batteries and and shift some of the space heating i.e. less charge in the storage heater and run a fan heater more.  Say that is a 4 kWh shift.

So night usage will become (with the latest price increase) 5 kWh x 15p/kWh = 75p, day will be 6 kWh x 30p/kWh = 180p.  Total a day 255p/day.

As I am currently using 9 kWh at night and 2 kWh during the day, I pay 195p/day.

So that gives us 60p/day to play with.

Battery system at £7500 divided by usable charge/discharge cycles of 5000 is 150p a cycle (say down to 20% and up to 80%, which is actually only 9 kWh of storage).

£1.50 will buy about 5 kWh of peak rate electricity.  Even if you get 10,000 cycles, it is still a greater amount, by 75p/day, than just being on bog standard E7.

And who knows what is just around the corner.  We all thought that the 'challenger' energy companies would reduce the prices paid, all it has done is added a few billion to the overall pricing.

 

Domestic batteries are a fun hobby, but quite an expensive one.

 

Edited December 19, 2023 by SteamyTea

[Square Feet]

Battery storage via a used EV really isn't that expensive. 15kwh would be about a third of what you are talking there Steamy Tea, ie £2.5k

[SteamyTea]

4 minutes ago, Square Feet said:

Battery storage via a used EV really isn't that expensive. 15kwh would be about a third of what you are talking there Steamy Tea, ie £2.5k

And how many charge/discharge are left, and what is the charging penalty for using knackered batteries 20%?

 

At the moment domestic storage is just not worth it.  Let the big power generators and DNOs deal with it and take the lower overall cost of power.

Store any local PV in water and bricks, you know it makes sense.

 

And it is kWh not kwh.

[ToughButterCup]

28 minutes ago, SteamyTea said:

...

And it is kWh not kwh.

 

That's us - you and me both (and a good few others) -  told off @Square Feet..... join the ever lengthening  queue of us CamelCaseIlliteratesWhojustCan'tGedditRight

You should listen and watch and LeArN fROm  the Maestro at work.

 

You BaD mAn . Phhhhhh.

[Simon R]

5 hours ago, Gaz Bancroft said:

I am planning to have 14no. solar panels on a south facing roof and an air source heat pump to power ground floor and first floor underfloor heating. The house is 144sq.m and I have been advised that an air source heat pump of 9kW would be appropriate. Will be three people in the house. We do not currently have an electric car but will be likely to get one if I'm getting free power for it through the solar panels! 

Well, I'm unashamedly in favour of home storage, even though it doesn't make much financial sense at the moment.

 

It's just a year now since my 9.5kWh battery got commissioned, with a 3.6kw solar installation. So I have a full years data to go on.

House is a well insulated 180m2, pretty much passive build. Our heating is from a 3.5 kW split air to air unit and has no problem keeping the house at 22c summer and winter.

 

Electrity consumption total from grid 2814kWh, of which 339 was charged at peak rate and 2475 at low rate..

So I've used 2138 kWh at low rate that I would otherwise have paid the higher for.

Of that 2,138 I estimate 1,600 ended up in the car so just 538kWh went into the house.

.3794 - .0714 = .308 x 538 = £165.70 which sounds pretty dire.

But that calculation ignores the 7mWh of solar generation. Of that 7mWh I only managed to use 4mWH the remainder went into the grid at of give away prices.

So adding the 4mWh of solar I was able to use all of which was generated during the day 0.308 x 4000 = £1232

Annual saving is in the region of $1232 + £165 = £1397 given my system outlay was 6.5K that's not such a bad return.

More importantly the energy exported added to the renewables component of other peoples electricity, helping to reduce co2 production.

 

Regarding battery cycles. It's still an emerging technology but the data on Lithium Phosphate suggests they are going to last a long time. 12 year unlimited charge/discharge warranty is not exceptional. The company I got my battery from is now trialling a battery to grid pilot to use home storage to help balance out grid loads. This has to be the way forward in decarbonising the grid. True 9.5kWh is tiny but hopefully in the future the 50kWh in my EV will also be able to help.

 

Tesla use Phosphate in the base model 3 and the data so far is suggesting that they may well last the 20 year life of the car. A figure of 2 to 4 times the number of cycles expected from the NCA batteries used in the other Tesla models

 

GivEnergy have an integrated inverted battery package (13.5kWh) for £6650 just as an example of the current solution costs.

[Square Feet]

10 hours ago, SteamyTea said:

And how many charge/discharge are left, and what is the charging penalty for using knackered batteries 20%?

 

At the moment domestic storage is just not worth it.  Let the big power generators and DNOs deal with it and take the lower overall cost of power.

Store any local PV in water and bricks, you know it makes sense.

 

And it is kWh not kwh.

Naturally I had taken battery degradation into account.  I prefer that term by the way, as opposed to 'knackered'. 😉

[CotswoldDoItUpper]

I’m running this quandary round in my head at the mo. When we can get such ‘cheep’ electricity off peak (7.4p/kW) why bother with solar? Just buy a big battery, charge overnight and use through the day? 
 

We are high electricity users (aprox 15kWh/yr) and only about 1/2 fits in to off peak. 
 

surely the maths doesn’t make sense for solar, but buying an old ev just for the  30kW battery (about £3k for an old leaf) and doing something like vehicle to grid (no idea on the technical side of how you could get it to work!) makes economic sense?!?

[SteamyTea]

9 minutes ago, CotswoldDoItUpper said:

15kWh/yr

Do you mean 15 MWh/year?

 

10 minutes ago, CotswoldDoItUpper said:

buying an old ev just for the  30kW battery

I am not sure, but the old Leafs had pretty rubbish batteries and this is why they are sold off cheap.

You may find that for every 10 kWh you put in, you only get 4 kWh out, and, a lot of low temperature thermal energy when changing.

I would like to see some real numbers in using a cheap EV battery, especially if the thermal management system is disconnected.

[RobLe]

We have a 15.5kWh Seplos Mason battery, it comes as a kit from fogstar here and it cost £2550:

https://www.fogstar.co.uk/collections/seplos-mason-kits/products/seplos-mason-280l-and-x16-grade-a-eve-lf304k-battery-bundle

You do need to be very careful assembling it, it's not for the faint hearted.  Personally I think this sort of thing belongs in a garage, even though they are the "safe" type of cells (Leaf ones are the dodgy "vent with flame" type btw)

 

It connects to a 3.6kW Sunsynk hybrid inverter, and for the moment there's no PV.  This has been handy as I've had a mid-certified in/out elec meter put inline, so I can calculate the efficiency.  In our case we get an overall 76% round trip efficiency.  While this sounds dreadful, this encompasses charge, discharge, and also standby stuff - I think it's to be expected.

 

There's 3 of us at home, in an all elec house with a gshp and elec car.  We use on average in winter £2.50/day of pretty much exclusively "go" electric.  Without the battery we'd pay another £4 ish per day in winter, but I'd need to try a lot harder to actually work out payback though.  And the entertainment value is worth it alone 🙂

[SteamyTea]

55 minutes ago, RobLe said:

In our case we get an overall 76% round trip efficiency.  While this sounds dreadful, this encompasses charge, discharge, and also standby stuff - I think it's to be expected.

 

That's interesting.

On what sort of depth of discharge is that.

 

Say it is a mean of 50% (down to 25% and up to 85%), then that is 7.75 kWh input. 76% of that is 5.89 kWh output. 1.86 kWh 'missing'.

I think my DHW cylinder is better than that.

Edited December 22, 2023 by SteamyTea

[CotswoldDoItUpper]

V interesting thank you @RobLe. That’s the sort of thing I was thinking of. Even at ~75% efficiency that would make our units 9.9p/kW. 
 

Significantly more efficient than a leaf both in use and also space!! Ours would go into the garage too. possibly in an insulated and ventilated box to try and keep the temp more stable.
 

 

Yes @SteamyTea, I do mean 15MWh, apologies for the error in units. 
 

we don’t have a DHW cylinder - combi LPG, so can’t use overnight to pump it up to 70deg or similar!!

 

effectively we’d be paying ~£3k to get approx 45% of our electricity at 10p/kWh. 50% at 7.4p/kWh and 5% at 29p/kWh.

 

current 50/50 split costs approx £2730/yr

new split: £1447

 

giving payback in 2.5years inc ‘lost’ interest on the original investment. Seems a bargain!

[SteamyTea]

2 minutes ago, CotswoldDoItUpper said:

‘lost’ interest on the original investment

Check the efficiency on a regular basis.

It will be interesting to see how the performance drops off over time.  Temperature may make a difference as well.

 

I should be passing though the Cotswolds tomorrow.