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Discussion Starter #1
Hi,
I'm currently on the Octopus Go tariff, and noticed they are trialing larger time spreads but for increased p/kwh.
That got me thinking about home power storage- Tesla Powerwall or something else.
Has anyone in the UK completed the calculations (Total Cost of Ownership) and worked out if its worth it purely from a financial point of view. Currently I don't have any solar panels. We live in a village with no mains gas, so house heating is currently bulk LPG. ~20kwh usage per day (not including car charging)
The ideal would be low upfront cost, and either get a cheap loan or lease the battery system. Hopefully the savings would more then 100% offset the finance.
Excel sheet calculation would be nice ?

BTW- Its a pity the BMW i3 we have can't feed it's remaining charge back into the house when the Mrs returns home each evening (normally at least 50% charge remaining), then fully charge it again overnight. One for the future.
 

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I did it years ago for Nickel Cadmium batteries. 4Ah D cells from Maplins were about £5 at the time, and were estimated to last 3000 charge/discharge cycles.

4Ah x 1.25v x 3000= 15kwh life for a £5 battery. Therefore any saving must be more than 33p per unit, which of course I don't think even now is likely.

Maybe do a similar calculation for the power wall.
 

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Hi,
I'm currently on the Octopus Go tariff, and noticed they are trialing larger time spreads but for increased p/kwh.
That got me thinking about home power storage- Tesla Powerwall or something else.
Has anyone in the UK completed the calculations (Total Cost of Ownership) and worked out if its worth it purely from a financial point of view. Currently I don't have any solar panels. We live in a village with no mains gas, so house heating is currently bulk LPG. ~20kwh usage per day (not including car charging)
The ideal would be low upfront cost, and either get a cheap loan or lease the battery system. Hopefully the savings would more then 100% offset the finance.
Excel sheet calculation would be nice ?

BTW- Its a pity the BMW i3 we have can't feed it's remaining charge back into the house when the Mrs returns home each evening (normally at least 50% charge remaining), then fully charge it again overnight. One for the future.
I am not sure that it warrants a spreadsheet unless you are into trying to double-guess energy inflation. My KISS analysis gave a 30+ year return based on today’s Go energy prices. Assuming that you will charge the i3 on Go and not off the battery, then if, say, your household usage is 5000kwhs/year then you will probably need 5000 x 1.2 kWhs to top up the battery at a cost of £300 per year. You will though save (15-5) *5000 or £500 per year (assuming Go prices of 5 and 15p). An annual saving of £200 per year.

In your situation, I would suggest that the maths get more complicated. Fitting a Powerwall requires DNO approval which might at the least require a fuse upgrade to 100 amps or, depending how many others have panels etc, require you to upgrade to a 3 phase electricity supply. if you intend to replace LPG with a heat pump, then I would confidently predict that heat pump use; EV charging and battery charging would require 3 phases.

The ‘good news’ is that solar plus battery makes much better financial sense ( sadly, long gone are the FIT payments. Looking at my figures for a 6.2kWp (limited to 5kW export) solar array plus PW2, I am getting a return of 19 years. This includes diverting energy for EV charging and water heating.

PS Heat pumps do attract FITs which is probably something to think about for someone with LPG heating.
 

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Discussion Starter #4
Thanks. I was also investigating replacing my LPG combi with one of the new LPG/Air Source Heat Pump combi's.
Costs don't work out until my current boiler packs in. I'm guessing within 3 years.

When I another months worth of data from Octopus Go, I'll do some calcs and post them here.
I have another thread with an issue with my Rolec charger only offering 16amps, so my charging is currently going outside the 4 hour window. Should be fixed early March and lower the electric bill a bit..
 

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Thanks. I was also investigating replacing my LPG combi with one of the new LPG/Air Source Heat Pump combi's.
Costs don't work out until my current boiler packs in. I'm guessing within 3 years.
The risk of delaying a heat pump for 3 years is that you might find that the Government has stopped the RHI scheme.
 

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The simplest calculation you can do is take all your electricity and LPG spend per year, and divide the cost of the powerwall by it. That will give you an absolute minimum repayment time. If you're thinking about a loan, calculate the length of repayments using your costs as the repayment value.

For me, I spend £670 a year on electricity and gas. I can't save any more money than that, so £7,500 for the powerwall would take £7,500 / £670 = 11 years 3 months to recoup. If it was a 3% APR loan, it would be 13 years 4 months.

Any more accurate calculation will only be extending that period (e.g. maybe I'm only saving half of my costs, so twice the recoup period). As 11 years is more than the guaranteed lifetime, I don't think it's economically viable for me.

Edit: I though that the wholesale price of batteries was fast approaching $100 per kWh. That'd be ~£1000 for the batteries in a Powerwall. Shouldn't they be about £2,000 retail by now? That's where they need to be IMHO.
 

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Discussion Starter #7 (Edited)
The simplest calculation you can do is take all your electricity and LPG spend per year, and divide the cost of the powerwall by it. That will give you an absolute minimum repayment time. If you're thinking about a loan, calculate the length of repayments using your costs as the repayment value.
For me the LPG is a linked but separate calculation, because I'd have to replace my existing LPG boiler with an Air Source heat pump or Hybrid Boiler. The air source heat pump could then use power from the powerwall.

Looking back on my Octopus bill- it estimated last year I'd use a total ~8000kwh over the year on a standard tariff. That was with a Golf GTE charging at any time of day (~1500kwh)[email protected] ~14p kwh
Now we have the BMW i3 with Octopus Go tariff. Assuming the i3 is only charged during the off peak time at 5p kwh. That leaves the rest of the house consuming ~6500kwh at ~14p/kwh that is £910.
If I could store that power in the powerwall at 5p/kwh then the cost would be £325. So a saving of £585py
I'd have to factor in upgrading the plumbing to a air source heat pump in order to displace the LPG usage for heating. + insulating the house more.
We have underfloor wet heating downstairs in the house, which I understand is good for air source.
I have a friend who's building a house with an air source heat pump, he thinks the heating will cost him nothing with the government subsidies.
I'll make some further enquires with him and a plumber friend.
I think a hybrid boiler would be the best, the boost of lpg when required, air source running in the background for space heating.
 

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The simplest calculation you can do is take all your electricity and LPG spend per year, and divide the cost of the powerwall by it. That will give you an absolute minimum repayment time. If you're thinking about a loan, calculate the length of repayments using your costs as the repayment value.

For me, I spend £670 a year on electricity and gas. I can't save any more money than that, so £7,500 for the powerwall would take £7,500 / £670 = 11 years 3 months to recoup. If it was a 3% APR loan, it would be 13 years 4 months.

Any more accurate calculation will only be extending that period (e.g. maybe I'm only saving half of my costs, so twice the recoup period). As 11 years is more than the guaranteed lifetime, I don't think it's economically viable for me.

Edit: I though that the wholesale price of batteries was fast approaching $100 per kWh. That'd be ~£1000 for the batteries in a Powerwall. Shouldn't they be about £2,000 retail by now? That's where they need to be IMHO.

The bit that I am struggling with in your calculation is 'how does a PW2 with a 13.5kWh output replace both LPG and electricity without some use of Grid Power or LPG'? You have also ignored the efficiency loss when charging a battery. The calculations that I have seen suggest that to get 1000kWhs out, 1200kWhs will be required to recharge the battery. The life of a PW2 is also interesting. The warranty states Unlimited Cycles when solar charged or under time-based control/load shifting (energy retention 80%) or in other circumstances 37.8MWh of throughput. There isn't as far as I can find a definition of a cycle.
 

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I did it years ago for Nickel Cadmium batteries. 4Ah D cells from Maplins were about £5 at the time, and were estimated to last 3000 charge/discharge cycles.

4Ah x 1.25v x 3000= 15kwh life for a £5 battery. Therefore any saving must be more than 33p per unit, which of course I don't think even now is likely.

Maybe do a similar calculation for the power wall.
Interesting way of doing it. Downloaded a Samsung 21700 datasheet, so lets see.

  • Telsa has said $175/kWh in 21700s, and each cell is about 20Wh. So that's $3.50 a cell.
  • A Cell is rated at just under 5Ah with a voltage of 4.2V.
  • After 500 cycles it has 80% capacity. Assuming that continues 1,500 would be 50%. That's probably lifetime.
4.2V * 5Ah * 1500 = 31kWh for $3.50 or $0.11 (8.5p) per kWh. That's more achievable, but:
  1. That's wholesale pricing for Tesla cells to Tesla.
  2. It assumes no lost capacity over those 1,500 cycles.
  3. The current average is 9p per kWh on Agile. With an EV charging overnight you'll probably bring that down.
I'm pretty sure home battery storage doesn't make economical sense right now.
 

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Assuming the i3 is only charged during the off peak time at 5p kwh. The leaves the rest of the house consuming ~6500kwh at ~14p/kwh that is £910.

If I could store that power in the powerwall at 5p/kwh then the cost would be £325. So a saving of £585py
If you are on Go or Go Faster, then your calculation is slightly flawed as most people would also put their washing machine; tumble dryer etc on when the cost of electricity is 5p/kWh. I am on Agile at the moment, I could just use the peak cost of 35p/kWh and justify buying two PW2s.
 

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The bit that I am struggling with in your calculation is 'how does a PW2 with a 13.5kWh output replace both LPG and electricity without some use of Grid Power or LPG'?
It doesn't, but you can't save more money than you actually spend. So if a powerwall magically made all your energy bills vanish in a puff of smoke you'd still only save what you were paying before hand. That's the upper bound on the calculation.

If the upper bound isn't economical, no amount of massaging the numbers will make it economical.
 

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I've just installed a heatpump to get off the old oil boiler. No need for RHI that just inflates the price.

I think you would be best off working out what the heat loss is on your house. Then calculate what size thermal store or sunamp you would require to store that heat for a day. Then what size heatpump you need to heat that up during the low tariff period.
 

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Looking back on my Octopus bill- it estimated last year I'd use a total ~8000kwh over the year on a standard tariff. That was with a Golf GTE charging at any time of day (~1500kwh)[email protected] ~14p kwh
Now we have the BMW i3 .....
Stop right there. You're going too complex.

Leaving the LPG out of it for now. You paid £1,120 in electricity before (8,000 * 14p). The best you could ever hope to do, is turn that to £0, a saving of £1,120. Installing a PowerWall won't save you 100%, but let's just say it magically would.

£7,500 / £1,120 per year is 6.7 years (6y9mo). That is the absolute minimum timescale to break even, and you can't save money until you break even.

Now you can say things like...
  • "Well, the powerwall will allow me to move all my electricity from 14p to 5p per kWh. That's basically savings of 60%". 6.7years / 60% is 11.2 years.
  • "Moving to Agile I reckon I can get an average of 3p per kWh. That's 80% savings." 6.7y / 80% is 8.4 years
The reason I said add your LPG in was to give you a minimum timescale if everything was electric, but maybe that calculation should be (Cost of Powerwall + Heat pump + Heat battery?) / (electricity + LPG).
 

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Discussion Starter #14
If you are on Go or Go Faster, then your calculation is slightly flawed as most people would also put their washing machine; tumble dryer etc on when the cost of electricity is 5p/kWh. I am on Agile at the moment, I could just use the peak cost of 35p/kWh and justify buying two PW2s.
True. I was making a rough calculation. I'm going to tell the current Mrs T that any future cooking and washing/drying happens between 00:30 and 4:30--- ermmmm no. You can't put a price on marital peace :)
Seeing my housebuild/ plumber mates tonight. Plumber friend told me in the past that air source only works well with well insulated houses. i.e. new build houses designed for it.
 

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My house was built in the 50s, it has cavity wall and loft insulation and a number of drafts. The heatpump has been fine. You keep the house warm the whole day rather than just when you are home. And you save more money by fixing the drafts and improving the insulation. It is a myth that they don't work in older properties.
 

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Discussion Starter #16
My house was built in the 50s, it has cavity wall and loft insulation and a number of drafts. The heatpump has been fine. You keep the house warm the whole day rather than just when you are home. And you save more money by fixing the drafts and improving the insulation. It is a myth that they don't work in older properties.
Would you mind sharing your heat pump calculations on this forum?
 

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Air source will surely work fine regardless of how old the house is, so long as the pump is big enough.

The main issue with air source is lower flow temps, which means the rads operating at a lower delta T, which in turn means the old rads often arent large enough.

So if you get a cowboy in who fits an undersize pump and dumps it on your stock rads, it'll be shit. If its done right, it'll be fine.
 

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Discussion Starter #18
What would be smart is using your EV as a powerwall, which I understand is being trialled around the world.
Our i3 rarely goes below 50% charge. If we could plug it in after work and consume the remaining battery (via smart notification to devices) then do a full charge out of hours.
 

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What would be smart is using your EV as a powerwall, which I understand is being trialled around the world.
Our i3 rarely goes below 50% charge. If you we could plug it in after work and consume the remaining battery (via smart notification to devices) then do a full charge out of hours.
Vehicle to Grid, or V2G. Only available on Nissan Leafs right now, but yes it would make sense. Especially good for peak shaving.
 
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