Home Power Walls

Are there any EV tariffs at the moment?

Run the numbers. If you can get a tariff where you can buy at 10p less than the daytime rate and you use the average 10Kwh per day then you save £1 a day (ignoring the losses from charging and discharging the battery)

So saving £365 per year. £3650 per decade. At which point the 10 yr warranty on your £10'000 battery has run out.

Maybe you can get a better tariff that will still be on the market in 8 or 12 years time when far more EVs are charging overnight. Say you can save £600 per year and the battery lasts 20 years. Over those 20 years you have saved £12k a net profit of £2k after the cost of your battery (assuming no maintainance costs).

Unless of course you have solar panels then the numbers may be a bit different.

The other thing to consider is loss of investment. When you spend £10k on a battery it is a 10 to hopefully 20 year investment to save money on electricity.

If instead you stick that £10k in a safe investment you could conservatively make 5% a year on average. So £500 a year. So £10k profit over 20 years.

IMO it isn't worth it. With the battery you are risking failure and maintainance costs after year 10.

You are risking overnight and daytime rates reaching near parity in a few years as more EVs are in use at which point cheap overnight tariffs won't be offered..

https://www.jojusolar.co.uk/batteries-smart-grids/tesla-powerwall/

If you can get a tariff where you can buy at 10p less than the daytime rate and you use the average 10Kwh per day then you save £1 a day (ignoring the losses from charging and discharging the battery)

So saving £365 per year. £3650 per decade. At which point the 10 yr warranty on your £10’000 battery has run out.

Current off-peak rate for Octopus Go is 7.5p per kWh (you either need an EV or con them into thinking you have one to get the rate) which is more than 20p cheaper than my current tarriff so closer to £900 per year saving. If you make sure the battery is islanded it will also provide back-up during power cuts.

Unless of course you have solar panels then the numbers may be a bit different.

I have a 5kW solar array which will soon be 7.5 kW and my Tesla Powerwall arrives in September after being delayed by 6 months. Unless your house is occupied during the day or you have an EV parked in the drive at peak solar to mop up any excess generation I think battery storage is a no-brainer to make solar work best. The payback period for my system assuming current 29p per kWh price is about 10 years and at the moment electricity prices are only going one way.

I find it amazing that if you buy a new car, new kitchen, new couch people will say "that's a nice new car, kitchen or couch but the first words out of their mouths if anyone buys solar is "what's the payback?"

Its not just about saving money its doing your bit for the planet. So OP if your arithmetic works out even roughly to give you an acceptable payback and you want the system just buy it.

Its not just about saving money its doing your bit for the planet.

Although generally, if you're not actually saving money you're probably not doing anything for the planet - eg the CO2 cost of making the Power Wall isn't small and unless it is genuinely saving more CO2 over it's lifetime, you've just made things worse...

@uponthedowns You are me.

I’m ahead of you on the PV, also looking at Octopus, Tesla PW on order too.

Crunching the numbers here, I’m getting a 6-8 year pay back on the system overall, though others will have different usage and therefore results.

PS the G99 (G59) connection with the ability to ride through a mains fault, rather than G98 (G83), is a bonus for me.

@uponthedowns @igm - Is there a minimum solar output which is required before your powerwalls will charge ? We currently have solar and 2 EVs, but the Zappi will only charge when we have a surplus of 1.3kW, so there are lots of periods when we are sending <1.3Kw back to the grid rather than utilising it.

the CO2 cost of making the Power Wall isn’t small and unless it is genuinely saving more CO2 over it’s lifetime, you’ve just made things worse…

Its a fair point. Estimates of the carbon footprint of a lithium ion battery vary widely but lets take an average figure of 175kg CO2 eq per kWh which would mean a Tesla Powerwall would have 2100 kg CO2 eq embedded carbon. Where I live in NW England there's about a 100g CO2/kWh difference between the carbon intensity of peak and off-peak electricity so I'd have to use 21000kWh of off peak electricity to compensate for the embedeed carbon in the battery which is about 5 years use for me. Of course, as I'll be using the battery to capture solar generation that carbon payback period for me will be less but even using a battery to timeshift electricity use to off peak it looks like a battery will save carbon. Also as we decarbonise our electricity the embedded carbon in things like batteries will decrease e.g. a Tesla battery made in Giga Nevada using solar already has half the embedded carbon of the same battery made in China. Battery lifetimes are looking better than most people expected and when they are exhausted the materials can be recycled into new batteries.

Is there a minimum solar output which is required before your powerwalls will charge ? We currently have solar and 2 EVs, but the Zappi will only charge when we have a surplus of 1.3kW, so there are lots of periods when we are sending <1.3Kw back to the grid rather than utilising it.

I can find nothing in the Powerwall specs on this but any usage graphs I've seen on Powerwalls shows the charging ramping up continuously with solar output i.e. there is no jump up to a minimum value.

When solar is marginal for getting the 1.3kWh charging minimum I use the Eco, rather than Eco+, mode on my Zappi which boosts the solar with mains to get minimum charging current. I figure its OK to use a bit of mains to capture as much solar as I can. Of course when my Powerwall is installed I won't bother doing that.

@marcus I don’t think so, but to be fair I don’t actually know that one.  We have a heat store central heating system (big hot water tank) so we’ll also be diverting anything the EV and the PW can’t use into the heating system when they fit the immersion controller.  I think the last answer is probably more helpful though.

I asked the question of my installer.

Battery I'm using not an issue.

Different chemistry to a car it's designed for low discharge/lower charge rates as oppose the NMC - which means it's also more thermally stable /less prone to fires than the PW2

Downside is I only get a max deliverable discharge of 3000watt from the battery but long as we are clever when it comes to use of high drain appliances it ain't an issue - we have played that game with solar input for a while so are wise to what everything draws on start up/ continuous so can stage cooking dinner so that the hobs/oven run out of sync.. their sustaining draw is a fraction of peak.

We have a heat store central heating system (big hot water tank)

This is next for us. Some evacuated tubes on the dormer roof and a 250l thermal store may or may not feed the solic in....but I think the evacuated tubes in the space I have will have the tank cooked long before I have excess solar to fire at it.

If you can only get 3000w from a battery, will an EV charger drop down to trickle, use the mains, or a combination?

If you can only get 3000w from a battery, will an EV charger drop down to trickle, use the mains, or a combination?

I viewed it as a moot point. As I'm not going to be trying to charge my car from a battery- there just isn't the capacity.

My solar won't heat my house/charge the car and run the house

The Cars a separate issue entirely. A car battery being 5-7 times the capacity of most home batteries.

The batteries and solar are(for me) about lowering the day to day reliance on the grid and gaining a more sustainable back up for the not so frequent but usually lengthy power cuts.

Other batteries that do more than 3000watt draw are availible your just onto more voilitile chemistry and want built in temperature stabilisation.

But say I did..... Well it would use the battery first and foremost to 3000w and then draw the rest from the grid same as if I turned on the toaster/kettle and boiled an egg on boost on the induction hob.

Other batteries that do more than 3000watt draw are availible your just onto more voilitile chemistry and want built in temperature stabilisation.

Tesla Powerwall can deliver 5kw continuous and 7kW for short peaks. Its battery chemistry is not "volatile" but it does have thermal conditioning. Tesla are transitioning to Lithium Iron Phosphate cell for all their non-EV energy storage products which is even more stable than NMC cells. LFP also has the advantage of being cobalt and nickel free.

An EV will charge at a minimum of 6A, about 1.4kw here, as that's the lowest the "power available" signal goes down to in the communications between charge point and car. Any lower than that would be pointless anyway really as the car would use more power just to run its computers and whatnot than it would put into the battery so the efficiency would be terrible.

Technically if you reduced the voltage you could make an EV charge slower, but it would be so slow it would be pretty pointless and wasteful.

The same doesn't apply to a home battery so I presume it can charge at whatever rate it likes right down to tiny amounts if necessary.

We're in the silly position of having 8kwp of solar panels on the roof... but no inverter or battery as the supplier hasn't received them yet! Can't wait until they turn up. It's not really a financial decision, although wanting to insulate ourselves from price rises helps - we're still on the octopus 5p evening / 13p day tariff so it's going to be a shock next year when it goes up - but it's more about wanting to do our bit towards reducing and greening our energy consumption.

You've put quotes around volatile like it's not a fact.

It's a very well known fact with NMC battery technology with many papers written on thermal run away of NMC batteries ....

The thermal conditioning doesn't detract from the chemistry facts it's a sticking plaster on the chemistry. Technology has moved on to safer options.

It's not just nom EV products that are ditching NME . All their standard range EVs are moving to it with a significant number already coming depending on your geozone.

The leadtime on the powerwall is also laughable right now.

But no doubt about it like Tesla's car - the powerwall is the most complete system/package on the market. No other battery or car ticks all the boxes for ease of use and performance so well.

But no doubt about it like Tesla’s car – the powerwall is the most complete system/package on the market. No other battery or car ticks all the boxes for ease of use and performance so well.

That was what reckoned when I looked into it both professionally and personally.  It will change of course, just like the cars that are starting to look a bit dated.

You’ve put quotes around volatile like it’s not a fact.

OK NMC is less stable than some other battery chemistry, and volatile might be the technically correct term to use, but to the lay person it makes it sound like the battery is going to explode at any time and plays to the FUD the fossil brigade roll out which is what triggered me.

Technology has moved on to safer options.

Yeah like I said in my post LFP tech which is more thermally stable than NMC- it also has a longer lifetime

plays to the FUD the fossil brigade roll out

Yeah but we can safely ignore them and their ignorance.

Your right though and it's no slur on NMC/Tesla because they have pushed the boundarys on what's possible. It's just that this markets gathering momentum at a Hella rate of knots. I'd advise anyone looking at it not to get het up in what's absolutely biggest and best but to size both to tech requirements and needs. You can keep costs manageable that way. I guess it all depends whether your looking to grid shift your usage onto low rate or if your wanting to store your excess solar.

Their cars may look dated but when the others are focusing on making a fancy looking car. Tesla's busy making a working integrated infrastructure. His power wall distributed node power station system is an inspired idea.

Many LFP batteries are in the region of 10000cycle expected life span which is 25ish years in normal use.

Perhaps @trail_rat but most of my colleagues who’ve had a Tesla wouldn’t have another.  Nice idea, but not that great as a car over the long term. The supply chain for real world use, bumps, fixes, build quality etc just did not work for them.

I’ll happily take the infrastructure and not the car.

His power wall distributed node power station system is an inspired idea.

To put that in perspective, we were doing that 5-10 years ago. He’s just packaging and selling it.

Nicely packaged though.

To put that in perspective, we were doing that 5-10 years ago. He’s just packaging and selling it.

Sinclair was doing electric cars in the 80s.

Tesla just do it better

This guy has three or four videos on his home battery storage projects. I bet he's a proper hoot down the pub but nevertheless some good info

There's very little 'new' about anything Tesla have done, it's just that what they have done they have done extremely well and made it very user friendly. Very very impressive mind.

Downside is I only get a max deliverable discharge of 3000watt from the battery

would’ve thought one of the main benefits would be charging up the battery during the day, then re-charging your leccy car overnight

I started with that thought then realised id need an array the size of my street with a number of powerwalls to do it ....

Footflaps gets Tesla. None of it's new he just breaka down the non joined up systems and implements them well kinda like apple

I started with that thought then realised id need an array the size of my street with a number of powerwalls to do it …

Footflaps gets Tesla. None of it’s new he just breaka down the non joined up systems and implements them well kinda like apple

so there are lots of periods when we are sending <1.3Kw back to the grid rather than utilising it.

Have you got an iSolar/solec thing in your water tank(s)? That's one of the easiest ways to use excess generation. We have the boiler off all summer.

out of interest – obviously not your typical day – how much solar did you generate yesterday (and from what size array)?

I have a 5kW array with 10 south facing and 4 west facing 375w panels. Yesterday it generated just over 30kWh so nothing special for a summer day which is to be expected as the current heatwave is nothing to do with sunshine. Best solar day this month has been 10th July with 33kWh

out of interest – obviously not your typical day – how much solar did you generate yesterday (and from what size array)?

25kwh but not my peak as they don't like the direct heat.

Peak was in may at 30kwh

That's a 4.14 array clipped at 3.68

IHN - We're on combi boilers so no tank. With 2 daughters, 1 of which can spend (literally) hours in the shower each week, the thoughts of free / cheap hot water is appealing though to consider retro fitting though. Thanks to others for input into the apparent ability of the wall to use even the smallest of surplus.