Tesla Power Wall?

Continuing the discussion from Battery for solar PV - thoughts on if it's worth it?:

I don’t have solar PV (my house is not suitable), however if I did I would also be interested in a Tesla Power wall to store energy for use after dark etc. I’ve been reading that Tesla has launched the power wall 2 with double the capacity (14Kwh) for £5400 in the UK.

I would be interested to hear what folk with solar PV think of this? Has anyone done some number crunching? Besides the potential financial rewards home battery stores would also reduce peak demand on the national grid which is a good thing. Diverting excess PV energy into battery store makes more sense to me than dumping high-grade electricity into a low-grade heat energy in a hot water cylinder.

If anyone does plan to or already has a power wall it would be interesting to learn what openAPI options there are to get the data into Emoncms e.g. charge state etc. :slight_smile:

I’ve looked into it Glyn but I reckon you’d have a very long payback time.

We have solar thermal and PV (1.8Kwh). The solar thermal went on before the PV, so we didn’t have much roof space for the PV. These are both used to heat a heat bank, PV diversion using a custom version of Martin’s code. The heat bank is used for HW through a heat exchanger and for the CH.

The heat bank also has a gas boiler to top up the temperature as needed. We also maintain the temperature at two different levels depending on whether the CH is on or not.

But back to the calculations, we use roughly 125W overnight. 100W background and a couple of fridges which come on every now and then to take it up to 200W. So 125W over say 8 hours is 1Kwh which the Powerwall would of course support. So 365Kwh per year approx. Which at 20p per Kwh is £73 per year saving.

During the day the Powerwall would supplement the PV for things like the washing machine, dishwasher etc. So let’s say we run one of these once a day and they use 2.5Kw for 30 minutes and 25% of the time there is enough PV to supplement this directly. That’s 75% of 1.25Kwh per day (I’m ignoring the fact that we haven’t enough PV to drive 2.5KWs). Roughly 1Kwh per day, so another 365Kwhs and another £73 saving.

I’ve not taken into account lights in the evenings and there are other things like the oven for Sunday lunch which uses a lot, so let’s say another KWh per day, i.e. another £73 saving.

So our saving would be just over £200, let’s be over the top and say it’s £250, so 21 years payback which is a long time.

I think I’d rather have a mobile battery (EV) and use that to supplement my electricity usage and to store excess PV rather than a static one like the Powerwall.

Tesla are doing some interesting things though, I heard they are developing a PV panel with integrated battery which sounds like a good idea and if you haven’t had a test drive of a Tesla you should do. They are great vehicles the acceleration is incredible.

Instead of a Powerwall we’ll wait for Tesla or someone else to bring out a convertible EV. My better half’s SAAB convertible won’t go on forever. Or better still VW to bring out an E version of the van, then I’d trade in our VW campervan for an electric one.


Hi Glyn,
this is a very interesting topic for me. There are some countries like Romania (I think Spain also) where in this moment it is not allowed to inject power in the system. To be more precise the meter installed by distribution company will count any energy as consumption so to build a diversion is mandatory in this case. I have 2KW PV system with ongrid inverter installed and extra power diverted by heater but I am not happy with this solution and i am looking to store in a 24v200AH battery the surplus of the production. I already did some test with a cheap dc-dc 900W boost converter and inject 120V dc in inverter input and the results are very promising for me because the output current and power can be controlled also. Also I ordered on aliexpress a charger equipped with a potentiometer for current regulation and i am planning to use a digital potentiometer to control the charging of the battery with the surplus of the energy production. In my case the main consumption of the energy is in the evening until midnight which is minimum 500W and not during the day because my family is at work or school in this period. Also i want to add that i will use ESP8266 with Espeasy firmware to monitor charging current and voltage with INA219 sensor (with external shunt) and also to control the charger.
Best regards

As I already wrote in another topic, if it is just to get off grid, yes it is interesting. If it is to gain money … nope we are far away from a pure financial positive ROI.
I have some PV and V** multiplus allowing me to charge a battery bank (600Ah/24V), mix use PV power, socket power and even re inject if there is nothing more to spend in the house.
I knew from day one I wasn’t ever going to have an positive ROI and the main reason I went for it … guess … we had a 3 week power cut a few years back after some trees pulled the line down (well not just here, south France got a nice storm) Anyway that’s the moment I said no way I even will have that again. So I calculated minimum needs and went for it.

But the Tesla model is thinking further into the future. His model is based on the fact our usage of electricity will triple. 1/3 is what we use now, 1/3 will be added since we will convert to an E car and 1/3 for more fixed consumption since we will convert from conventional heating to E heating (though that is to me not really efficient today)

If we go along that thinking yes these kinda powerwalls will be needed to store huge amounts of energy during daytime (we are mostly not there to consume what we could produce) and use it night time when our needs are biggest.

His glass tiles seems to me an interesting idea as it is cheap to make (though I’m wondering the efficiency when the sun is backing on there with no cooling). The battery packs … not so sure it is the way to go. We all know it is expensive to make these with pretty rare materials. Plus it’s environmental balance is not what you could say very positive. I’m still wondering why nobody is more exploring the hydrogen option. Some small cracking system (bit like a oil heating system) producing hydrogen and oxygen and in winter heat can even be used to heat the house) … 2 storage tanks and a fuel cell … today this is still expensive but if simplified and mass produced I’m sure it can be lower then battery packs. Finally it’s all about efficiency with each conversion.
Plus passing hydrogen from a local storage tank to a car tank (hydrogen car) is more efficient then pulling electricity from a powerbank to a car’s battery.

My ROI calculation is easy because I have a year of Emoncms data, which shows almost 1300 kWh exported in the last 12 months. (3kW panels, no diverter, but often charge the car during the day in summer).

If I optimistically say a battery will let me reduce exports to zero, my annual consumption will come down by 1300 kWh. That’s worth £130 at today’s prices, or maybe £150 in the near future. So if a battery has a 10-year lifespan (also optimistic) the most I can pay for it is £1500.



The issue I see is storage is not great. Also, I’m not a big fan of discharging a large batteris of today regularly on a heating element such as a heater or cooking range. I have seen what a fast draw does to a battery. also during spring fall and winter months daily production will not keep up every day. So to hurt our ROI in most regions it is likekly the battery will be charged by the grid than by solar.

Just some things to consider. Also with Tesla’s marketing model, I don’t want to be on the ground floor with the first expensive floor model car. I want the already proven, and economicly sound version

I have a Solaredge system with optimisers, which is compatible with the powerwall. However, with 18 months accumulated Emon data, I can confidently say Tesla would have to reduce the price to 10% of the current £6000 (installed) to make it worthwhile for me… (and I don’t have room for one, Solaredge photo below!)


Hi Glyn,

My deposit down is and am keen on getting the new Powerwall installed when it is available.

I’ve done some very rough calculations (6.27KWh system) as to if its going to make its money back, and for me it may do over 20 years.

I’ve not factored in the cost of electricity rising or the additional electricity load, but for this and the Solar panels its not just about the money.

As for monitoring, I’ve just re-joined the forums to see what my options are, hopefully the community here will be able to help me with getting the data onto Emoncms and possibly even adding a battery storage option in the apps menu.

I also looked into this a while back and having gained some data from emoncms I figured it would never pay for itself! At the time when they had the 7kWh and 10kWh banks, the limitation I saw was that they could only input 2kW and output the same - not very useful if the oven is on say! I think the newer one states 3.3kW input/output so this might be more suitable. The website is so woolly there’s no mention of specs for the 14kWh - “just give us £400 as a deposit and life will be great” kind of marketing :wink:

I have similar figures to Bramco - background use varies from 100W-300W, a bit more in the winter if the boiler is going and the TV in on plus lights etc but only between 5pm-10pm roughly. I also divert any excess into the hot water, which cost me very little to achieve as the wiring was there and the hardware was DIY - this saves roughly £60/year in gas and has potentially reduced my PV payback down by about 6 months. Now as Glyn says, this is a crude use of energy, gas is 3p/kWh, electric 15p/kWh on my current tariff so storing the electric would be a good thing - and there’s the crux of the problem with PV - we have a net export in the summer due to the fact that the immersion can only take about 10kWh, on a good day we might produce close to 30kWh - the Powerwall (new) could take another 14kWh but there’s no way we’d get through that overnight so we’d be exporting again the next day and so on. On the flip side, at this time of year leading up to the winter solstice, on a good day we may just get enough hot water i.e. zero export, but on a bad day with cloud and rain, as is frequent between Oct-Mar the PV output can be so low that even the background use isn’t covered! So the crux of it is that unless someone invents a battery system that can store in the order of 100s kWh over the summer to then use over the winter, for me it’s not a good investment, as much as it pains me to export in the summer and import over the winter :frowning:

My PV installer in now Powerwall approved and is putting together some prices but I can’t see it being worthwhile financially - fortunately we have a SolarEdge inverter with optimisers which was a good decision on that front as they are compatible. Otherwise I think it’s just a gimmick if you use make use of any PV as best as possible e.g. using high power stuff when it’s sunny, which in some cases is unavoidable but then on a relative scale it doesn’t cost a huge amount to import if done infrequently. My 2p worth :wink:

Wow, the power wall is much bigger than I thought! Is that the latest gen (2nd?) gen power wall?

Tidy install :slight_smile:

Does the power wall connect to your WiFi to give you stats via a mobile app? I would be interested to see how this works? I wonder if they have an open API you can use to log the data to Emoncms :wink:

That’s definitely not my installation. My calculated payback time for a Powerwall is 25-30 years (NOT including battery replacements), so no sale here! The photo was sent to me by one of the local companies trying to sell a Powerwall upgrade. I think it’s one in South Wales somewhere, and definitely the first version. I was surprised by the size of it, because on the Solaredge site, they show photos of inverter and powerwall. The powerwall photo is scaled down to make it look just a bit bigger than the inverter. Do Tesla realise that UK houses are a lot smaller then US ones?

Solaredge docs here: http://www.solaredge.com/products/storedge#/

I’d heard a while ago that there was only one Powerwall installation in the UK. So maybe the one in the photo is the one and only.

And given the payback time other folks have worked out it may well be one of the few that are installed.

A Google image search for “Tesla powerwall UK” suggests there may be more than one in service. It also found someone who’s built something similar using 18650 cells from laptop batteries - UK DIY Powerwall, UK DIY POWERWALL - YouTube

If you scroll down on the Powerwall page it gives you some basic stats, 5kW continuous with 7kW peak with a 13.5kWh usable capacity.

Thanks, yes I spotted that later on when I was browsing the site. That would be more feasible for me but no mention of how the 2nd gen version integrates with existing inverters (it seems to have one of its own) - cost per kWh then seems very good but the size needs to match the circumstances IMO

Hi Chaps,

Tesla have sent me the technical specifications for the Powerwall 2. First thing to note is that there are 2 versions, an AC version and a DC version. Here are the headlines from the specs.

AC Powerwall
230v Single Phase
AC energy 13.2kWh
Power 3.68kVA (charge and discharge)
Round trip efficiency 89%

DC Powerwall
DC energy 13.5kWh
Power 5kW (charge and discharge)
Peak Power (10s) 7kW (charge and discharge)
Round trip efficiency 92%

The AC Powerwall is standalone and wires into the distribution board. It looks like it requires a control unit that monitors the distribution board and charges/discharges that way.

The DC powerwall can either be wired into a compatible hybrid inverter or you can get a separate battery inverter. There doesn’t seem to be a control unit on the layout diagram so I am assuming that the inverter (either hybrid or separate) looks after the charging and discharging.

So basically it looks like the Powerwall 2 DC the same as the original Powerwall, but double the battery density (14kWh instead of 7kWh). And the Powerwall 2 AC is also double density, but also has a 3680 inverter built into it.

EDITED to correct the inverter

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Don’t you mean power?

Sorry yes. :slight_smile:

Does it handle a power cut ? Will it act as a UPS?

Where I am we get the occasional power cut - so having a battery be able to work - supply power & charge etc is rather dam handy!

Sorry to resurrect an old thread. I’ve just had a Powerwall 2 installed this week. Tesla are actually out of stock as they’ve diverted al theirs to Puerto Rico to help re-establish the grid over there following the hurricane(s). My local installer had one in stock however. I just popped back into the forum to look for help on how to program my feeds for the Powerwall connection, my additional EmonTx and CT clamp arrived this morning.

Anyway, on to some of the issues raised in the thread. I’m quite sanguine that I don’t think it’s economical but I have the spare cash at the moment and it’s sort of a way of paying now for my energy I’ll use in a few years when I’m retired. I also find since I got my PV installed in the summer that I resent seeing the energy exported and getting only the very small FiT payment for it.

As you’ve established the PW2 is 13.5kWh usable with a max charge/discharge of about 5kW. Only the AC is available, not sure if that will change. The software currently only allows the unit to soak up excess solar but there’s an update imminent that will allow you to program it to charge during the night during Economy7 or TIDE tariffs. Ultimately it will be able to act as a UPS but not yet, that will require an additional gateway board and of course require an isolation switch so you don’t energise the grid when it’s down.

The gateway unit has a sim card and connects to Tesla’s servers so you can monitor it via their phone app, it can also connect via your domestic wifi.

With regard to Paul Cardelli’s comments about problems with rapid discharge of batteries and degradation, Tesla of course have a little experience of using large batteries with high output and fast charging - I think my Model S’s peak power is about 350kW and I regularly see 115kW when charging via supercharger. A Model S used for a limo service in the States reportedly showed 6% battery degradation @ 200,000 miles, the car has since gone on to 300k miles.

Anyway, my solar PV is a 9.4kW system with just over half facing east, the remainder facing west. I already monitor that and my Tesla car charger using an EmonPi and an EmonTx and have just added a clamp to the powerwall cable. My question is about the best way to capture this given that I want to capture the charge flow separately to the discharge flow? I’m having trouble getting my head around the formulae I should use in the feed list.