Solar PV & Tesla Powerwall.

Discussion in 'OT (OFF Topic) Forums' started by Chris-S, Nov 14, 2018.

  1. Chris-S

    Chris-S Hardcore MB Enthusiast

    Messages:
    588
    Joined:
    Dec 10, 2016
    Car:
    C350e
    This is to discuss (or ridicule or whatever takes your fancy) Solar PV & Tesla Powerwall installations. Prompted by a somewhat OT drift on the C350e forum I'm guilty of perpetrating...

    @Dodgy & I were discussing it. I had a system put in about 18 months back, triggered in part by buying the C350e, although I'd thought about it before, off & on.
     
    Dodgy likes this.
  2. OP
    OP
    Chris-S

    Chris-S Hardcore MB Enthusiast

    Messages:
    588
    Joined:
    Dec 10, 2016
    Car:
    C350e
    @Dodgy , re the E7 and charging the PW overnight...at first when I saw this feature was coming I thought it would be fantastic (for those with E7) but after the event, it struck me that really, you probably need to keep a close eye on the weather forecast, or you'll end up putting energy in for no good reason. I'm sure that with some experience, you can get the feel for how much to charge it as the seasons roll on, but as soon as Tesla implement the Crystal Ball option, it'll work a lot better ;)
     
  3. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    My installer tells me there’s a “minimise cost” option in the Powerwall settings that allows it to predict whether taking on some night time charge is the best option, but that it takes a while to start making good predictions. I’m doubtful.

    The documentation I have assumes I will put very little into the battery from the panels between November and February - most of it will be used. I’m doubtful about that as well - no-one is in during the day, so I can’t see us using that much power. We’ll need to get used to running the dishwasher, washing machine, etc during the day, but I’d be amazed if the battery isn’t used quite a lot.

    One thing I am clearer about is that in December and January, I’m only expecting about 3kWh per day from the panels, but the battery holds 13kWh.
     
  4. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    I need to think harder about that winter solar output. The average per day might be 3kWh, but I have no idea what the range will be. I definitely don’t want to be putting 10kWh of economy 7 into the battery and discovering that the following day I get 6kWh of solar power and nowhere to put it.
     
  5. rockits

    rockits Hardcore MB Enthusiast

    Messages:
    1,595
    Joined:
    Dec 31, 2009
    Location:
    Hertfordshire
    Car:
    1999 SL500;2007 Maserati 4200;2005 Alfa GT 3.2 V6;LR FL2 SD4 HSE;XC90 V8;306GTi-6;TVR Chimaera
    Can't you get on a FIT deal and get paid to feed the excess back into the grid?
     
  6. SPX

    SPX MB Club Veteran

    Messages:
    14,734
    Joined:
    Aug 29, 2010
    Location:
    Ap Norf
    Car:
    That one
    Just to be clear - you’ve currently got PV but no battery?

    As much as I love what Musk has done for the industry, RoI on Tesla equipment is poor, £ for £ BYD makes the best battery with the longest life expectancy for the initial cost.

    Grid trading is going to be the biggest growth market - Duracell to launch AI-enabled home battery system next month for UK market | Solar Power Portal - My business is an official partner of that but we don’t see it taking off until next spring.

    Also worth noting that prices per kWh on PV is collapsing by the day, we anticipate a cost of less than 20p per watt very soon.
     
    Dodgy likes this.
  7. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    FIT deals are avvailable until the end of March. I haven’t checked it out in detail, but the word is they won’t be available for new installations from that point onwards.

    The point of getting solar panels is to use the electricity instead of buying it from the grid. We have timers on some the devices that use the most electricity (washing machine, dishwasher and tumble dryer), but we won’t be able to use anywhere near all that we generate. The battery changes that - for 7 months of the year, we’ll use everything the panels produce. For the other 5 months, we’ll use about 90%.

    The FIT deal is strange. You’re paid for generating and you’re paid for exporting, but there’s an assumption that you export half of what you generate. I’m not sure if it’s difficult or expensive to measure it, but the amount you actually export doesn’t matter.
     
  8. SPX

    SPX MB Club Veteran

    Messages:
    14,734
    Joined:
    Aug 29, 2010
    Location:
    Ap Norf
    Car:
    That one
    Reading between the lines your system is problem too big for its intended use but a good way to use up the extra electricity is to have an air or ground source heat pump installed - the RHI in most cases covers the cost of the kit so you should be quids in.
     
  9. OP
    OP
    Chris-S

    Chris-S Hardcore MB Enthusiast

    Messages:
    588
    Joined:
    Dec 10, 2016
    Car:
    C350e
    No, I have PV and a PW2. With such poor FIT payments now (and potentially none in future), I thought a battery made sense.

    I did look at the alternatives at the time, but thought the Tesla offering was the safest given their battery experience.
     
  10. OP
    OP
    Chris-S

    Chris-S Hardcore MB Enthusiast

    Messages:
    588
    Joined:
    Dec 10, 2016
    Car:
    C350e
    @Dodgy re the FIT measurement, as part of the installation, you have a “generation meter” installed. This is a small device that measures the total amount generated by your PV system. Assuming you opt for “deemed export” (best bet with a battery system), you just read that meter every quarter and send the number to your chosen FIT tarrif provider.

    You can opt for an actual export tarrif if you want, but you then need an export meter as well but I don’t think it works out as well for battery users.

    Using air sourced heat to burn the excess would be great, but it isn’t that long ago I put a new boiler in.

    When we get good sun, we can go for days or weeks without using any power from the grid at all. We have adjusted our habits a wee bit to try and make best use of it, things like not charging the car if the PW is low and the forecast is poor on a given day. Being retired gives us more flexibility with that of course.

    It is a fast evolving field though, so as always, there’s the early adopters risk to think about.
     
    Dodgy likes this.
  11. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    I thought about a heat pump, but I’m not sure it works for me. Am I right in thinking that heat pumps aren’t very powerful, so whilst they’re ok at maintaining a steady temperature, they struggle to increase the temperature quickly? We let the house cool down overnight then heat it up again in the morning and we don’t really want to change. Have I misunderstood?

    On the sizing point, why is it too big?
     
  12. SPX

    SPX MB Club Veteran

    Messages:
    14,734
    Joined:
    Aug 29, 2010
    Location:
    Ap Norf
    Car:
    That one
    The Tesla battery is Panasonic (nothing wrong with that)

    BYD make the iPhone batteries and are arguably the biggest manufacturer in the world. I know the European ops director and their technology is ahead of the competitors, the link I posted to earlier which has a Duracell battery is effectively a rebadged BYD.
     
  13. rockits

    rockits Hardcore MB Enthusiast

    Messages:
    1,595
    Joined:
    Dec 31, 2009
    Location:
    Hertfordshire
    Car:
    1999 SL500;2007 Maserati 4200;2005 Alfa GT 3.2 V6;LR FL2 SD4 HSE;XC90 V8;306GTi-6;TVR Chimaera
    All interesting stuff and I'm sure we can all see that there will be lots of changes over the next few years. Lots of horses and tech headlining but what will be the horse to back moving forward I'm not sure yet.

    We have 16 solar panels on the back of the house the previous owners installed on the original FIT tariff that we carried on with. At the moment at that rate the return we receive pays for 90% of what we buy back from the grid. It isn't worth us spending money to revise the system to use the energy rather than feed it back in at the moment. The cost outlay would receive a return for years based on only a likely 10% gain.

    Does anyone know if the original FIT tariff I'm on will reduce or end?

    Seems odd that the govt would stop FIT payments full stop. Surely a good sustained green source of cost effective energy wherever it comes from is a good thing.

    If I don't get permission to build a house I might look at the numbers on creating a 2 acre solar farm. I wonder if there is a cost effective sweet spot to create a small solar farm. Mmmmm.
     
    Dodgy likes this.
  14. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    I believe existing FIT contracts are for 20 or 25 years and adjust in line with RPI. It’s the new ones that are being pulled, partly due to austerity and partly due to reducing prices for solar installation. There’s a load of information about large installations, some of it here: ROO-FIT: Large installations
     
  15. Piff

    Piff Hardcore MB Enthusiast

    Messages:
    1,947
    Joined:
    Feb 24, 2005
    Location:
    Suffolk
    Car:
    Mitsubishi L200 Barbarian, A207 E250CDi Cabriolet Sport, R171 SLK280
    At installation we were told 25 years of FIT payments. The original tariff was £0.433/kwh for generation and an assumed 50% export to grid at £0.031/kwh
    Ours were installed in Feb 1012 so we should get payments until Feb 2037 (assuming I'm still around!)
    Installers provided spreadsheet calculations showing increased FIT tariffs over 25 years against reduced output over that period
    6 years in, the FIT tariffs are £0.5275 generation and £0.0372 export.
    Over the last 4 quarters we have received £1935
    The prediction by the installers for year 6 was that the tariffs would be £0.553 and £0.040 but their predicted income was £1693. So we are obviously generating more than their prediction of 2958kwh
    Checking my records for the last 4 quarters we have generated 3590kwh
    Installers also estimated annual electricity saving of £282 in year 6. I could be saving more as I have surplus over a certain level diverted to an immersion heater and we have an air source heat pump.
     
    Dodgy likes this.
  16. SPX

    SPX MB Club Veteran

    Messages:
    14,734
    Joined:
    Aug 29, 2010
    Location:
    Ap Norf
    Car:
    That one
    The older versions weren’t the most efficient of kit, the modern ones are far superior and will kick out about 4kwh of heat to every 1kWh electricity.
    General rule of thumb is your battery size should match your PV kw.
     
  17. OP
    OP
    Chris-S

    Chris-S Hardcore MB Enthusiast

    Messages:
    588
    Joined:
    Dec 10, 2016
    Car:
    C350e
    Re the battery tech, it was more that Tesla have a lot of real-world experience of large battery systems, with thermal management, in the exact application of interest.

    I'm getting about 4p per kWh I generate and another 4p for half that amount for the feed-in component. A far cry form the 50p tariffs of old, sad to say. Then gain, my system was relatively cheaper to install and the panels are more efficient and have a good guarantee as well, so I guess it works out broadly comparable.

    I absolutely accept that it's a gamble. Having already had the PW develop a fault and need replacing, there is the risk I will end up facing a choice of get it repaired out of warranty or just revert to PV-only at some point after the 10 years is up. The panels have a 25 year guarantee (including output guarantee), inverter & panel optimisers are 12 years.

    Battery capacity vs. array - mine is roughly speaking 2:1 battery:array, and I think it's about right for me. I say that, because in summer, I can easily charge the PW as well as run the house, charge the car, heat the water and still have a significant excess, so could in theory use more battery capacity to soak up that excess, but even if I doubled the battery, it would still be full by the end of the next day, so really, not that useful and very expensive. In winter, I cannot generate enough to see me through a 24 hour period so could in theory use more array capacity. Downside would be cost of course, roof isn't big enough and in summer, I'd have way too much array capacity to make use of.

    If the battery had half the capacity, it would be marginal in terms of seeing me through the night for a lot more of the year. Could be balanced with charging it from E7 if that was available though.

    I think a net tariff would be best though, rather than FIT payments etc.
     
    Dodgy likes this.
  18. SPX

    SPX MB Club Veteran

    Messages:
    14,734
    Joined:
    Aug 29, 2010
    Location:
    Ap Norf
    Car:
    That one
    We’ve worked with all the big manufacturers and BYD have the best, longest lasting battery. Again, don’t get me wrong, what Tesla and Musk have done for the industry has really opened so many eyes and all of us in the renewables sector have benefitted but the truth is their batteries are not what I’d recommend to anyone I personally knew.

    It’s not that your battery size isn’t useful, you can never have enough storage in theory, it’s the RoI that doesn’t necessarily stack up because of the high initial cost.
     
  19. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    I don’t think the battery size question is easy to answer. The changing season make it a serious compromise. In the winter, my battery will charge from the grid overnight (currently 6p per kWh), which could reduce my winter electricity bills by 20-25%.

    On the merits of BYD, their website doesn’t correctly use kW and kWh - it’s painful to read, like marking school work from a pupil who hasn’t understood. It always worries me when an engineering company allows this kind of error in their marketing.
     
  20. Dodgy

    Dodgy Hardcore MB Enthusiast

    Messages:
    466
    Joined:
    May 11, 2016
    Car:
    C350e
    Scaffolding is up and the fitting starts today...
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.