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Solar PV and home battery = crazy electricity bill!

edited 27 June at 6:12AM in Energy
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gzoomgzoom Forumite
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edited 27 June at 6:12AM in Energy
A bit of back ground to our set up:

We bought our current house in 2017 it has a 4KW solar panels split 50/50 on SW/SE facing roof. It qualifies for the FIT scheme and pays back about £500/year.

In November last year we got a Tesla PowerWall fitted, essentially a home battery storage system - 13.5kWh usable of energy.

We also have an EV which we charge at home, and in recent months have been commuting more on my pedal bike but still doing about 400 miles a month in the car.

The solar PV setup is also connected to the immersion heater, so when the sun is shinning if the PowerWall is full the solar energy goes into heating the hot water rather than be exported back to the grid.



Our bills since the summer has started have really surprised me, here they are for the last two months. Effectively around £1/day INCLUDING standing charge, don't forget that also includes 'refuelling' our car!!





You can see our daily electricity consumption has dropped from 25kWh a day to 1kWh a day!!!

The two main contributors have been the addition of the Powerwall and driving our EV less. The PowerWall really has taken our solar PV setup to the next level. This is what our home grid electric usage has been like in June, even with all the rain there was only one day we pulled significant amounts from the grid :smile:



I'll report back once I've had a full 12 month with the system, but currently really really surprised how big a difference the home battery has made to our grid electricity usage when the sun is out.

Bulb has also sent me our annual electricity consumption chart for the last 12 months, the effect of be Powerwall/lock down is pretty obvious. What is also interesting is our consumption in Dec to March was lower than Sep to Dec - the Powerwall was installed 22nd of November. 


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  • edited 27 June at 6:30AM
    gzoomgzoom Forumite
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    edited 27 June at 6:30AM
    This is how the PowerWall has really made an impact on our bills. The Powerwall starts the day nearly empty as I knew it was going go sunny during the day.

    It has enough energy stored to meet the needs of morning pre work usage, I think the car started charging very briefly too. The house is empty all day as we are both at work, and by 4pm the Powerwall is fully charged so the excessive solar electricity is sent to the hot water tank.

    We get home and use what ever electricity needs are in the evening, the car is also charges later in the day. Without the PowerWall you can see nearly all of our solar electricity generated would have gone back to the grid.


    Costsaving wise the numbers work out roughly as below:
    Summer period, assuming 12kWh a day solar PV saved so 12kWh a day reduction in electricity bill at 16p per kWh = £230 over 4 month.

    For Winter, I use the PowerWall to 'load shift' so I used cheap E7 electricity at 8p instead of 16p. Assuming 12kWh usage a day saving 8p per kWh, over the remaining 8 months, that's a saving = £234.
    So roughly speaking our Powerwall will save us about £460 a year in electricity bill.
  • matelodavematelodave Forumite
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    Excellent figures but they are for a fairly exceptional month or so. More to the point will be how they performed in the winter so as you say you do need 12 months worth of data to see how much you will save. I note that you've had the Power-wall since November, what sort of performance did you get during the winter
    What was the cost of the solar panels and the power-wall and how long have you calculated for the payback. - it's the sort of info that other people need to try and work out the economics of doing something similar.


    Never under estimate the power of stupid people in large numbers
  • edited 27 June at 7:13AM
    gzoomgzoom Forumite
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    edited 27 June at 7:13AM
    Excellent figures but they are for a fairly exceptional month or so. 
    May was exceptional, but June has been fairly average, even so we are only using just over 1kWh a day of grid electricity this month. 

    Winter usage is harder to quantify without 12 months of data but this is what the PowerWall enabled us to do nearly every day even in winter - We are on E7, so essentially zero 'peak' grid usage, so we were paying 0.08p per kWh for the entire winter period rather than the full 16p per kWh 'normal' electricity rate. 


    Sadly I cannot help much with overall costs of the installation, the house already had solar PV + water immersion heater system setup, and the PowerWall was 'free', so I can only report the benefits, my best estimate is we are currently saving £400-450/year on reduced electricity bill due to the PowerWall excluding the FIT tariff.

    Including the £500/year FIT tariff I recon we will effectively be cost neutral for our gas/electricity bill including charging the EV until the FIT tariff ends in 2035   B).
  • edited 27 June at 7:25AM
    matelodavematelodave Forumite
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    edited 27 June at 7:25AM
    I'd assume that most people (unless they are all electric - like me) would use around £500 a year in leccy, so it looks as though you are fairly close to saving almost all of it, which is pretty good. Coupled with a FIT payment of £500 then it's equivalent to around £1k per year
    .
    On another thread the OP was offered solar-panels and 5kwh battery (which is less than half your 13.5kw unit) for £11.5k which on your figures could yield a payback of around 11-12 years, however as FIT payments aren't available anymore the payback extends to over 20 years
    Never under estimate the power of stupid people in large numbers
  • edited 27 June at 8:03AM
    TalldaveTalldave Forumite
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    edited 27 June at 8:03AM
    @gzoom many thanks for the really interesting posts. If I owned a house at the moment I would be doing exactly as you have done. Time shifting E7 is awesome. I wonder if you've looked into lowering that E7 buy-in cost? E.g an EV tariff that's well below 8p?
    I also don't think this is all about getting your money back over n years. It's fine to actually spend some money to get a system that's both innovative and gives you a certain level of self-sufficiency. If there's ever a power cut, you'll be laughing and fully powered - those with panels only will be scratching their heads wondering why the sun isn't helping.

    I look forward to a time when your car either is your powerwall or becomes a temporary extension of the powerwall when it's at home.
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  • gzoomgzoom Forumite
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    On another thread the OP was offered solar-panels and 5kwh battery (which is less than half your 13.5kw unit) for £11.5k which on your figures could yield a payback of around 11-12 years, however as FIT payments aren't available anymore the payback extends to over 20 years
    I think the overall payback period will change as the tech gets cheaper, £11K for solar PV + battery installed is alot cheaper than what it would have cost even last year.

    I've been really surprised how much more 'useful' our solar PV setup has been with the addition of a home battery, take right now for example, its actually drizzling with rain, but the solar PV setup is still generating enough current to store. Its not a lot but it all adds up!!



  • MWTMWT Forumite
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    In part though the savings are good because the tariff is bad, 16p/8p isn't hard to beat by a considerable margin in most regions.
  • gzoomgzoom Forumite
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    Talldave said:
    @gzoom many thanks for the really interesting posts. If I owned a house at the moment I would be doing exactly as you have done. Time shifting E7 is awesome. I wonder if you've looked into lowering that E7 buy-in cost? E.g an EV tariff that's well below 8p?
    I have, especially with Octopus Agile offering 'plunge' tariffs recently where you get paid to use grid electricity, so you could set up a script to tell the PowerWall to charge during plunge prices so basically start generating income. 

    For now though our bill is so low am not in a rush to change, E7 is very predictable and time shifting works well with no effort at all from me. 

    Later on this year I will review the situation, but its a win-win either way, having the battery storage really opens up option.  
  • edited 27 June at 8:18AM
    gzoomgzoom Forumite
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    edited 27 June at 8:18AM
    MWT said:
    In part though the savings are good because the tariff is bad, 16p/8p isn't hard to beat by a considerable margin in most regions.
    Which suppliers offer better than 16p/8p for E7 in EastMidlands? 

    The savings in the last 2 months has been from not having to use much grid electricity,  infact the standing charge of 18p per day makes up 50% of the bill for May and June. I suspect July and August will be the same!!
  • MWTMWT Forumite
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    Sounds like you already know the answer, with your setup you would almost certainly be better off on Octopus Agile, not so much for the 'plunge pricing' but the ability to take advantage of the lower prices that occur throughout the day as well.
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