Smallholders Insurance from Greenlands

Author Topic: Rethinking cost/benefits of solar and domestic batteries  (Read 2514 times)


  • Joined Mar 2012
  • Carmarthenshire/Pembrokeshire Borders
Rethinking cost/benefits of solar and domestic batteries
« on: November 04, 2019, 08:49:00 pm »
Pricing in this area has changed a lot over the last 12 months or so.  The FIT on new solar installations is no more, and the 5% VAT has also been ended so new solar installations are a whole new set of economics. Fortunately uf you already have installed then the FIT goes on for a good few years.

Big change is the price drops in domestic batteries. Be careful on capacity though, many are only 3-4Kw which isn't going to get you very far.  The Tesla powerwall appears to be the best value in terms of £/kw capacity - we are looking at £7-9000 installed for the 13kw unit. OK - a lot of money, but there are other factors at play here. The latest model includes the ability to continue to supply power (from the battery) when your grid connection fails (eg. powercut) so you get standby power which is a benefit in remote parts of the world like SW Wales. Then add the new tariffs and smart meters to the mix and a whole new world opens up. We signed up to Octopus' recently introduced electric car tariff, which needs a smart meter to record half hourly usage (which they kindly paid for and the installation). We now get 4 hours of power between 00:30 and 04:30 at the ridiculous price of 5p per unit. So charging our electric car now costs under £1 rather than £3 or so before. Most modern appliances (dish washer, washing machine, tumble dryer etc) can be set to start at a preset time in the future, so they all work at the same 5p/unit overnight.

Having done the calculations I reckon I can use the low rate power to charge the battery overnight (probably about 50p) then run off the stored power (along with any solar generation if the rain ever bloody stops) all through the day until it's time to charge again. Most solar generation not immediately used will go to charge the battery (free electricity) and reduce the amount i need to fill the battery at night. I still get paid FIT for 50% of my generation, even though I will probably barely export anything at all - so money in the bank on that one.  Bottom line I can go from buying any power over and above solar generated for 5p rather than 15p per unit and avoid "wasting" power by storing in the battery rather than exporting for a pittance. This looks like a pretty useful confluence of technologies to me.  Warranty is 10 years and payback should be less than that (we are heavy electricity users) without factoring in the inevitable increasing on electricity prices, I have standby power (just sold the 5kw generator we had) without the hassle. OK you have to invest, but then you should save.

Will report more once all installed and operational


  • Joined Jan 2013
Re: Rethinking cost/benefits of solar and domestic batteries
« Reply #1 on: November 05, 2019, 08:39:05 am »
will be watching this with interest. Current usage is around £450pa from the grid so payback does not look too good at around 14 to 18 years but if the EV became a reality then may improve things. Currently the panels produce around 3650kwh PA and we use around 1600 with the balance going into the thermal store significantly reducing the number of times the oil boiler fires up during the summer when we do not want to light the WBS.


  • Joined Jun 2013
  • South Wales .Carmarthenshire. SA18
Re: Rethinking cost/benefits of solar and domestic batteries
« Reply #2 on: November 23, 2019, 01:50:58 am »
Thanks for that Steve,  I came to similar conclusions as you .

 Can you set things so if neeeded the EV is charged off the Powerwall in day rate hours , instead of having to wait for night rates to recharge ..

Whilst not yet having a Powerwall  I have our EV on an over rideable time clock to charge in the day if needed ( rarely ) .

 Spent the powerwall savings on triple glazing & high spec door for the bungalow instead .. ( due to be fitted any day now ) .
 :thumbsup:Powerwall … 2021...... perhaps .
Strong belief , triggers the mind to find the way ... Dyslexia just makes it that bit more amusing & interesting


  • Joined Mar 2012
  • Carmarthenshire/Pembrokeshire Borders
Re: Rethinking cost/benefits of solar and domestic batteries
« Reply #3 on: January 25, 2020, 11:40:22 pm »
Not sure charging the EV off their power wall during the day makes sense since the car would drain the full 13kw of battery and still not be full (if it started close to empty). Since we get 120+ miles on pure electric (and another 100 on the range extender charging overnight works well for us.

Have now ordered the Powerwall and awaiting delivery so the die is cast as it were. We are heavy power users (running the farm as well as the house) with annual usage in excess of 7000 kWh. Capturing the “lost” 50% or so of solar production (even in summer) has to be worthwhile but only time will tell. I must admit that as a confirmed gadget freak, the onlinemonitoring of power used, stored, in/out to grid and solar all in one place with the Powerwall management system is going to be informative as well as fun.

We already have double glazing everywhere and the 3 foot thick walls provide a level of thermal insulation streets ahead of modern cavity walls - we are cool in summer and warm in winter

Will keep updating this as the year progresses. You cannot be far from us, so you are welcome to come over later in the year and see things in the flesh as it were.


  • Joined Jun 2013
  • South Wales .Carmarthenshire. SA18
Re: Rethinking cost/benefits of solar and domestic batteries
« Reply #4 on: February 10, 2020, 01:41:06 pm »
Like it .
Today the fitters have arrived to start installing our triple glazing , it has the thermal reflector in it as well as being inert gas filled .   
 So far they have only done the two bath room windows & one of  the big 7 x 4 foot ones in the lounge .
The reduction in road traffic noise is amazing ,so is having a UPVC window in the lounge that fully closes & stops me being blown out my armchair when the wind gets up . 
Strong belief , triggers the mind to find the way ... Dyslexia just makes it that bit more amusing & interesting


  • Joined Mar 2012
  • Carmarthenshire/Pembrokeshire Borders
Re: Rethinking cost/benefits of solar and domestic batteries
« Reply #5 on: March 02, 2020, 12:40:12 pm »
Well the Powerwall 2 was installed Tuesday last week (excellent service from My Contribution in Narberth). Despite the rather variable weather over the last week it has exceeded my expectations. We have an EV oriented tariff from Octopus which gives us 5p/kwh between 0030 and 0430am and 14p the rest of the day. Over the last week we have managed to fully charge the car and the powerwall during the off peak period and then with a bit of help from solar can make it through until late evening without drawing any power from the grid on peak tariffs, and I mean ZERO. It will be interesting to see the bill at the end of March now if this carries on. Only time will tell if the investment was worthwile, but there is an immense satisfaction from viewing the app and seeing that zero grid input all day! The other interesting thing is simply how much solar energy was previously going to waste - never really realised it. Since we get paid through the FIT for 50% of our production whether we export or not, the ability to capture it all (and again, zero solar export to the grid now) is an additional saving.  As I sit here at just after noon, the sun is shining (for once, and it isn't even raining!!!) and the solar is generating 3.5kw, the house (which includes the farm) is consuming 0.6kw and 2.7kw is charging the powerwall (which is currently showing 91% full). Time to go put the dishwasher and washing machine on! :thumbsup:

Will provide a detailed analysis at the end of the month


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