Solving high electricity usage and cost using solar + batteries to store off peak power

elrao

We currently use ~ 15,000 KwH of electricity a year and we are looking to buy an EV which would add ~3000 KwH a year on top.  We don't have solar at the moment but have a roof that can accommodate a 6.2KW system (predicted annual output 5,619 KwH) for which I have been quoted £9,550 to supply and install 16 395w panels with a solar edge system (due to some shading) and a Zappi EV charger.  This does not include a battery, which is what led me to this forum!

The original plan was to use a modest 6.5KwH battery to store unused Solar.  At £4k this would leave annual costs at around £3,750 (inc. the EV) based on a standard rate tariff. Working out the costs on a dual-rate tariff proved more complex, as while I would save £700 on charging the EV during the off-peak hours, I  would be paying a premium for the majority of my electricity.  There was also the consideration of charging the battery every night off-peak and using the power during the peak period.  If my rough calcs are correct, then the net effect of the dual-rate would be an annual cost of roughly £3,000 a year.

This then made me think how much more I could save by buying a bigger battery and buying more of my electricity off-peak and I realised this is essentially what Tesla does on a commercial scale with their Tesla Tariff on Octopus.  Which has led me to a comparison of the Tesla option vs going it alone and trying to work out the optimum battery capacity to balance upfront cost against the potential savings  Either way, with an annual demand of ~9,500 KwH (12,500 KwH with an EV) there is potential for significant savings...i just need to decide which option to go for (and how big a battery).

Tesla Powerwall

Costs £10,800 and opens up the exclusive Octopus Tesla tariff where they buy and sell electricity for 11p per KwH.  The projected annual cost would be ~ £1,375 a year meaning that an extra £7k now potentially saves me £1,625 a year.  

I like this option as it is simple and flexible.  No stress about working out the right size battery, just stay within the 9500KwH limit for daytime usage (should not be an issue) then you never pay more than 11p per KwH (current prices).  So you pay more for off-peak but a lot less for peak!

26 KwH Non Tesla Battery

Installation cost is between £10k - £11k based on 26KwH batteries (26KwH provide an annual storage capacity roughly equal to my annual demand of ~9,500 KwH (exl EV which would charge off peak only)).  The projected annual cost could be anywhere between £925 and £1675.  So an extra £6k to £7k now potentially saves me between £1,325 and £2,075 a year.

My problem is that it is hard to properly compare them as I don't know my detailed usage to determine how many days I would exceed the amount the batteries could store + available Solar that day and have to buy from the grid.  The non Tesla option should cost less, and has the potential to save £300 a year more ... or to cost £450 a year more!

Anything else to consider?

If I go down the non Tesla route, then I am being recommended GroWatt batteries (had also been looking at GivEnergy when not considering Solar Edge, but the installer favours the GroWatt with the Solar Edge)

EcoScruples

Don't forget about the lifespan of the batteries, they don't last forever.
What about getting an EV with V2H capability? 

2nd_time_buyer

Very interesting. With your high usage, there seems to be a better than normal case for a battery.

The problem is that it is often very tariff dependent. That Tesla tariff is a bit of an outlier - I am guessing if it was pulled it would change things a lot. 

Several suppliers are offering cheap overnight rates for BEVs so their availability might be slightly more secure. 

Another option is to use Octopus Agile outgoing for export. That has had an export rate around 2-3 time more than the fixed rate over the last year. But it can not be used with Octopus Go (or presumably the Tesla tariff). Again, it is an outlier.

The one common theme on this forum, seems to be to put the maximum number of solar panels on your roof you can. It feels for your usage you are a little light. Can you add any more e.g. on an outbuilding?

elrao

EcoScruples said:

Don't forget about the lifespan of the batteries, they don't last forever.
What about getting an EV with V2H capability? 

Yes, payback period is well within the lifespan of the battery warranty.

Did think about a V2H EV, but does limit your options.  Might help reduce the chances of needing to buy power at peak times 

EcoScruples

The only reason I mentioned V2H is that if you can have it at home during the day it'll save you the cost of a battery.

If you have the cash to invest then just do it, the feel good factor against the rising energy costs will make it worthwhile 

elrao

2nd_time_buyer said:

Very interesting. With your high usage, there seems to be a better than normal case for a battery.

The problem is that it is often very tariff dependent. That Tesla tariff is a bit of an outlier - I am guessing if it was pulled it would change things a lot. 

Several suppliers are offering cheap overnight rates for BEVs so their availability might be slightly more secure. 

Another option is to use Octopus Agile outgoing for export. That has had an export rate around 2-3 time more than the fixed rate over the last year. But it can not be used with Octopus Go (or presumably the Tesla tariff). Again, it is an outlier.

The one common theme on this forum, seems to be to put the maximum number of solar panels on your roof you can. It feels for your usage you are a little light. Can you add any more e.g. on an outbuilding?

Yes.  The thought of the Tesla tarrif being pulled is a concern. You can drop to a regular dual rate, but obviously lose the main reason for choosing Tesla over a regular battery.  

I also think (hope) EV tariffs are here to stay, they seem to be very popular.

We are using the roof on an outbuilding, you can't see it from the house or garden so perfect for solar. Can't put solar on the house.  Garage might be an option, but would be a separate system, so quite expensive to add.  

razord

Would also be worth checking with your DNO if they will let you have a battery with that much capacity before you go too far down the planning route.

QrizB

razord said:

Would also be worth checking with your DNO if they will let you have a battery with that much capacity before you go too far down the planning route.

I don't see why they'd mind you having as big a battery as you want. Any restrictions will be on how fast you can charge or discharge it.

elrao

razord said:

Would also be worth checking with your DNO if they will let you have a battery with that much capacity before you go too far down the planning route.

Yes.  Did think about being able to draw enough power during the 4 hour window to charge the battery and an EV.  

Would be 54KwH in 4 hours, so 13.5Kw or about 56A for 4 hours!  Obviously the Tesla route avoids that as its less power and you don't need to cram it into 4 hours.

elrao

QrizB said:

razord said:

Would also be worth checking with your DNO if they will let you have a battery with that much capacity before you go too far down the planning route.

I don't see why they'd mind you having as big a battery as you want. Any restrictions will be on how fast you can charge or discharge it.

That's what I thought. I understand limiting the amount we can draw or feed in.  As the battery would not feed to the grid, it would only be a potential restriction on how much I can charge.  

Bigger battery not really any different to an EV with a big battery.  Only difference is the charger being limited to 7KwH.

Octopus Intelligent Tariff gives you six hours off peak, but you need a compatible EV.

michaels

Have you had any more solar quotes, we got ours 6 years ago and it cost just over £4k for 4kwp of panels with solar edge inverter so your quot sounds expensive.

Our usage is lower but we are looking to add a heat pump and off peak hot water tank heating and use our EV for V2H which will hopefully allow a shift from gas at 13p/unit (come October) to electricity at 4.5p per unit overnight.