Yet another battery / no battery system

ABrass

I've got a couple of quotes that i'm trying to choose between

8kW panels (20x400) - probably split over S and W facing
6kW* Growatt hybrid inverter
£8,500 installed

Or the same as the above but with a 6.5kW battery installed for an extra £3k.

I work from home and use a minimum of 400kWh a month during the summer and 650kWh during the winter. No EV and probably none for 5 years or so. I've checked PVGIS and i'd probably getting between 200kWh a month over winter and up to around 950kWh in the peak of the summer.

Assuming 20% utilisation and at 32p per kWh, no battery breaks even at 11.6 years. With a battery assuming 30% utilisation it is at 12.5 years. (or 15 and 17 if prices went back to 20p per kWh).

So i'm trying to work this out, I want a battery but i don't want to spend money just for the pleasure of having it if it'll never break even. So I need someone to check my workings, and help me explore the options and any others that I haven't thought of:

1. Solar and never battery (£8k)
2. Solar and hybrid inverter but not battery now (£8.5k)
3. Solar and 6.5kWh battery now (£11.5k)
4. Solar and <battery I buy separately and configure myself> now (???)
5. Other

*I'm not sure about the 6kW inverter, with a S/W split then it probably works out fine and i'll only experience meaningful clipping when i'm exporting to the grid, or as i like to think of it 'subsidising Bulb' so any lost power then i can live without.

EcoScruples

Where did you get your utilisation figures from?
What did the installer give you as a payback period As they will factor in SEG payments too.


I use 4200kw a year, my system cost £10,520 and I'm now looking at just under 6 years payback with the prices from October and beyond.

If you have a battery then you'll use much more of the power you generate.

ABrass

The installer assumed 100% utilisation (albeit with a 2x bigger battery). I had said I was a high electricity user but I'm not that high a user and the battery wasn't that big.

That's never going to happen as in the summer my panels could easily produce 40kWh on a good day, when I am expecting to use 13kWh on average a day. On those days it would be impossible to get higher than 31% utilisation. At least 550kWh would either be exported or lost in the sunniest month. In the winter with a battery I'll use every watt, without I'd probably still use nearly every watt.

So I picked a realistic (maybe edging to pessimistic) utilisation based on browsing the forum here.

Saving £1750 a year on 4200kWh usage seems too good to be true. But congratulations if it really is delivering that.

ABrass

I did a theoretical check of PVGIS against last year's electricity usage and I would have generated more in March through October than I would have used. Assuming my battery was able to smooth out use over an entire month I'd only manage a maximum utilisation of 60%. In reality it wouldn't get close to that even with a larger battery.

EcoScruples

There are people on here with far more detailed stats going back over several years than me. Mine are just projections and maybe aren't going to come to fruition.
I'm only 2 months in and with my battery I'm using 98% of what I capture, without a battery that would likely be 40% of what I generate. 
I am also using around 400kw since my panel install as I'm heating my water now with electric.
My yearly bill will be roughly £2,500 a year for electric alone where it was based on £600 when they were installed.
Just remember to factor in what you'll earn selling to the grid. As well as it's quite a number especially with your potential 8kw set up.

QrizB

Both those prices - for panels, and for battery - look pretty good to me.

ABrass said:

I work from home and use a minimum of 400kWh a month during the summer and 650kWh during the winter. No EV and probably none for 5 years or so. I've checked PVGIS and i'd probably getting between 200kWh a month over winter and up to around 950kWh in the peak of the summer.
Assuming 20% utilisation and at 32p per kWh, no battery breaks even at 11.6 years. With a battery assuming 30% utilisation it is at 12.5 years. (or 15 and 17 if prices went back to 20p per kWh).

I think your utilisation numbers are a bit low, especially for the with-battery case.

You've got a bigger array than me, but also a bigger demand. This year is turning out a bit better than average but I've only had nine days since the stary of April where I've made less than 2kWh/kWp; for you, that would mean generating 16kWh or more for all bar nine days. If your 6.5kWh battery will see you through the night (and I don't know how your energy use varies through the day), you'd oinly have had nine days in that period where you'd need to buy any electricity at all from the grid. That's 2400kWh used during the sunny half of the year.

During the dark six months, you should be able to use most of what you generate. That's another 1200kWh (or more). 3600kWh is 45% of your generation for the year.

Magnitio

ABrass said:

I did a theoretical check of PVGIS against last year's electricity usage and I would have generated more in March through October than I would have used. Assuming my battery was able to smooth out use over an entire month I'd only manage a maximum utilisation of 60%. In reality it wouldn't get close to that even with a larger battery.

What assumption have you made for the rate you receive for exporting excess energy? Getting a good rate makes a significant difference. For this month using Agile Outgoing Octopus, my exports have averaged 25p/kWh and an income of over £150 which is certainly helping with the ROI.

As mentioned above, the price you've been quoted looks very good in the current market. Although I'm happy that I have a battery and the backup it provides in the event of a power outage, it's not yet clear whether it was a sensible decision financially.

ABrass

At the moment I'm assuming 5p per kWh. The Agile tariffs do seem to make sense, but they complicate the calculations a lot.

If I can switch to one and improve the financial return the  great, but I think I need it to make sense on a conventional plan to go ahead.

1961Nick

It looks as if gas is going to cost over 10p/kWh from October so diverting surplus solar to DHW during the summer is another possibility if you have a cylinder or thermal store?

Off peak battering charging during the winter is another potential earner.

ABrass

We're on a combi boiler so nothing useful there for us.

Reed_Richards

QrizB said:If your 6.5kWh battery will see you through the night (and I don't know how your energy use varies through the day), you'd oinly have had nine days in that period where you'd need to buy any electricity at all from the grid. 

My 6.5 kW battery has it's output limited to around 2.5 kW.  I've come to believe that this is a bit lower than it should be (it's controlled by my inverter) but nonetheless there may be times (cooking?) when your demand for electricity exceeds what your battery can supply and you have to buy extra from the grid, even though you don't drain your battery overnight.