Adding batteries to existing solar system 2 April 2021 at 7:11AM in Green & ethical MoneySaving 90 replies 5.4K views

nish2020

I have a 4kw solar PV system thats about 10 years old and was toying with the idea of adding battery storage to it.  I have a quote for a 4.5kw battery and hybrid inverter £5500. The company say that using it I will pretty much not use any electricity from the grid.  Seems a bit far fetched to me.  What kind of saving on my electric bill can I expect with a battery system?

EricMears

Guess that depends on how proactive you are with 'making hay while sun shines' (or at least running your washing machine etc then) !

If you have a smart meter (and thorough records) you should be able to find out how much you've exported in the past and that multiplied by your import rate would give your saving (or perhaps reduce the saving figure by e.g. 20% to allow for conversion losses).

If it's any help,  I exported 60% of my generation in August but only 25% in December.  That was with trying to shift as much as possible of the heavy usage to sunny periods but without automatic diversion to immersion heater etc.  Absolutely no idea how typical that is.

Reed_Richards

A battery will not normally discharge below 10% so a 4.5 kWh battery will give you 4.05 kWh of use on a charge.  During the darkest months of the year you won't get enough sun to fully charge your battery (well I don't).  In summer you will be able to charge your battery, use some charge, then recharge it.  Let's be over-optimistic and assume you get an average 4.05 kWh of use every day, so that is about 1480 kWh.  So the cost of 1480 kWh is what you might hope to save in a year.  But you'll still end up paying for electricity in winter, even if you buy less than 1480 kWh per year.  And your battery won't give you enough power to boil a kettle on its own so "not using any electricity from the grid" is rubbish.  

Solarchaser

How practical is it that you won't buy electricity again, its not, not at all. 
Given the company is feeding you BS right out the gate, id say avoid on principle. 

2ndly as you will be on a very good FIT rate, you wouldn't really want a hybrid inverter as you will lose some of your FIT with every battery cycle, and then wouldn't be able to use a time of use tarrif to help make the batteries make sense, you would be far better with a stand alone battery inverter in your circumstances. 

3rdly the quote seems very poor value, id expect the inverter to be under £1000 and the batteries to be under £2000, you are very very unlikely to have the proposed system ever pay tor itself.
At the most it can save you 60p a day (4kw @15ppkwh) so assuming you can save that every day from electric (you can't, not a chance in winter) then it would take over 23 years to recover the cost.

By quick comparison a quick eBay search gives a standalone system with 4.8kwh batteries (4kw useable) for £2200
https://www.ebay.co.uk/itm/264235136644

Reed_Richards

Solarchaser said:

2ndly as you will be on a very good FIT rate, you wouldn't really want a hybrid inverter as you will lose some of your FIT with every battery cycle, and then wouldn't be able to use a time of use tarrif to help make the batteries make sense, you would be far better with a stand alone battery inverter in your circumstances. 

I don't understand this bit because it does not apply to me; I only have the one inverter, my battery has no impact on my FIT.  But my battery is on the DC side of the inverter, along with the solar panels.  Is that not the norm?  

Ectophile

I have 3.4kWp of panels and a tiny 3.3kWh (3kWh usable) battery.  Throughout spring and summer, my usage of grid electricity drops to less than 1kWh per day.  The standing charge is more than the cost of the units I use.

But I am a light user, and I try to use more electricity during the day, especially on sunny days.  The battery needs to be matched to how much electricity you will use in an evening.

Of course, through the autumn and winter, the generation can be very patchy.  Sometimes the panels only generate enough to cover the base load in the house, and the battery never even gets to charge up.

Whether you will ever recover the cost of installing the system is another matter.  I needed to get a new inverter as the old one had failed.  If you're throwing away a working inverter to get a new one with batteries, then the costs may never work out in your favour over the expected life of the battery.

For the first time last week, I got to use "islanded" mode.  I was working from home, and suddenly there was a power cut.  When it became obvious it wasn't a momentary outage, I crawled into the meter cupboard and hit the switch to go off-grid.  But it's not often you get a power cut on a sunny day.

Ectophile

Reed_Richards said:

Solarchaser said:

2ndly as you will be on a very good FIT rate, you wouldn't really want a hybrid inverter as you will lose some of your FIT with every battery cycle, and then wouldn't be able to use a time of use tarrif to help make the batteries make sense, you would be far better with a stand alone battery inverter in your circumstances. 

I don't understand this bit because it does not apply to me; I only have the one inverter, my battery has no impact on my FIT.  But my battery is on the DC side of the inverter, along with the solar panels.  Is that not the norm?  

Charging a battery is never 100% efficient.  So if you use 1kWh to charge up the battery, you may only get 900Wh back out again.  That's 100Wh of power that never gets to go through your generation meter, so you don't get the FIT for it.

Exiled_Tyke

Solarchaser's thought  are the same as mine: 

1. They've lied to you so stay well away. 
2. The cost is far too expensive. 
3. Even if you get a great deal, a battery still won't pay for itself currently. 

nish2020

Reed_Richards said:

Solarchaser said:

2ndly as you will be on a very good FIT rate, you wouldn't really want a hybrid inverter as you will lose some of your FIT with every battery cycle, and then wouldn't be able to use a time of use tarrif to help make the batteries make sense, you would be far better with a stand alone battery inverter in your circumstances. 

I don't understand this bit because it does not apply to me; I only have the one inverter, my battery has no impact on my FIT.  But my battery is on the DC side of the inverter, along with the solar panels.  Is that not the norm?  

I think that just means PV generation is flowing into the battery rather than through the Solar Meter.

nish2020

Thanks for the replies everyone, I kind of thought it would be to good to be true, and thinking about it the batteries are only ever going to be any use for six months of the year. Thanks helping me make up my mind to avoid this option!!

EricMears

Reed_Richards said:
I don't understand this bit because it does not apply to me; I only have the one inverter, my battery has no impact on my FIT.  But my battery is on the DC side of the inverter, along with the solar panels.  Is that not the norm?  

Not sure if it's the norm or not but if you're charging batteries from DC that portion of the energy won't reach the generation meter so you won't get a FIT payment for it.