# What is the largest capacity battery I can charge from my 2.5kW solar panels?

1. Aug 7, 2017

### bob4866

I have solar panels fitted, with a capacity of 2.4 -2.5 kW. I am considering buying lithium battery storage for evening use. What would be the largest battery system I could charge from this setup?

2. Aug 7, 2017

### Staff: Mentor

Assume:
1. Your panels produce close to full power for 5 hours per day.
2. You consume zero during those 5 hours.
2.5*5 = 12.5 kWh of batteries is the largest amount you can charge.

3. Aug 7, 2017

4. Aug 9, 2017

### Rx7man

You can go larger, though you won't get them charged in a single day if you run them right empty.

5. Aug 9, 2017

### sophiecentaur

If you use much bigger capacity (but don't go wild!) and you use less than full capacity on most nights then you would have extra for rare occasions.

Note: the 'capacity' of a battery is not a well defined figure and many are advertised over-optimistically. Look at this link to see what I mean. A rule of thumb for Lead Acid Cells is that you can use only half the rated capacity is you want a good battery life. Lion cells are better behaved than that but you need to be careful with those shiny new batteries you have bought. Buying 'more than enough' could make good economic sense as it can avoid damaging them by discharging them too much.
If this is a serious exercise, a proper battery management system would be worth while considering which would keep a running total of charge / discharge and prevent damage. Much cheaper than a replacement set of batteries.

6. Aug 9, 2017

### bob4866

Thanks for the replies Rx7man and Sophiecentaur. This is a serious exercise as local sales reps use sales techniques quoting 3kWh, 7kWh and 10kWh battery packs, hence the question. I would definitely buy a management system, some of which have the option to trickle charge low batteries overnight when the cost is lower in the UK.

7. Aug 9, 2017

### sophiecentaur

There's the issue of actual kWh available every day. That can be very disappointing if you compare it with the peak kW output of your PV panels.
If you are considering co-existing with the Grid and the possibility of supplementary charging then that could well be the cheapest solution and you could live nearer the edge of performance, having your backside covered by the Electricity supplier.
I would be interested to know how you plan to use your PV energy if you are not going On Grid with it. The UK government have been talking about encouraging battery development for local storage but it's still very expensive, as far as I can see. PV was, for a time, the obvious way to go but the Feed In Tariff is rubbish now. Pity there's not more know-how in Government circles.

8. Aug 9, 2017

### Staff: Mentor

Please enlighten us. What happened to the Feed In Tariff in the UK?

9. Aug 9, 2017

### sophiecentaur

I have a friend who had the right house at the right time and the right money. The feed in tariff at the time was ludicrously high (to attract customers and to make Blair look good). My friend's FIT income pays for all his house services. And all the other Electric supplier's customers are subsidising him. Now I have an appropriate house and some cash, the deal is rubbish as the FIT is falling and falling. There seems to be no incentive any more.

10. Aug 9, 2017

### Staff: Mentor

Ah yes. Incentives are very difficult to right size when politicians get involved.

11. Aug 9, 2017

### CWatters

FIT was around £0.44 per kWH in 2010 but is now down to <£0.04 per kWH I think.
Stand rate electricity (eg to buy from the grid) is around £0.18 per kWH so storing excess and using it yourself at night saves you around £0.14 per kWH

12. Aug 9, 2017

### Rx7man

I think the most reasonable FIT would be the same as the cost to buy.. the "subsidy" would be the cost of maintaining the grid... In our case we're looking at hydro generation for about 2kW 24/7.. If the costs were symmetrical for buying and selling, I'm sure that would cover our needs quite nicely without the need to have expensive storage systems

13. Aug 9, 2017

### sophiecentaur

Once you have a reservoir of water, that's the best and cheapest storage medium.

14. Aug 9, 2017

### Rx7man

terrain doesn't allow for storage, but there is a reasonably steady flow

15. Aug 10, 2017

### sophiecentaur

Bad luck. But good luck that you actually have water flowing through!! I have never had that good fortune. From what you say, you probably have a large flow with not much head of water (?).
Water storage can be 'good value' when it's available. A tank of water, 2.5X4m and 1m deep, at a height of 10m will store about 1MJ, which is worth about 2.7kWh, which is almost half the capacity of a Tesla Powerwall Battery. There would be only one Energy Conversion involved (then there's the 80% efficiency for a small installation). Lifetime of an installation would be a lot better than Lion battery life and you would need the original generator in any case.
All those parameters to consider - no wonder people pay for some money grabbing 'consultant' to take away the brain ache.