When reading previous' PV + battery' threads the aim has been to hold enough power to last a few days in winter meaning you need >10s of kwH and it all gets rather expensive.

I was musing from a different perspective.

We are on grid.
Even during overcast winter days we are still exporting >100W through most of the day.
During this time it is the short bursts of power hungry appliances that result in us buying in electricity.

This makes me think that a relatively small capacity battery system could dramatically improve our self usage.

So does a long cycle life battery system with ~1kwH capacity and max output of ~3kW exist (if not which part of this requirement is the limiting bit).

If it exists how much would it cost and many cycles would the batteries be expected to last?

cheers

John

Posted By: jms452
This makes me think that a relatively small capacity battery system could dramatically improve our self usage.

What's your aim of doing this? Cost or being green? Batteries in a grid situation tend to fail on both counts. The grid is a greener battery than you think, it's also likely to be lower or similar costs, and it's definitely less hassle and safer.

You have to balance the storage capacity of the batteries with their power delivery. The kWh of the battery tells you very little about it.

So how do you get the UPS to provide power from the battery when there's nice mains it can provide it from instead?

Like jms452, I'm thinking cost. Even when the energy in a battery is replaced for free the cost of the cycle in terms of depreciation of the life of the battery is likely to approach, if not exceed, that of mains electricity.

<blockquote><cite>Posted By: SteamyTea</cite>Have you made a profile for your energy usage</blockquote>

I have the data (although not on me) but the salient points are.

We buy on average 3kwH/day although approximately 30% during daylight hours, 30% evening and 40% economy 7
Peak load is ~3kW
1kwH total available storage would greatly eat into the first two 30%s

I should have also said:
average generation is just over 10kwH/day.
Average export ~ 10kwH/day
Daytime/Evening electricity ~20p/kwH

<blockquote><cite>Posted By: SteamyTea</cite>Only at 100% cycling efficiency. If you took a very hopeful 80% cycle efficiency the numbers are not so good.</blockquote>

I'll have a 1.25kwH system then please.

<blockquote><cite>Posted By: SteamyTea</cite>Just knowing how much energy is needed is only a small part of it. The when is important as you need time to recharge the batteries. No good depleting them at sundown</blockquote>

I'm happy with estimating how much cash it could save the part I am struggling with is how much it will cost!

Probably over simplistically I could buy a 1.2kwH deep discharge lead acid battery for £60 which would payback in ~300 cycles or as little as six months.

It is the non battery kit I am struggling with.

Posted By: jms452: “Probably over simplistically I could buy a 1.2kwH deep discharge lead acid battery for £60 which would payback in ~300 cycles or as little as six months.”

Rule no 1: so-called deep-cycle batteries (leisure batteries) aren't. If you cycle one down to 100% DOD (depth of discharge) regularly it'll die - probably well before you get to your 300 cycles. Generally you want to keep them to 10 to 20% DOD to have any reasonable life at all.

Traction (fork-lift) type batteries are a bit more tolerant of deep discharge. Still, they like to be recharged fairly promptly. I suspect leaving them full discharged overnight regularly would be less than ideal.

In other words, with lead-acid batteries you really need a lot more nominal capacity than you actually use.

There are a few people who do this sort of thing: on-grid but with PV-charged batteries. Two blogs on the subject:

http://solarbodge.blogspot.co.uk/
http://green-power-gen.livejournal.com/165925.html

Both do it for ideological and tinkering reasons rather than because it makes immediate financial sense though I think it probably saves enough that it's a cheap hobby. In both cases they run specific appliances or circuits off the off-grid system and switch things back and forth depending on the time of year, etc.

You might like to talk to the owner of the first of those blogs (“Outtasight”) who you can track down via the last comment on his last blog post. I've had an email conversation with him about other stuff (http://edavies.me.uk/2013/07/lifepo4-charge/) and know he's been doing some commercial work in this area.

I agree, there is some significant potential for distributed power storage to shave the peaks off the grid and it would be good if people could have it at home. I just don't think the battery and other technology or the grid pricing is right for it yet - particularly not for somebody who's not very familiar with the problems.

If you were a company that used compressed air or low vacuum (and you could get rid of the leaks), then this could be used for storage.
Now there is a thought.

Posted By: jms452I think its been thought!

Yes it has, as a dedicated storage system. But if you need compressed air anyway then it becomes more viable.
But as Ed says, the domestic FITs is not set up to encourage this.

http://energy-solutions.co.uk/self-consumption-solutions/

There is a Dutch firm called NEDAP who make a battery/inverter system which can kick in to "top up" your PV supply when a bigger load is switched on - hence minimising import from the grid. It also works as a UPS in power cuts. I think there are other manufacturers too - Sunny Island?

Posted By: jms452It is the non battery kit I am struggling with.

Missed this line

What you need is some sort of controller that can divert excess PV to a storage system, then when you need more power than your PV can supply a controller to deliver from the storage
What you have there is a standard off grid system

So how about some kind of controller that can connect or disconnect the grid coming into your place when there is either not enough storage (connect to grid) or export excess when the storage is full (connect to grid). The normal state would be disconnected from grid.

I think you could make your own from a large contactor to switch the mains coming into the place. The controller would just have to sense the voltage on the storage (cuts the grid supply at over say 10.5V and reconnects it at less than 10.5V, connects to grid when over 12.5V or whatever is needed).
That can't be hard to design.

and also reconnect the grid if the load exceeds a certain, probably not very high amount

Wouldn't like to see what happens if you turn a simple kettle on - taking 3Kw load from a 12v battery is erm 250 amps, assuming 100% efficiency - not sure how quick that would melt any reasonably sized low voltage wiring

I think as stated above the only practical way to do this is run dedicated devices permanently from the UPS/battery (things with constant low load) and manually/auto switch the mains in if the battery fails

all you would then needs is a "sunny whatever" immersion controller to steal the excess energy generated to charge the UPS , and a mallet to smash the UPS "beep" alarm into a million pieces when it thinks the mains power has been lost!