Battery system for Solar PV

TrickyDicky101

Hi everyone

Today I was visited by an outfit trying to flog a battery system for my solar PV (3.5kW system installed in 2011).

Fortunately, I had my relevant numbers in mind (c.3200 kWh annual generation, current electricity costs, broad split of generation across the year, electricity consumption).

I knew I was in for an amazing deal when the chap quoted me annual savings - using their battery system - of £1980 per year. Given my entire electricity bill is currently no more than £450 pa the batteries must literally be the alchemist's dream!

It got even better though - I know that for approximately 6 months of the year my generation is, on average, c.4.5kWh per day (this is only an average - so some days less, others more). Size of the batteries they wanted to install was 6.5kWh. However, battery chap assured me that it would take "no longer than an hour to completely fill the battery even in winter!". Bad news for the physicists amongst you as we may need to revisit the first law of thermodynamics.

At this point I had tired of the sell by someone either so clearly prepared to lie or so stupid as to not to know any better and asked him to leave.

The system cost was £4k in case anyone wants to know.

I do feel sorry for some of my neighbours who I fear may actually fall for the patter and end up splashing out cash that they might ill afford in the hope of making savings that in all likelihood will fail to materialise.

Martyn1981

Wow. Nice post.

So savings equate to avoiding around 16,000 to 20,000 kWh of import! Shirley that's lifetime savings?

Yet if the whole 6.5kWh is useable, that comes to 2,372.5kWh per year. Technically you could process more using repeated mini cycles through the day, then 6.5kWh for the evening, night and morning, but that's a lot of consumption, and the theory is very unlikely.

Still £4k isn't a bad price for 6.5kWh installed ...... well, it's a terrible price ...... but not bad at the moment considering some of the other figures out there.

At least we have a figure on the scoreboard now, so your time wasn't all wasted.

Mart.

PS. Are you suggesting your spare winter generation isn't 6.5kW? Mine is ..... if you move the decimal over one.

TrickyDicky101

Hi Martyn

In many ways it is a shame as I am genuinely interested in a storage solution.

Quite possibly I might also be interested in using the surplus to power an electric car but, right now, the economics simply don't make sense financially.

I am extremely grateful to this board as I read up some of the storage posts in advance of this visit so I had a good idea what I needed to know.

I was surprised to find my electricity costs per year were so reasonable - given I think the savings the PV bring are no more than £150 to absolute max £200 pa I think the £400-450 pa I actually suffer is pretty low. Maybe I should stop going round the house insisting kids and wife turn the lights off when not in use all the time?!

Alan_Brown

I'm keeping a close eye on batteries as I'd love to get off grid electricity. I was looking at the Power vault, which has a max of 6kwh. With the batteries not being allowed to drain down past 25%, that takes the capacity down to 4.5kwh. At an average of 1.5kwh consumption per hour, that provides 3 hours of power once the sun goes in. In summer time with long days, it might be doable to get from dusk to dawn, especially with some early nights. It's definitely not going to be possible in winter or even the shoulder months.

I'm still interested though, especially if capacities rise as costs fall, so in the meantime I'm looking at ways to eek out that limited power as much as possible. The electric kettle, toaster, tumble dryer will have to be replaced by alternatives, so too will the cooker (electric ovens and induction hob).

The other electrical items we have are media equipment (TV, surround sound, stereo, ipads, laptops and iphones), lights, fridge, freezer, microwave, washing machine).

We can fit more energy efficient appliances, but unless we are prepared to go back to the 18th century, I don't know how we can reduce our energy seage to the point when solar & batteries alone will fill the gap?

Any suggestions?

Martyn1981

Alan_Brown wrote: »

We can fit more energy efficient appliances, but unless we are prepared to go back to the 18th century, I don't know how we can reduce our energy seage to the point when solar & batteries alone will fill the gap?

Any suggestions?

I don't think it's possible in the UK, unless you have a load of PV and batts (plus a back up gennie).

Looking at PVGIS for Spain, you can see that 5kWp of PV will give around 7,500kWh pa, and 450 to 850 per month. A pitch of 50d can narrow that spread a bit more to about 500-750 pm. that would probably suit most households, combined with an appropriately sized batt to cover a few bad days. Plus again a gennie for exceptional periods, but use will probably be very low.

In the UK, we need wind in the mix, but domestic scale wind is nearly useless unless you can mount it high enough and there are no obstructions to stir up the air within 100-200m in the direction the wind usually blows from in the winter.

We do have a good grid here, so you could invest in a large scale, or community wind turbine/farm. You wouldn't be off-grid, but get the investment size right, and you might effectively be exporting all year round, rather than just being a net exporter from PV.

The trouble with small scale wind is that it suffers badly from economies of scale, whereas PV is nearly immune (though economies of volume are important).

Mart.