UKC

Alright all,

My OH and I are having PV panels fitted at the end of the month and are due to have a battery installed at the same time. We're getting 6 panels and have signed up for a 2.6kw battery but have now been offered a 5.2kw for a moderate amount more (~£700).

My initial thought was it seems like a great deal as more = better, and allows for potential future changes but am now wondering about whether this is actually the case. My concern is if they are just trying to upsell a larger battery that we aren't really going to use, resulting in it barely being discharged and being a waste of money. 

I don't suppose anyone has any experience of calculating what a rough appropriate battery size is likely to be? I've been looking online but while there's a couple of websites they rely on postcodes in other countries to work out relevant bits like solar yield per day. I've got predicted yields for the panels and average daily electricity use which implies it could be a waste of cash but would like another opinion on my fag packet figures.

6 x 330w panels  

The panels are predicted to generate: 2,198 kWh /year so  6 kwh / day

We have been told we'll be able to use around: 1,011 kWh /year so 2.8 /day

but our average usage over last 12 months: 4.5kw / day.

Theoretically at least we should generating enough every day to cover our usage but obviously this relies on using power as it is generated. I am out most days working and OH works from home 2 days a week so most power is consumed of an evening although to what extent is difficult to know.

I could ask the company involved what they think but frankly as a business it's in their interest to flog the battery so I can imagine what their answer will be.

so, anyone got any ideas if this is a good plan or not? I await any wisdom available.

The economics of battery load shifting is soooo dependant on your usage habits and your tarrif.

Assuming a good aspect and no shading, their predicted annual power seems reasonable.  But it’s more like 12 kWH per sunny summer day, 6 kWH per shaded summer day and 0.5 kWH per claggy winter day; perhaps only assume 1/3rd of days can load shift 6 kWH per day.  As that’s something like £100 worth of electricity it hardly seems worth having a battery at all. (Edit; this “worth it” decision depends on how much you’re paying for the initial battery and what you’d do with the money otherwise).

Not convinced linking battery size to solar capacity is the key thing - if you get a provider with very cheap night rates, filling the battery at night, then using it during the day for peak load (way more than your PV can supply) and letting the PV export when you’re not consuming could be more financially beneficial with daytime export rates perhaps beating some night time import rates.  This kind of arbitrage could save you more money than shifting your solar.  But it’s subject to the whimsy and madness of the energy markets so it’s hard to predict beyond the end of the next crisis.

Just to repeat, there’s no way that solar array is going to meet your daily needs for much of the winter.  Sun lower in the sky (worse angle to panels) and shorter days, as well as lots of cloud.  Batteries can load shift a few days worth at most, a domestic seasonal battery would be stupidly expensive - at least until aluminium ion batteries are common hopefully ten years hence.

The other biggie for me - does their battery system provide an islanded microgrid mode?  This would be a mode where, if the grid fails, the batteries provide a micro grid for your house and allow the panels to feed your demand and the batteries.  Ask them if your household wiring is compatible with this.  If the system does this, I’d absolutely get a bigger battery at the price offered, if it’s lithium phosphor.

Have you predicted how long it might take to recoup the costs of the battery and installation? I have a 4 kW system and looked at batteries. It seemed like a very poor investment to me in purely financial terms.

Averages aren't too helpful in your calculation. No battery is much use when you are generating sweet FA during winter. As wintertree says maybe you can count on a third of the year when the battery/PV combo is self-sustaining.

Anything based on battery technology with a forecast payback after >5 years is a dubious investment I reckon.

Thanks for the answer and the thoughts.

"there’s no way that solar array is going to meet your daily needs for much of the winter." I'll well believe this. I was working using averages over the year as it seemed like the simplest way of just ballparking it. It was good to have pointed out again though - obv' the panels are generating least at the time of year when you're using most power and the difference will be greatest. 

"does their battery system provide an islanded microgrid mode?" This is an excellent question and tbh I just assumed that in the event of the grid going down would be the case. I'll give them a shout tomorrow although frankly I'm dubious that I'll be able to get a real answer on that one from anyone on the phone :-/

I think it's looking like we'll get a small battery (we're contracted so we'll have to now) and leave the larger one. 

Check out Octopus's Agile tariff. You can charge your battery very cheaply overnight and draw on it through the day.

"Anything based on battery technology with a forecast payback after >5 years is a dubious investment I reckon."

This was one of my concerns with building future proofing in. Buy a bigger battery now, not really need it or use it and by the time you might actually need it something better/cheaper is out there and you're stuck with old tech.

One thing is for sure -  battery tech is evolving rapidly...

My very poor understanding is that a lot of the challenge is actually in the manufacturing rather than the chemistry at this point.

I'll have a look at that. Thank you.

So we are getting a 3.8KW system installed next week with a 5.2KWh battery - we had the option of upgrading to 8.X KWh for about another 500 quid. Interestingly the guy sort of talked me out of it basically saying we'd probably not really charge it from our system.  Looking at the smart meter , we use between 8 and 12KWh a day at the moment but that should halve soon when our heating goes on and all cooking and water comes off the solid fuel. I suppose one thing we are getting is an iboost so that should feed into hot water when battery is charged so probably negates the need for a bigger battery and some would argue a battery fullstop but should hopefully work well in the summer when we need it.

A lot of the payback has been sold on the basis of using the agile tariff but looking at it at the moment the cost per KWh is very expensive all the time so doesn't quite add up. There is also the 'go' which gives you 4 hours of 5pence leccy at night. and 15pKwh at other times. I might look at this and charge the battery with the cheapo stuff in the winter. 

We didn't opt for the tech to run 'key circuits' when power goes off as it well basically rarely happens and if it becomes a feature of the UK i guess it could be retrofitted.

Thats that then I think - decision is done. Small battery or nowt!

"Interestingly the guy sort of talked me out of it basically saying we'd probably not really charge it from our system."

A completely honest salesman - capture him! We must study and clone him.

I think the problem is that he was the engineer - probably programmed to be honest.. I have also heard many good things about the company from different sources so maybe they are just an honest company? One of the reasons I went with them.

Thinking purely in financial terms: there are three things you need to consider: how much excess PV will you have each day, how much import could you save by storing and using it in a battery, and how many pence will each kWh save you?

You have a 2kW PV system.  My perfectly-south-facing 5.7kW system generates about 6000kWh every year so their estimate that you will get 2200kWh is slightly generous but certainly not ridiculous.

But it is extremely seasonal.  My daily average is 16kWh/day across the whole year... but on average this is near 30kWh in high summer and maybe 5kWh on average in January.  I have known days when the total output was less than 2kWh.  So you can probably look forward to maximum days of around 12kWh and minimum days of basically zero.

For probably a third of the year, you'll self consume almost everything that comes off the roof in the daytime, especially if you are canny about when to run the dishwasher and washing machine.  That means charging your battery from excess PV will be more or less negligible.

What do you get paid for exporting energy?  I get approx 5p/kWh.  So every kWh you save in the battery actually costs you that export payment.  Then when you use that kWh, you save the price of importing it (for me that's about 13p/kWh).  So the net benefit of cycling a kWh is around 8p.  It doesn't actually matter that your solar system is small, because you can mitigate this by switching supplier to Octopus Go and buying 5p electricity in the middle of the night.  Helpfully the 5p cost of a kWh from Octopus is identical to the 5p cost of not exporting a kWh of PV so you can pretend they are just the same.

Imagine you use your 2.6kWh battery perfectly every day for 365 days a year, filling it with PV and then using it all later.  That saves you 2.6kWh x 5p x 365 days = £47 per year.

Doubling the battery size to 5.2kWh will save you a further £47 a year (assuming you could use it perfectly every single day, which is is unlikely as you won't require a whole 5.2kWh every day, but let's run with this assumption).  The additional £700 would have a payback time of ~ 14 years.

If you do not get any payment at all for export then the figure are better because you're filling he battery for free when the sun is shining... but not very significantly.

Are you sure you're using the right firm if you think their interest is just in selling you as much as possible?

I'd worry you're being ripped off. The late upsell, the overstated generating capacity, the poor choice of battery with a very small PV system.

Back when there was feed in, people started with 4 kW. I'd have loved to go for 16kW but didn't want the extra faff with the DNO. I assumed I'd expand later and haven't. But I'd up my capacity and buy and EV before I considered a battery.

Just do some checking around before you commit.

I suspect batteries are where solar panels were 10 years ago - the cost of the battery is so high that any excess capacity is money wasted. Thus you're better to get a smaller one you use completely.  The only question mark over that is battery lifespan - if you completely flatten the battery all the time are you going to kill it prematurely, especially if it's allowed to charge up quickly and get hot?  Are you better off getting some kind of smart charger and using the battery in an EV instead?

I am definitely holding off on a battery because of this.  It seems crazy for me to spend thousands on a few kWh of dedicated battery, when I have 64kWh sitting on my driveway 90% of the time doing stuff all.  Surely tech is going to catch up with this soon enough..?

OP, are you committed to a battery of some sort?  If it's very cheap as part of the package it might make sense.

In reply generally:

I haven't double checked but we've paid deposits which I believe normally constitutes a contract (but I may well be wrong). I have to ring them tomorrow so I'll ask out of curiosity

The whole deal was organised as a bulk buy with associated discounts via Devon County Council so I trust the company is legitimate. The deal compared to other  we've been given is pretty favourable so we're happy on that front. Compared to other similar setups the cost is quite a lot better (1/3 off if memory serves) due to economies of scale. I am, however, old enough and cynical enough to wonder when someone tries to upsell you just about anything whether they've considered if you actually need it or not. 

As to using an electric vehicle I'd love to but I think currently the cost of those make it a pipe dream. 

This thread is giving me slight battery buying regret. Although i still like the idea of using more of our own generation so I can probably justify the cost as the whole investment was never really about payback time anyway. However I do think the cost of batteries will come down and so holding off going big does seem sensible. It reminds me of when i was at uni they had a grant to buy processors to run a model to basically fine tune structure of earth. It was quicker to wait a year and then buy the processors which were getting faster per £ than if they had just started the model on current technology. At some point you have to bite i guess and i suspect 'recycling electric car batteries' or hook up to electric cars is the key.