Battery for solar PV - thoughts on if it's worth it?

I had PV installed in mid-August, and at the time the installer said a battery wasn’t worth it because we’re only 2.56kWp (yeh, tiny I know!). And I think he was right, since that was the Tesla Powerwall that they sold. There’s no way we would have made the money back on that.

But now I have some data from OEM, I see that We generate basically spot on the amount we use. Sure, we’re heading into winter so that will start to drop off. But still, it’s encouraging. Of course we only end up “using” roughly half of it. Some gets diverted to the HWT, but it still pains me a little that we’re not making full use of the electricity.

Some data. If I look at the entire time I’ve had OEM, which is from around August 25th 2016, I have this:
Generated: 325kWh
Total house usage: 325kWh
Diverted to immersion: 131kWh
Exported: 88kWh

Does anyone have any experience with batteries for solar PV and how much you need to be generating above your house’s base load to make it worthwhile? I’m an investor in Powervault and considered chatting to them about getting one of their units which are a lot cheaper than the Tesla, but obviously aren’t as good.

Or am I just being mad in thinking I should invest in a battery right now?

To give you a rough idea of annual solar gen, my solar is 3.5kw, 18 panel system sited in sunny Yorkshire, and over the past 12 months has generated in total 3020kWh.
Since Aug 25th, I’ve generated 508kWh.

From now until April, very little will be fed back to the grid (or available for battery storage) as I’ll be very lucky if the solar even meets my household & hot water needs.

Also depending on how your system is setup and you might also need a charging inverter with the batteries. I have a 14.6K PV system and am expecting at least 22,000 kWh in 12 month in Kennewick Washington State USA with at least 300 days of sunshine.

Still like Paul Reed in the winter months on rainy days I’m unlikely to keep up with my power demand. Lucky for me I netmeter at market rate with credits.

Now if power was sold back at wholesale diverting to battery might make sense to minimize my power bill.

I keep toying with the idea too!

We have a 4kW array that has produced a little shy of 4000kWh’s in the last 12mths. the split of imported and self-use is around 50/50 and we export around 30%. But that 30% is only during the summer months. From here through to spring/early summer there will be negligible exporting due to 2 diverters, one on DHW and the other on a couple of small electric heaters that “take the chill off” most cold days and the GCH is only needed on the coldest winter days.

So the lack of surpass in the winter months reduce the appeal even though we could alter our priorities and timings to make it more efficient, the gain would be quite minimal.

The use of an inteligent battery system can also be used to shift demand from peak pricing to off-peak pricing regardless of generated energy, but despite the off-peak electric being something like a third of the peak price it still won’t justify the outlay.

I was hoping to see other advances in this area, specifically a couple of years ago I was made aware of a concept that used “silica beads” dehumidifier type technology to make the energy storage cycle seasonal, during summer month surplus energy was used to “dry out” the filling (could be in the walls, or in the furniture or dedicated heater units etc), during the winter months the absorption of moisture generated heat, the concept sounded quite good for our climate but I never heard anything more on it, but there again I don’t frequent the “eco sites” so may just be oblivious to any advances.

Recently I had to contact a supplier for some tech info on their inverters and spotted this product on their website,

I looked at the product site ( battery storage ) I thought it looked quite good at first glance, although I haven’t looked into the details. It’s quite modular which could help spread the investment and the physical placement, apparently a master is from £1895 but I didn’t see any ref to slave pricing so couldn’t calc the ROI on any larger systems, although numbers should improve with each additional slave, I guess.

(I am not affiliated with or even recommending the product, just discussing the concept as an option)

I have a very small setup based on a few panels (still need to install the others one day)
It is based on a multiplus from Victron
They have so many options it is hard to tell what you really need but I can say I have the multiplus and you can use a little dongle to pc to reprogram it the way you want
some whitepapers

you can go from simple to expandable to huge systems … off grid, re inject or not … offload …
Have AGM batteries paired to 24V, about 600 A and I even managed to read the data from the multi-plus via node-red

As said you can start basic and grow or if you’re nerd you can go for a full set … depends on the budget :slight_smile:

Given that you have exported 88 Kwh, I assume you are grid tied with net metering. So the question is: Are batteries a better choice for energy storage than net metering?

Net metering:

Unlimited storage capacity (thousands of Kwh)
Unlimited demand capacity from storage (hundreds of Kw)
Unlimited rate capacity into storage (complete output of typical 20Kw array)
Store for months (store in summer, use it in the winter)
No maintenance.
No upfront capital expense.
No depreciation.
Still free in most states.


Pretty much the reverse of the above points.

If you could buy a storage system with the attributes of net-metering, what would you be willing to pay for it? Let’s say your state allows for a cost of net metered energy (most do not). If it’s as high as 5cents per Kwh, you would spend about $4.40 to store your 88 Kwh for months and use it anytime you want, at any demand rate you please.

If the concern is for backup in the event of power failure, you may find that that the money is better invested in integrating a backup generator for those relatively rare events.

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Unfortunately net metering is not (currently) an option for us here in the UK. even if export was metered we would get paid only a very small fraction of the import price so using the grid as a storage solution is a costly affair.

But since we tend to be paid a set 50% of our PV production rather than metering export, exporting has no impact financially where as importing is costly. The more we can self-use the better, unfortunately the way it is set up it doesn’t matter how inefficiently you self-use, if not exporting 10kWh’s can save you buying 1kWh it’s better on the pocket despite not being so good on the environment.

I’m not familiar with the UK PV incentives. Can you elaborate on how this works?

That depends on where one lives. I’m in rural Oklahoma. The power here is flaky on its own, rarely.a month goes by without it dropping off-line. (usually, for only a few seconds) Throw in a severe weather event, and it’s very likely it’ll go off-line. e.g. We had two ice storms back-to-back, 30 days apart last year. Commercial power was down for 3 days in each case.

Three years before that, an ice storm knocked out the mains from one week, to as much as 30 days, depending on where one lived.

The main issue with a generator is fuel. Or more correctly, fuel shelf life. Gasoline and Diesel both have relatively short shelf lives. That makes Propane or Natural Gas a better choice for long term fuel viability. One advantage with NG, it’s usually the last utility to go off-line in a severe weather event.

I’m not saying that a generator isn’t a good idea (I have one!) just that it, like the other solutions, has its weak points too.

So in the UK we have the Feed In Tariff and the Export Tariff. The feed in you get paid for every kwh you generate, whether you use it or export it to the grid. Then the export tariff you get paid for anything you export to the grid. Usually that is deemed to be 50% of what you generate. You don’t need to meter your export - you just get assumed you will export 50%. So if you generate 100kwh, you get paid 100 times your feed in rate, and 50 times your export rate.

The kicker is that the export rate is much less than the import rate. AND, even if you use 100% of your generated energy, you still get paid for the 50% export, even though you’re not actually doing it.

So as Paul says, to get the best economic benefit from solar PV in the UK, you should use every single unit you generate. Of course, the environmental benefit is there. And I personally am driven by that as well as the economic benefit to me. So i don’t want to do the battery thing if the battery is going to lose so much that I’m actually negating a large portion of the environmental benefit of having PV.

Hope that helps!

And thanks for all the discussion here - some interesting things to go on! I like that PowerFlow thing - I’m going to look into it.

I’d still call that relatively rare in the big picture, but if you have natural gas (I am rural and do not), a generator is not only good for grid backup, but for co-generation when there’s a heat demand.

Back to the battery issue, I think an elegant solution is what Nisan has been developing in Japan. A Leaf has something like a 36Kwh battery and the average home there uses less than half that in a day. They have bi-directional charging stations that can reverse and power the home in the event of a power failure.

I think Tesla is working on a similar concept to integrate their Solar City acquisition.

generally batteries are not worth it unless your are looking for security. depending on the application… example i have a group of lithium batteries on on a solar water pump that provides water to my cattle and two houses. . I have a couple blocking diode preventing power from the batteries powering a GTI and since the water pump does not run that much ( it pushes 3000 gallon per hour in full sun. all it does is pulse every 10 - 20 minutes for maybe 60 seconds to recharge the pressure tank. . but for me electricity is a bit unstable i can have power outages that can last days but generally hours- but in the middle of winter at -40c things freeze up fast so i need constant running water…

I also applied the same thing to my main solar panels and wind gen. . and those lithium and a small electric utility car is charge by it. - but incase of power outage it drops a couple relays and powers a specific breakers in my house. the electric utility car has 9 kw of stored power. and the lithium ones about 4 kw – instead of tesla wall you could by BYD power storage system 1/2 the cost of the tesla wall for a bigger capacity .

curious if net metering not an options . batteries are generally an expensive route unless you are considering security benefits. but if Matt is exporting 88 kw in 2 months . for all intent and purpose he also pay the Utility company to up load into the grid if net metering is not an option , his cheapest option is to simply add in a second water tank. and when the primary tank reach max temp a simple relay to switch to the secondary nothing wrong with too much hotwater. I have ~200 gallons of thermal storage. and in the the spring and fall basically act as large thermal mass keeping it quite toasty . hence why i only heat fro 3-4 months out of the year even though my climate is considerably colder ie my average temp lately is about 5c yet my house temp is 25C currently . and everyone else has either started burning wood or turned on thier heat. i will not turn on my heat until the end of Nov generally( average outside temp by then - 10c)

Just to throw in my £0.02 here…

Check what your bill breaks down as; for example, I do not have any wind / PV here, and 1/2 of my electricity bill is the standing charge (Ovo customer) - I don’t dispute the following;
Ovo are not the cheapest
Their standing charge is high, while the per unit cost is low (comparatively speaking).

So while I am seriously considering PV, PV+Storage would be costly (assuming I maintain the same supplier) considering the possible further reduction in my bill…

Hope that if of some help to someone.