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A question about solar panels and battery storage

Pitlamp

Pitlamp

Well-known member
I keep wondering about investing (ouch!) in battery storage and some panels. I'm starting from scratch here (knowing very little on the subject) but no doubt there are folk on here who are well clued upl. So forgive me if this has nothing to do with caving (other than a tenuous connection, perhaps, to clubs which have hostels and may also be thinking about going in this direction).

Anyway, here in the Dales we get power cuts and isolated communities are the lowest priority for being reconnected after severe weather. (There were some houses near me which didn't get their sparks back on for a week after Storm Arwen, for example.)
So, here goes; in a power cut:

1. Is it possible to energise normal household electrics direct from battery storage?

2. Is it possible to energise normal household electrics direct from solar panels? (Obviously not at night!)

3. Does any extra equipment need to be installed to do this?

4. What sort of cost implications does this extra equipment bring?

I'd be really grateful for anyone replying who can point me in the right direction.
 
We invested in solar panels 3 years ago and they have been excellent. Even the sleepy Cotswolds gets regular and unpredictable power cuts.

To answer your questions in (hopefully) one go.

1. You will need a battery and an inverter as well as the solar panels. Obviously the battery stores excess solar energy generated and the inverter converts from DC to AC for use in the house.
2. In the event of a power cut the inverter can be configured to supply the power to house. However, this power isn't infinite and a full household usage is likely to drain the battery quickly.
There are two key elements here:
i) Ensure that there is always minimum battery available for mains failure (rather than general use). We set the battery level at 20% in summer and 30% in winter. This means that for normal (non-power cut usage) we can't access the remaining 20%/30% which is used in the event of mains failure.
ii) Use one or two critical circuits on the mains failure system. In our case these are the socket circuits that power the fridge and the oil boiler starter/pump - so we keep our food fresh and can always have hot water.
3. We get a glut of power on sunny/clear days which means that if we are not using the electricity in the house the battery charges quickly and we end up sending excess back to the grid. Note the feed-in tariff is dreadful and you need a SMART meter.
4. For us, it really works as we run 2 x EV's (our only cars), so we end up with "free" mileage in summer and can ensure nothing ever gets sent back to the grid. I've calculated that having the EV's will reduce our pay-back time by half.
5. It is the batteries and inverter that add significantly to the cost.
As a guide. We have 4.2kW of solar panels which generate well into the mid 20kWh each summer day and full yearly generation is generally about 3.8MWh on largely south facing aspect.
 
We have 11 panels and a Tesla battery. The whole set up integrates with our mains input, we don't, normally, have to do anything. We set the battery minimum discharge level at 15%. In the weather at the moment the house uses power from the battery overnight, the fridge and freezers lower the battery level to about 80/85%, so next morning the panels recharge the battery and then feed the surplus power to the grid. We are with Octopus and get 0.15p per kw exported. At any time we use the cooker (electric oven), washing machine, tumble drier the power comes either from the panels or the battery. All automatic.
In winter the panels generate very little, so the battery stays at 15% unless we raise the threshold. If there is a power cut the battery automatically cuts in. We have never had this happen but we reckon that, with a full battery, we could go 2 to 3 days off grid without cutting back on usage much. For our 4 bed detached, modernish house (1985) our electricity costs us about £50 a month. We've earned about £300 in exported payments so far this year, but it has been a sunny summer. This offsets our energy bills. We reckon that we will break even after 10/11 years, but for us it's not just about that.
 
Reckon on the average daily solar yield in midsummer being around ten times what you get in mid winter. For sizing battery capacity, work out what your typical daily consumption is and how much you can reduce it too. Big consumers are fridges and freezers, assuming you aren't running heat pumps. Electric cooking, anything that heats or cools. Charging an electric car would have to stop while off grid in winter. Everything else, lights tele etc is tiny in comparison. That will give you an idea of how many days you can run without the grid and what you should turn off to keep, say a freezer full of food safe if it looks like the grid people aren't going to get you hooked up again soon.
Lithium batteries make home storage practical in a way that lead acids were not. A game changer. The prices are dropping rapidly. Solar panel prices have already dropped by an order of magnitude over the last fifteen years.
My experience of this is on boats, not houses and I have fitted several systems, so can't comment on specific house equipment, but the principals are the same.
 
To answer your specific questions: yes, yes, yes, probably in the order of £1000.

We've had 3.6kW of panels since 2011, but no battery as they weren't well developed then. I'm looking at adding more panels and a battery.

Our panels don't work in a power cut because the inverter only works in grid tied mode, and they'd be at risk of feeding back into the grid, making it live while people are repairing it. To use panels in stand alone more you need a changeover switch, which has to be hefty enough to carry the maximum current the house might use, and the inverter has to be capable of creating it's own regulated AC sine wave without having the grid to sync to. Most inverters can be set up like that and my impression is it might cost an extra £1000 for the kit and the installation.

The panels generate DC and the battery stores DC, so it can be more efficient to have the battery on the DC side of the inverter, which is then called a hybrid inverter as it's inverting the supply from the panels and the battery. It's also possible for the battery to have it's own inverter and rectifier making it independent of the panels. That's probably what I'd have, to work with my existing panels as well, but I don't see why a new system shouldn't all be on the DC side.

A solar inverter actually does a lot more than invert DC. It also manages the current and voltage through the panels to optimise power; current heats up the panels and if they get too hot they lose efficiency, but voltage also needs to be managed before it gets inverted.

I haven't got into this yet but my understanding (from the posts above and from my brother who has recently had such a set up) the circuits that you can run from battery or panels won't necessarily include everything. Maybe just some of the circuits in the consumer unit, or maybe just a couple of sockets on the battery.
 
Thanks; I've learned loads already!

Being a potholer I can cope with no mains power except for one thing; I want to be able to run the boiler on low heat so pipes don't freeze. That's my primary wish.

But - eventually it looks like we're all going to have to use electric cars so I'm also thinking ahead. Comments above about free motoring for part of the year definitely appeal!
 
Don't waste money charging your car from your panels. I get 15p per KWH selling energy to Octopus. Their night time car charging rate is currently 7p per KWH (you have to have an electric car to get this rate). You can also fill your battery overnight at 7p per KWH and sell any surplus back to Octopus at 15p per KWH during the day. This is hypothetical as I don't have the car rate yet. but I do get 15p per KWH for selling to Octopus.

My electric is 23p per KWH whereas my gas is 6p. If you have gas it is better to heat your hot water using it and sell the electricity.

Occasionally you will be offered free electric, time to fill the battery and heat your water.

For your battery to run your house when there is a power cut you need a special isolator fitted. You wouldn't want to accidently energise the grid while someone is mending it. My battery has 2 x 3 pin plug sockets attached so you can plug stuff into it in a power cut.
 
Aha - so you can just run an extension lead from the battery to the boiler plug?

Is it a Tesla battery? I'm told the 3 is better than the 2. (Same capacity but greater draw possible.)

I've been an Octopus customer for quite some time now. They're one of the best companies (of any kind) I've ever dealt with.
 
I'm hoping this conversation proves useful to other forum members, not just me.

Incidentally, if a certain person from Bentham is considering chipping in for my benefit; don't worry, as I'll pick your brains next time paths cross!
 
Another point, if you can install a system yourself this would save prob well over 50% in costs or if you know an electrician personally who is willing to have a go, will also save a fair bit. The electrical side is pretty easy, the hard part is the donkey work of fitting the mounts and getting 27kg panels up on the roof, this is actually were a caver work party would be very handy!

I installed mine a few years back and managed to bypass the Microgeneration Certification Scheme (MCS) and get an import tarrif with octopus. I installed a 3.2kw system with 6 panels, grid tied, no battery. Since Nov 23, I've generated 4.8MWhrs, this is on a south facing roof that only has a 11deg slope, so not ideal, but still impressive. As wellyjen says, I have logged about 10% of solar panel wattage in winter on average, but if the sun shines, it ramps right up as well in winter - can get +2kw.

I only got round to doing the feed in this spring and that is a big factor of making a system pay for itself. In terms of getting accepted for a feed in tarrif you can only use octopus doing it this way as they have a scheme that does not require a MCS cert.

- Getting a DNO was easy, just simple form filling, import thing here is to be below 3.86kw odd as above that there are a few more technicalities. DNO is about notifing your electricity network supplier so they are aware you will be feeding into the grid and them accepting it.

- I got a electrician I know to do the Part P on the AC side. £120.00.

- Found a friendly local solar installation company that did the Build Regs Cert for me (they could not do the MCS as I had non standard roof mounts due to importing a system for shingles from the USA, for mounting on my log cabin), they did not charge me as they were impressed with my install, but this was a fluke + being very lucky. I guess this could be in the £120 region.

- £250.00 for application to octopus, NOTE: not guaranteed acceptance, they are trialing a non MCS route so that more electricians can do installs rather than having a MCS certed company (I think they charge around £6-700 for that pleasure).
 
The big difference is between having solar power & batteries / not having solar power & batteries.

Kit is changing fast so anything a year old may be out of date.

I treated it as buying a tin of beans and it all worked out OK.

I did splash out on chicken wire round the panels to stop the pigeons nesting.

When I fitted mine having more than 5kw going into the grid caused problems. Otherwise more is better.

If you get them installed in February you'll spend the next 2 months wondering if you have made a mistake.
 
With battery storage you will need to have your own earth connection to provide you with an earth if a power cut removes the supplier's earth as well as the live.
It's usually a rod driven into the ground.
Getting a low enough resistance is often easy enough, but depends on what's under your back garden. If it's limestone a couple of feet down it might be more difficult.
 
Two things I forgot. As MrMike mentions, DNO approval: the DNO is the Distribution Network Operator, in effect they own the infrastructure that Octopus supplies you over. They will be concerned, principally, that you and your neighbours aren't going to export enough solar to destabilise the grid in your area (all to do with power factor and inertia, technical electrical stuff). Generally, up to 4kW approval is nominal, above that they might have to take a harder look.

Second thing is, any power you put into the grid will be described as export - feed in tariff was an earlier scheme, not now available, where you were paid simply for generating solar power, whether you exported it or not ("feed in" was a bad choice of name). The purpose of it was to get the market going, to break the deadlock where it was hard to get panels fitted because there were few experienced installers, and there were few installers because not many people wanted installations. There was also an idea that it would help scale up production of panels and get the price down, but that was largely pointless as the panels were all German or Chinese and the UK market made minimal difference.
 
Being in the Dales, the amount of juice you'll get is noticably lower than further south, like on Mendip. S Wales would too, except they never see the sun through the rain clouds. :)
 
John, I’m not able to answer your specific questions but here is my experience from having solar panels installed in September 2024. There are 16 panels on a west facing roof linked to an inverter and 2 batteries.

In August 2024, our estimated annual usage was 5,554kWh but now our estimated annual usage is 1,705kWh, so a saving of 3,849kWh from the Grid or about £1,014 (which might vary depending on the tariff that you are on). Sadly, it has no impact on the daily standing charge. Our last monthly bill was £1.35 for electricity and £16.63 for the standing charge (ouch!).

Since installation, about 2,980kWh have been exported to the Grid, which at about 19p per kWh, has reduced the electricity by £566. However, after the first year this will drop to around 5p per kWh. That said, the greater saving has always been from the electricity taken from the Grid.

As I understand it, the panels have an anticipated life-span of about 25 years but the inverter has an anticipated life-span of about 10 to 12 years, so that may be a cost to factor in to any calculations.
 
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