Solar: With an Import/Export tariff difference of only 7p/unit, does it make sense to get a battery?

mikecrum

Please can I test my thinking with you?
I've tried reading the recent article in MSE and this forum.

We have had solar panels for 2.5 years now, average production 5-6 MWh pa.
Somehow we consume 4MWh pa despite having no electrical heating - but just about to go ASHP

My current thinking is that even if we could turn all of current import from grid into drawing from battery, it would save £200 pa, and if you limited to that to excess production each month it is more like £100. So with batteries costing £4-8k and lasting 10-15 years that doesn't stack up.
This small saving is due to the energy cap (Octopus 22p/unit import, 15p/unit export)

So whilst it is annoying paying slightly more without the battery, the big up-front of the battery makes it not worth it.

Any thoughts? Not least whether 4MWh pa makes sense when heating/water is not on electricity (oil) - how can I find the culprit devices?

tim_p

4MWh does sound high, very high. We’ve had solar (4kWp - 2014 & 4.92kWp since 2023) and our total annual elec consumption is around 1.2MWh. Doubling the battery size earlier this year should, I’m hoping, see that go down further. You can probably halve that 4-8K for batteries, prices are dropping. I’ve no regrets about mine, importing virtually no electric between Mar and mid-Sept. 

Might be a good idea to wander around the house / garage / stables etc and take an inventory of what’s in use. Immersion heater left on? Panel heater somewhere? 100w incandescent bulb left on in loft?

mikecrum

tim_p said:

4MWh does sound high, very high. We’ve had solar (4kWp - 2014 & 4.92kWp since 2023) and our total annual elec consumption is around 1.2MWh. Doubling the battery size earlier this year should, I’m hoping, see that go down further. You can probably halve that 4-8K for batteries, prices are dropping. I’ve no regrets about mine, importing virtually no electric between Mar and mid-Sept. 

Might be a good idea to wander around the house / garage / stables etc and take an inventory of what’s in use. Immersion heater left on? Panel heater somewhere? 100w incandescent bulb left on in loft?

Thank you. Stables no, but we do have a Klargester.

I reckon there is base load of about 250W (2.2 Mwh pa) before we turn kettle etc on. The Klargester pump is rated at 50W, so there is a fifth of it. I'll do the walk round with mySolarEdge monitoring consumption and turning things off.

But I am not sure getting base load down affects the argument for a battery too much, if anything maybe using less electricity means less opportunity for savings from a battery. I think the situation would be very different if the import/export gap were wider. But I will see what difference the ASHP makes, which presumably wants to run more when we are not producing.

matt_drummer

You are looking at this the wrong way, as many people do.

The idea with batteries is to store electricity at the cheapest cost possible for use when you need it.

Ideally the batteries supply all of your electricity demand at the lowest rate you can get.

Your base load does matter, as does everything else. 

The longer you can keep your consumption within the limit the the battery's inverter can supply the less you will need to import.

The lower your electricity usage, the smaller the battery capacity you need to supply all of your usage at the lowest rate possible.

You are already paying virtually double what you need to with a heat pump at 22p per kWh 

mmmmikey

Hi, if you have a battery system and combine it with a time of use tariff, then you can use it to:

(a) reduce the cost of your imports by buying electricity when it's cheap and storing it in the battery whenever it's convenient

(b) increase the income from your exports, by storing your solar generated energy in the battery and selling at during the evening peak when export rates are much higher

This changes the economics significantly. For example, over the last 90 days on Octopus Flux in the East Midlands, I've paid an average of 14.71p/kWh for import and received 17.85p/kWh for export. This is just an example but you can work out what it would mean for you by looking at how much you import and export, and when. The size of your battery and the time you use energy both limit the extent to which you can do this. It's probably harder to estimate how much you'll save than actually do what you need to do to make the most of the battery.

You also have to bear in mind that the battery is a long term investment and polish up your crystal ball before deciding what to do. Given the rapid growth in the amount of solar energy being generated (very obvious if you live in Lincolnshire) it seems likely to me that the value of solar energy will go down significantly when the sun is shining so the opportunity to make more by selling it at other times is likely to increase. But that's just my crystal ball, when you look into yours it may tell you something different.

My gut feeling is that a battery is still a sound investment (I upgraded mine earlier in the year). You may not make a fortune but are unlikely to lose anything, and the more self-sufficient you become the better placed you will be if/when the next energy price shock comes along. It's a bit uncomfortable saying this knowing how much of a challenge energy prices have been for a huge number of people in recent years, but the truth of it is the worse the general situation became the better it became for many of us with solar panels and batteries.

So not really answering your question but hopefully gives you another way of looking at it and something to think about?

mikecrum

matt_drummer said:

You are looking at this the wrong way, as many people do.

The idea with batteries is to store electricity at the cheapest cost possible for use when you need it.

Ideally the batteries supply all of your electricity demand at the lowest rate you can get.

Your base load does matter, as does everything else. 

The longer you can keep your consumption within the limit the the battery's inverter can supply the less you will need to import.

The lower your electricity usage, the smaller the battery capacity you need to supply all of your usage at the lowest rate possible.

You are already paying virtually double what you need to with a heat pump at 22p per kWh 

Thanks.
Are you saying the key thing is: that with a battery I can buy at a night-time rate and consume during the day? I will admit I have been mainly thinking of using a battery for what the solar panel produce.

mikecrum

mmmmikey said:

Hi, if you have a battery system and combine it with a time of use tariff, then you can use it to:

(a) reduce the cost of your imports by buying electricity when it's cheap and storing it in the battery whenever it's convenient

(b) increase the income from your exports, by storing your solar generated energy in the battery and selling at during the evening peak when export rates are much higher

This changes the economics significantly. For example, over the last 90 days on Octopus Flux in the East Midlands, I've paid an average of 14.71p/kWh for import and received 17.85p/kWh for export. This is just an example but you can work out what it would mean for you by looking at how much you import and export, and when. The size of your battery and the time you use energy both limit the extent to which you can do this. It's probably harder to estimate how much you'll save than actually do what you need to do to make the most of the battery.

You also have to bear in mind that the battery is a long term investment and polish up your crystal ball before deciding what to do. Given the rapid growth in the amount of solar energy being generated (very obvious if you live in Lincolnshire) it seems likely to me that the value of solar energy will go down significantly when the sun is shining so the opportunity to make more by selling it at other times is likely to increase. But that's just my crystal ball, when you look into yours it may tell you something different.

My gut feeling is that a battery is still a sound investment (I upgraded mine earlier in the year). You may not make a fortune but are unlikely to lose anything, and the more self-sufficient you become the better placed you will be if/when the next energy price shock comes along. It's a bit uncomfortable saying this knowing how much of a challenge energy prices have been for a huge number of people in recent years, but the truth of it is the worse the general situation became the better it became for many of us with solar panels and batteries.

So not really answering your question but hopefully gives you another way of looking at it and something to think about?

Thanks. That line of argument about timing of export is new (ish) to me but makes sense.
And yes, we have solar farms going up here in Buckinghamshire as well.

matt_drummer

mikecrum said:

matt_drummer said:

You are looking at this the wrong way, as many people do.

The idea with batteries is to store electricity at the cheapest cost possible for use when you need it.

Ideally the batteries supply all of your electricity demand at the lowest rate you can get.

Your base load does matter, as does everything else. 

The longer you can keep your consumption within the limit the the battery's inverter can supply the less you will need to import.

The lower your electricity usage, the smaller the battery capacity you need to supply all of your usage at the lowest rate possible.

You are already paying virtually double what you need to with a heat pump at 22p per kWh 

Thanks.
Are you saying the key thing is: that with a battery I can buy at a night-time rate and consume during the day? I will admit I have been mainly thinking of using a battery for what the solar panel produce.

Cheap isn't necessarily night time, but yes, charge batteries at the cheapest times.

Your solar production isn't always the cheapest. You have to take account of charging losses but on some tariffs you can buy at 7.5p or less. Charge the batteries and run the home with this and then export your solar at 15p

You need to work it out yourself based on how much you use and when.

There isn't a set way that will suit everybody.

It is still best to use as little electricity as possible, that will always be cheapest.

Qyburn

mmmmikey said:

(b) increase the income from your exports, by storing your solar generated energy in the battery and selling at during the evening peak when export rates are much higher.

I was thinking exactly that for myself but with the recent changes in Flux rates I'm not sure I can make it work in the Summer. To break even against the fixed 15p I would need 22% of our export to be at peak rates, and there still needs to be enough left in the battery to last until the cheap import time. 

mmmmikey

Qyburn said:

mmmmikey said:

(b) increase the income from your exports, by storing your solar generated energy in the battery and selling at during the evening peak when export rates are much higher.

I was thinking exactly that for myself but with the recent changes in Flux rates I'm not sure I can make it work in the Summer. To break even against the fixed 15p I would need 22% of our export to be at peak rates, and there still needs to be enough left in the battery to last until the cheap import time. 

Yes - I'd agree that the best way to do things depends on a number of factors and what works for me wouldn't be the best for everyone. I've decided to stick with Flux for a bit because it's very easy to work with in terms of timings and the potential savings to be had through, for example, Agile and the 15p/kWh make it hardly worth the effort. I'm monitoring it all the time though.

tim_p

I’m filling batteries overnight at 7p / kWh then using all day ready to fill again at night. EV gets charged every few days at the same bargain 7p. Works for me, particularly through the winter months.