Hybrid inverter or AC coupled on Solar/Home Battery system

QrizB

Turnipfarmer said:

Do you think going for smaller battery might be better if not getting full cycle charge at certain times of year on the larger battery or is that bit of false economy?

All other things being equal, a smaller battery that you cycle every day will pay back faster than a bigger one you cycle every other day.

Imagine for a moment that 1kWh of battery costs £500, 1kWh of electricity costs 18p and there are no other marginal costs. The battery costs the same amount as 2800kWh of electricity, so you will need to cycle it 2800 times for pay back. That's every day for 7 years 8 months, or every other day for 15 years 4 months.

It's also important to size the battery to match the array and your likely consumption.

Real world figures: I have a 2.72kWp array but no battery [yet]. During January I generated about 70kWh but bought 400kWh from the grid. In June I generated 300kWh but bought 100kWh from the grid. I could've used everything I generated in January, in June I was only importing 3-4kWh per day.

If I'd had a 3-4kWh battery I could have stored pretty much everything I generated in December, and still have used most or all of that capacity overnight in June.

If I'd had an 8-10kWh battery it would be mostly empty throughout December, and mostly full throughout June. The extra capacity would rarely be used.

In your particular case, it might be worth considering a relatively small battery now, but expanding it if/when you get an EV (or heat pump, or hot tub, or whatever).

Turnipfarmer

QrizB said:

Turnipfarmer said:

Do you think going for smaller battery might be better if not getting full cycle charge at certain times of year on the larger battery or is that bit of false economy?

All other things being equal, a smaller battery that you cycle every day will pay back faster than a bigger one you cycle every other day.

Imagine for a moment that 1kWh of battery costs £500, 1kWh of electricity costs 18p and there are no other marginal costs. The battery costs the same amount as 2800kWh of electricity, so you will need to cycle it 2800 times for pay back. That's every day for 7 years 8 months, or every other day for 15 years 4 months.

It's also important to size the battery to match the array and your likely consumption.

Real world figures: I have a 2.72kWp array but no battery [yet]. During January I generated about 70kWh but bought 400kWh from the grid. In June I generated 300kWh but bought 100kWh from the grid. I could've used everything I generated in January, in June I was only importing 3-4kWh per day.

If I'd had a 3-4kWh battery I could have stored pretty much everything I generated in December, and still have used most or all of that capacity overnight in June.

If I'd had an 8-10kWh battery it would be mostly empty throughout December, and mostly full throughout June. The extra capacity would rarely be used.

In your particular case, it might be worth considering a relatively small battery now, but expanding it if/when you get an EV (or heat pump, or hot tub, or whatever).

Many thanks for the advice and tips.

I've been thinking about what I'd do in winter and thought I'd go to octopus go and charge cheap at night and use during day or stay on Octopus agile and do the same. Also get outgoing on Agile too.

Reed_Richards

At some time in the late afternoon or early evening the amount of power generated by your panels drops below what your house consumes.  Then at some time in the morning the reverse happens and your panels start to meet and exceed the needs of your house.  If your battery has enough storage capacity to see you through from the evening to the morning then you'll get near-free electricity until the days grow too short.  In my case at the moment I get from about 09:00 to 18:00 to charge my battery then the remaining 15 hours to run it down.  So if you read your meter at 18:00 and again at 09:00 the following morning then the power consumed plus 10% might give a rough idea of what battery storage capacity you could make use of.  The 10% is because you never fully discharge your battery so the actual capacity is at least 10% less than the nominal capacity.     

Petriix

Turnipfarmer said:

Solarchaser said:

Yeah I had some bother with my growatt, connection issues,app issues, solar issues, but most of all battery issues.

None of your quotes are too out there, roughly 1kw per £1k and in terms of batteries 2kwh per £1k which all seems pretty reasonable. 

If your roof has shading, id say solaredge would be a great benefit, but if it doesn't have shading, I dont really see the point. 

Of all your quotes, the solis inverter has the best reputation, but that is possibly because GivEnergy is still quite a new guy on the scene.

Looking at you more personally if you use 3500kwh a year that's roughly 1200 between April and October and 2300 between October and April,  based on most usage patterns saying when the sun is out you use less electricity. Unfortunately solar works the other way and you get your best 6 months of generation when you need them least.

I'd imagine for ease of application to dno that all the inverters will be restricted to 3.6kw, and probably all provide you with around 4000kwh /year (all rough theorising)

Assuming you use the GivEnergy 8.2kwh battery at 100% dod (datasheet says its a 100% discharge battery) then you could theoretically use 8.2kwh a day with it.

Solar in your best 6 months, let's say is 2700 of the 4000 so over 182 days is 14.8kwh/day.

If we take your brighter 6 months then over 182 days using 1200kwh = 6.6kwh a day, so the battery won't cycle fully, but let's say that the sun gives more than 8.2kwh a day during the brighter months as per the average 14.8kwh and in theory your inverter covers everything in the evening (they usually don't, as tumble dryer and washing machine combined are usually above 3.6kw, not to mention putting on the kettle/oven etc) then in that 6 months assuming a kwh cost of electricity of 15p which appears around average In the UK atm then you would save £180, pretty good right?

Now if we take your duller 6 months you generate about 1300kwh and use 2300kwh, now it's not linear, some bright days you will buy no electric, in the duller days your solar will do almost nothing, but as an average you *could* use all of your solar with the battery so 1300 x 15p = £195

So in theory your combined saving is £375 a year.  Plus you have roughly 1500kwh of solar that you don't use, but can export and get paid for at 5p/kwh so that's another £75 bringing you to a grand total of £450 a year, great right?

However your lowest quote is £8100, so your return on investment is 18 years.

If you get an ev, that will improve roi for solar, but it will make no difference to your battery roi.

If you take the solar on its own, without battery at around £4000 for 4kw with an ev and say a zappi ev charger, your roi will be much better as you will still have a similar savings pattern, but lose much of the cost, though its harder to put that into figures as it very much depends on your indivual usage patterns 

Awesome many thanks for the workings out, very much appreciated. Just been looking at my Agile use and my average unit cost is nearer 18p so will save a bit more on ROI. I do believe our energy usage will go up in few years when my children get older.

I understand the points raised, makes complete sense. Do you think going for smaller battery might be better if not getting full cycle charge at certain times of year on the larger battery or is that bit of false economy?

I'll steer clear of the growatt then by the issues you've described. Out of the rest would you go for the Solis AC coupled setup? It's a little bit more but think the AC coupled setup will be better. Would my monitoring be done through Solis or givenergy if I went this route?

I won't have any issues with shading thankfully and it's a South facing roof so not sure if Solaredge is worth it but I like their monitoring software but is £1k more and won't be anymore efficient in my case and adds onto my ROI.

Rather than basing your ROI on the inflated price you're paying on Agile, switch to a cheaper supplier and base your calculations on that. 

I keep running the numbers on batteries based on real generation and usage data but they would have to become a lot cheaper (under £2k for 10kWh) for the ROI period to become shorter than the warranty.

On the other hand, my Zappi has already paid for itself with the EV charging from surplus solar.

I'd go for the solar without battery at this stage. You should look to pay around £4k for a 4.5kWp array. You can always add batteries later if the numbers change. 

Solarchaser

You know it occurs to me that I just cracked on and decided you were thinking of solar plus batteries purely for roi, and so I plodded on with figures, but perhaps thats not why you are doing it?

In general I pretty much agree with petrix, if its purely for roi, then with your usage as low as it is, I can't see batteries making sense any time soon.
I'd advise going with a purely solar install and forget about batteries for the moment. 

If you were using 8000kwh then I think you have a good case for them, as that's what I use and batteries work for me, but to be fair, 20kwh is too much for me if I was chasing roi, I'd probably be better with say 12kwh.

I reckon around 60% of your daily usage is around the sweet spot for roi on batteries for a higher user.

For me though it wasn't really about roi, it was about reducing my bills as much as possible, providing my own green energy as much as possible, and really as a wee project for myself.

I'd definitely suggest switching to go or even go faster as my usual price is 15ppkwh, and the go faster rate is 3 hours of 4.5p between 20:30 and 23:30 which is actually topping me up right now as the wife has rattled over 30kwh today between her various white goods... there should be an award for that 🤬

Anyway I digress, if you are set on getting a battery then yeah a smaller one that you can cycle more often is likely to have a quicker roi.

But to contradict myself, a larger battery will save you more in the winter months if you are charging it on the off peak tarrif to save yourself buying as much peak electricity. 

As regards which software would you use between the solis and the GivEnergy,  sorry I've no idea,  in theory since they are both modern inverters, both should have data.
But I'd assume the solis would only have generation data and the GivEnergy would have battery data.

Solarchaser

Reed_Richards said:

At some time in the late afternoon or early evening the amount of power generated by your panels drops below what your house consumes.  Then at some time in the morning the reverse happens and your panels start to meet and exceed the needs of your house.  If your battery has enough storage capacity to see you through from the evening to the morning then you'll get near-free electricity until the days grow too short.  In my case at the moment I get from about 09:00 to 18:00 to charge my battery then the remaining 15 hours to run it down.  So if you read your meter at 18:00 and again at 09:00 the following morning then the power consumed plus 10% might give a rough idea of what battery storage capacity you could make use of.  The 10% is because you never fully discharge your battery so the actual capacity is at least 10% less than the nominal capacity.     

While that's true for the pylontech that alot of us use, and alot of other ones do 80% dod like the older growatt Darton batteries, the GivEnergy 8.2kwh battery is 100% dod

https://midsummerwholesale.co.uk/buy/giv-energy/giv-batt-8300
(Note price is not including vat or delivery)

Turnipfarmer

Reed_Richards said:

At some time in the late afternoon or early evening the amount of power generated by your panels drops below what your house consumes.  Then at some time in the morning the reverse happens and your panels start to meet and exceed the needs of your house.  If your battery has enough storage capacity to see you through from the evening to the morning then you'll get near-free electricity until the days grow too short.  In my case at the moment I get from about 09:00 to 18:00 to charge my battery then the remaining 15 hours to run it down.  So if you read your meter at 18:00 and again at 09:00 the following morning then the power consumed plus 10% might give a rough idea of what battery storage capacity you could make use of.  The 10% is because you never fully discharge your battery so the actual capacity is at least 10% less than the nominal capacity.     

Gonna have a look at this tonight and look at my usage in the summer and winter time and see how many kWh I use between 1800 and 0900. Might give me a clearer indication on battery sizing.

Will post my findings on here later on 

Turnipfarmer

Just had a quick look at two days worth data from 1800-0900, one in march and one in June. Used roughly 4kwh in that time so basically half of the 8.2kwh battery I'm thinking of getting. I think that tells me that the 8.2 is the right one to go for. I know it's only two days and probably should look at more which I will but our usage hasn't changed much between march and now.

Also was thinking that battery dependency will be more in winter due to less solar so this number will effectively be higher.

Reed_Richards

You're right about battery dependency being greater in the winter but unfortunately your ability to charge the battery will be greatly reduced.  You know this because you generated 70 kWh in January which is 2.26 kWh per day on average.  If you get a big battery you will struggle to charge it fully in winter so you would need one that does not mind spending a lot of time in a state of high discharge.

bewidds

Thanks for all these comments. I am in a similar position as the original poster. Considering a 4,130 kw array with possible the 8.2kw Givenergy battery. From what I am reading above a battery doesn't make loads of sense, although I was wondering if the energy price hikes make much of a difference to the equation.

I am considering getting a EV within 12-24 months, so it seems like the battery may well become redundant.

That said I am also looking to electrify my house gradually, i.e. install German style electric radiators instead of a gas boiler. My current annual usage is about 4,000KWH, but i expect this to increase.

Any advice or does the above advice still hold for now?