Help with changes to current set up

bintyandthebeast

Hi,

We bought a house last year with solar and battery set up and I am keen to get some help to work out some changes please.

Existing system:

4.86kw Solar PV comprising of 12x Black JA Solar 405w panels

Growatt 3600TL Inverter (in attic)

MyEnergi Eddi hot water diverter

Fox ESS 3.7kw (non-hybrid) AC coupled battery charger with 10.4kwh of Fox ESS HV2600 battery storage (battery and charger in electric cupboard by front door with garage immediately behind wall, so can be moved to other side)

We are a high usage household as don't have mains gas, so use oil fired central heating. I currently heat water overnight via immersion on British Gas EV tariff (7.9p per unit between midnight to 5am), so effectively get a COP of 3. Excess solar (virtually none between now and March) heats water throughout day and oil boiler boosts the hot water late afternoon if needed. 

Other points to note:

- we are in Perthshire, Scotland

- we have a very large hot tub that is regularly used

- we have a 6kw sauna

- we will be getting our oil central heating replaced by a 10kw air-to-air heat pump this winter

- i work from home in a garden building which runs a 3kwh blow heater. I may or may not swap this for an air-con unit when we install the heat pump

- we currently have diesel cars but I will be swapping to either plug-in hybrid or full EV car next summer

Current issues:

- PV limited 3.6kw due to inverter

- draw from batteries limited to 3.7kw, so even if we have plenty of reserve in batteries we are not able to use oven/air fryer when induction hob is on, or hot tub heating (16a) without drawing full price power from the grid.

- Summer months are fine, but 6-8 months of the year we are drawing expensive power from grid, so would be great to shift this to battery which is charged on overnight cheap rate

Can I please ask for advice on whether I cam correct in thinking the best way forward would be:

- swap out the Growatt inverter for higher rated model

- increase battery storage, which raises more questions. 

Should I be looking to increase the HV2600 batteries (can they handle a higher output than 3.7kwh) or should I be looking to replace the charger for something allowing a higher throughput and a different battery system. Please remember the inverter is in attic and batteries 2 floors down, with cabling concealed through the house. We could get a single cable drawn through the house but will be a pain. Ideally, we would want to use existing cable from attic if possible.

It's worth saying that this is a cost saving exercise so while I would be delighted to have a couple of Tesla Powerwall batteries, I don't expect this would be a suitable option due to cost.

Many thanks in advance for your help!

QrizB

Here are a couple of quick ideas while we all look at the big picture:

bintyandthebeast said:

we have a very large hot tub that is regularly used

See if you can get a heat pump for your hot tub.

bintyandthebeast said:

Excess solar (virtually none between now and March) heats water throughout day and oil boiler boosts the hot water late afternoon if needed. 

When you have excess solar, you're likely to be better off selling it to the grid rather than using it to heat water. Your oil boiler (and your future heat pump) should cost less to heat water than the price you'll get for exporting electricity.

bintyandthebeast said:

PV limited 3.6kw due to inverter

I don't know if your inverter has an app that gives you a generation graph, but the amount of generation that you're losing is likely to be smaller than you think.

Here's a typical sunny-day-in-June chart from my PV system:

You'll see that while my panels are rated at 2.72kW, the peak is lower than this (because solar panels are less efficient when they get hot).

If you double all the numbers on the left, we can pretend this is your system. Your curent 3.68kW inverter would be a little bit below the "2" line on my chart, so you're only losing the tip of the hump. Less than 10% of output, and that's on a sunny day in June.

bintyandthebeast

Thanks for the reply!

Regarding the hot tub, we have a Hot Spring Envoy. This uses a constant low flow filter and water is heated when passing through this. Unfortunately, no option for a heat pump for this without invalidating the warranty. I had thought about a separate drop in immersion for overnight to heat higher than we would normally to use the cheap rate but I wasn't able to buy or design anything safe to do this. It can then cool through the morning and start reheating in the afternoon.

Please see below a screenshot for a random day in May. Panels on our house face due south and perform very well. It may well be that the inverter swap would mean we would never recoup what we will get back in generation, but it's frustrating to see 3.6kw generation when we could have another 30%. I cannot fathom why it wasn't installed that way but maybe inexperience from the last home owner or an installer that didn't want the DNO check. It could equally be my thinking being wrong!

Export is just over 15p with British Gas, so I agree about the heat pump being better for heating water than our excess solar diverter and we will do this when we get the heat pump installed. Currently we use the Eddi to heat the immersion on a timer overnight.

Thanks again for your reply.

QrizB

bintyandthebeast said:

Thanks for the reply!

Regarding the hot tub, we have a Hot Spring Envoy. This uses a constant low flow filter and water is heated when passing through this. Unfortunately, no option for a heat pump for this without invalidating the warranty.

That's a shame. When does the warranty expire?

Hot tub heat pumps usually plumb in just after the filter/pump.

bintyandthebeast said:

I cannot fathom why it wasn't installed that way but maybe inexperience from the last home owner or an installer that didn't want the DNO check.

I suspect the latter. Less paperwork forthe installer. Although it's possible the istaller applied to the DNO and was refused anything larger.

Looking at your chart, I would guess you've lost no more than 2kWh on that day, so about 30p. Keep in mind that it's going to take a lot of 30ps to cover the £600 cost of a larger inverter.

bintyandthebeast

Thanks again QrizB,

Warranty runs for another 18 months, so 2 more expensive winters to go!

Next door is at a 45 degree angle to ours so have a 2 string PV array with a 7.8kw inverter, so going with lazy installer.

That's a fair point about cost for the Growatt inverter replacement. 

It would be great to have some advice on battery upgrade, and potential controller (charger / inverter) upgrade, as this is where the savings will be made.

QrizB

Quick thoughts on batteries:

• You can charge your battery overnight on your EV tariff. Even if we have a bright sunny day, your solar export will earn more than it cost to charge your battery.
• You could add more batteries to your current system (example link) but at £260 per kWh they're not particularly cost-effective.
• If you decide to replace the battery inverter, you could look at moving to a different system - but again, that's expensive.

I'd suggest looking at your charging routine first, then thinking about other options.

One more thing (channeling my inner Colombo):

bintyandthebeast said:

We are a high usage household ...

How much electricity per year are you currently buying from the grid? Do you have enough space on your roof to add more panels?

bintyandthebeast

We currently charge batteries to full overnight to maximise savings, I should've mentioned this in my initial post.

Your comments on battery and change of battery inverter is what I am questioning, I guess. I've seen some high capacity Fogstar batteries which sound cheap. £2k for 16kwh but only 80% useable. I thought maybe 2 of these would be good but not sure if I'd need a new charger.
https://www.fogstar.co.uk/products/fogstar-energy-16kwh-48v-solar-battery

I expect we will use somewhere around 4,500 units per year, but that will increase as heat pump is going in.

We do have space for more panels on the roof but not a huge amount more. Maybe 4 panels without shading only, so 1.6kw additional. I'm not sure we would recoup the cost, particularly with the 3.6kwh inverter.

QrizB

bintyandthebeast said:

Your comments on battery and change of battery inverter is what I am questioning, I guess. I've seen some high capacity Fogstar batteries which sound cheap. £2k for 16kwh but only 80% useable. I thought maybe 2 of these would be good but not sure if I'd need a new charger.

Your current system used "high voltage" batteries, not the 48-volt ones that many systems use. As a result it's only compatible with Fox ESS HV batteries. I've got a 16kWh Fogstar battery (not quite the one you link to, but similar). But theyre not compatible with your current inverter.

Once you start thinking of replacing the entire Fox ESS system, you then might as well fit a complete hybrid battery storage system to replace your Growatt solar inverter as well. But for that you're looking at >£10k once you've bought all the kit and paid for fitting. (You might be able to get a single Powerwall installed for £7-8k or so, but with 13.5kWh of storage it's not much of an increase. You could get the same additional storage for ~£1300 with your current system.)

bintyandthebeast

That makes sense, thanks QrizB.
I guess I have 2 options:
1. Sell off the existing kit and replace with something that has higher capacity. (Ideally I'd like 25kwh of useable storage) - do these have a second value worth pursuing this option? or
2. I've read I can use a max of 8 batteries with existing system, so could add another 4 batteries and another BMS (around £4-4.5k?) 

I had a thought last night about vehicle to home but may be a few years too early for this to be a viable option.

Heedtheadvice

Some great support to your thread there from. QrizB.

There is no practical reason ( on the face of it ) why you cannot add extra solar panels ( if possible S, E and W) to maximise your solar generation and these could be using an independant hybrid inverter and batteries. Maximising  solar collection through the day is very useful especially early and late when  South facing generation is lower. Particularly useful spring and autumn when heat pump use starts to increase. Optimally steeper angles facing E and W could be useful.

I have not got any costed options for you.

Eggs and baskets.

However ( this is not answering your question! ) I would consider the risk of moving entirely to a sole Heat Pump from oil. Certainly green ( very laudable) but it will put you at much more risk from power outages. Consider a grid failure ( not sure of the risk in Perth but more Rural Scotland can be higher risk)

Battery backup for a oil pump etc is fairly easy, cheap and requires lowish storage capacity for the battery it can be totally independent of the green system and grid I.e. UPS style. With grid power off ( no solar no heat pump it keeps you with heating (and easily) for several days.

Relying on a fully electrical system that requires high power levels for heating ( HP)  leaves more vulnerability and your batteries if used in a UPS mode or via manual or automatic switch over will not power a heat pump in winter for much more than a day, if that. Heat is not stored. ( maybe see Solarchasers thread on his heat storage system ? https://forums.moneysavingexpert.com/discussion/6377030/ditching-gas-going-electric-immersion-only-a-wee-project/p1 if you are not aware, aong thread in the end but demonstrates what storage is needed for his houshold)

It might be possible to add in an Air to Air heat pump and retain the oil system -if you think it is a risk worth addressing. Our current storm with the grid out of use for some households is not the most severe nor in the coldest of months but I well remember a winter outage over a few cold days and was glad of a gas fire!! Even though our winters are not as cold as I remember, bad storms are more frequent.

bintyandthebeast

Thanks for your input Heedtheadvice.

I am not certain the cost to add to the panels on our roof will be recouped. I will try and get some costings. Our neighbour has some (SE & S/SW) so may look to get some further information from her on what she's generating with the extra panels compared to us.

Understood on the reliance of being all electric. In a twist, our house we sold last year went without power for 24 hours but all cables in our area were buried (new estate) so we should be good unless the HV lines go down. We do also have a log burner in the house, but hear what you are saying about a total outage. I am disabled and work from home most of the time so this may be something I need to give more consideration, but I feel at this point a UPS would be more than sufficient, if we go ahead.

I think I am feeling slightly cautious as think in time a more modular system using an electric car and V2H technology will be relatively common and would give us the significant requirement we need for winter months without having to pay twice (for car and lots of battery storage separately). If I spend £10k now I don't think I will recoup in under 8-10 years at current prices. No crystal ball available, but would hope prices will stabilise and become more affordable in real terms during that time with technology progress and high inflation. Am I being trying too hard to fix a problem and should I just throw money in a high interest account?! 

I am an overthinker, clearly!