Considering adding another battery - inverter options

ed110220

ed110220 said:

QrizB said:

ed110220 said:

I've just had a call from National Grid (ex WPD) and they said I could have the full 17 kW.

That's a result, then 🥂

16 kW inverter, 7 Seplos kits for 112 kWh of storage, import 100 kWh each night for £6.70 and then export 90 kWh each day for £14.85

No, realistically am looking at an 8 kW inverter and another Seplos kit.

Joking aside the marginal cost of the Seplos 16 kWh kits probably justifies arbitrage on the EON drive tariff. If you can put 15 kWh in at 6.7p/kWh and export 14 kWh at 16.5p, that's £1.31/day, £478/year, just over 2.5 years to pay back the £1320 of the battery. I don't think I'd go crazy with inverter sizing to do that but it does make the economics of oversizing the battery compared to your use better.

michaels

ed110220 said:

michaels said:

ed110220 said:

michaels said:

ed110220 said:

Martyn1981 said:

ed110220 said:

Just had a reply back from National Grid (DNO, was Western Power Distribution) and they've said I can have 17 kW total and export! Can't quite believe it. Not that I'd be installing anything like that... Possibly 8 kW?

Not sure why it's so high, possibly because I know the local substation has had work on it recently and I'd estimate the number of PV systems in the area is lower than average (many housing association properties who unfortunately haven't had any, lower income, quite a few HMOs etc).

Damn I'm jealous. I got 5.9kW from WPD in 2012, and then when I asked about V2G in 2019, they told me no chance, and I think the choice of words when I rang was "no idea what idiot gave you 5.9kW". They did say they weren't minded at this time to take away my approval ..... so I've never asked anything since, don't rock the boat. 

Maybe ask again, surely they can't take away what you already have? 

I forgot to say that my supply has been unlooped since the solar was fitted. The neighbours were going to get PV too so they had their supply unlooped from ours, but in the end they didn't get any.

Looking at PVGIS I'm half thinking about having an identical or similar array on the NE facing side (the roof is symmetrical). Given the shallow pitch the yield isn't actually that much lower, though of course the yield in winter is quite a bit lower and export prices will probably come down.

Does anyone have an idea what kind of cost to expect for an inverter move/install these days? Currently it's in the loft which I know isn't ideal. So when I did internal renovations I ran some DC cables from the current inverter position down to the utility room so that it could be moved in the future. Was thinking of buying an inverter myself and getting it fitted as they seem to come up fairly cheap eg bought for a project but not used.

Our PV array is about 10 degrees south of west.  We were thinking of adding panels to the East facing roof and putting it through the same inverter and accepting about 10-20% clipping for an hour or so around mid day on peak output days, seems to work out better return than adding another inverter - however there are FIT implications as the inverter output goes to the generation meter.

No FIT here to impact (2022 install), I'm mainly interested in a larger inverter to be able to charge up enough batteries on cheap off-peak electricity to run an ASHP. Though it has made me think about another 4.8 kW of PV on the NE side of the roof, but that would be a project for the future. 

On that subject, how would you go about calculating how many kWh I'd need to store per off peak slot for an ASHP? I've looked through our daily gas consumption for the last two winters and the highest was 50 kWh, but it was rarely above 40 kWh and much more often below 30 kWh. 

50 kWh of gas = 16 kWh ASHP consumption at a COP of 3. Currently our 15 kWh battery lasts us till the next off peak slot so I'm thinking ~ 30 kWh would be sufficient. We don't use has for anything other than the boiler (no gas cooker etc). Therefore with a 5 h off peak period we'd need a 6 kW charge rate... Increase that to 8 kW inverter to be safe (higher than expected consumption, off peak tariff with a shorter period in future). Sound about right or too crude?

Can you get your half hourly usage for the gas as that will obviously show you how much was used in the peak hours rather than the daily total.  With a heat pump you are likely to use a smaller (or even no) night time set back which transfers some of your usage from peak hours to off peak hours, specifically any morning peak where the gas boiler gets the house back up to the daytime temp.

I would probably chose worst day and assume a cop of 2.5 on that day rather than 3 - but against that it may not be cost effective to go for 'zero' peak import if you then have to add another 5-10 kwh of battery that you will only use on 2 or 4 days a year (although there might be some offsetting of the cost if you get a good SEG rate and can make a profit on the round trip on the days where you don't need to use all the storage for your own consumption)

I have got the half hourly records but I haven't been through them all because the amount of gas we use from 00:30 to 05:30 is insignificant as we never have the heating on at those hours. Using a smaller night time setback is going to increase overall consumption - any idea how much?

Here's the half hour consumption for a typical colder winter day: 

I think it makes less difference than you would think, obviously heat loss is higher because of the bigger inside Vs outside differential but against that the hp will be a bit less efficient when working at full pelt to regain the lost temp.

Do you also not heat whilst at work? Our typical day would see about 12kwh for an hour or so first thing then flat usage throughout the day, peak day would be about 7.2kw constant reflecting a heat loss of about 6.6kw accounting for boiler efficiency - our boiler would modulate down to about 2kw so it would be a constant burn until the heat load was lower than this.

ed110220

michaels said:

ed110220 said:

michaels said:

ed110220 said:

michaels said:

ed110220 said:

Martyn1981 said:

ed110220 said:

Just had a reply back from National Grid (DNO, was Western Power Distribution) and they've said I can have 17 kW total and export! Can't quite believe it. Not that I'd be installing anything like that... Possibly 8 kW?

Not sure why it's so high, possibly because I know the local substation has had work on it recently and I'd estimate the number of PV systems in the area is lower than average (many housing association properties who unfortunately haven't had any, lower income, quite a few HMOs etc).

Damn I'm jealous. I got 5.9kW from WPD in 2012, and then when I asked about V2G in 2019, they told me no chance, and I think the choice of words when I rang was "no idea what idiot gave you 5.9kW". They did say they weren't minded at this time to take away my approval ..... so I've never asked anything since, don't rock the boat. 

Maybe ask again, surely they can't take away what you already have? 

I forgot to say that my supply has been unlooped since the solar was fitted. The neighbours were going to get PV too so they had their supply unlooped from ours, but in the end they didn't get any.

Looking at PVGIS I'm half thinking about having an identical or similar array on the NE facing side (the roof is symmetrical). Given the shallow pitch the yield isn't actually that much lower, though of course the yield in winter is quite a bit lower and export prices will probably come down.

Does anyone have an idea what kind of cost to expect for an inverter move/install these days? Currently it's in the loft which I know isn't ideal. So when I did internal renovations I ran some DC cables from the current inverter position down to the utility room so that it could be moved in the future. Was thinking of buying an inverter myself and getting it fitted as they seem to come up fairly cheap eg bought for a project but not used.

Our PV array is about 10 degrees south of west.  We were thinking of adding panels to the East facing roof and putting it through the same inverter and accepting about 10-20% clipping for an hour or so around mid day on peak output days, seems to work out better return than adding another inverter - however there are FIT implications as the inverter output goes to the generation meter.

No FIT here to impact (2022 install), I'm mainly interested in a larger inverter to be able to charge up enough batteries on cheap off-peak electricity to run an ASHP. Though it has made me think about another 4.8 kW of PV on the NE side of the roof, but that would be a project for the future. 

On that subject, how would you go about calculating how many kWh I'd need to store per off peak slot for an ASHP? I've looked through our daily gas consumption for the last two winters and the highest was 50 kWh, but it was rarely above 40 kWh and much more often below 30 kWh. 

50 kWh of gas = 16 kWh ASHP consumption at a COP of 3. Currently our 15 kWh battery lasts us till the next off peak slot so I'm thinking ~ 30 kWh would be sufficient. We don't use has for anything other than the boiler (no gas cooker etc). Therefore with a 5 h off peak period we'd need a 6 kW charge rate... Increase that to 8 kW inverter to be safe (higher than expected consumption, off peak tariff with a shorter period in future). Sound about right or too crude?

Can you get your half hourly usage for the gas as that will obviously show you how much was used in the peak hours rather than the daily total.  With a heat pump you are likely to use a smaller (or even no) night time set back which transfers some of your usage from peak hours to off peak hours, specifically any morning peak where the gas boiler gets the house back up to the daytime temp.

I would probably chose worst day and assume a cop of 2.5 on that day rather than 3 - but against that it may not be cost effective to go for 'zero' peak import if you then have to add another 5-10 kwh of battery that you will only use on 2 or 4 days a year (although there might be some offsetting of the cost if you get a good SEG rate and can make a profit on the round trip on the days where you don't need to use all the storage for your own consumption)

I have got the half hourly records but I haven't been through them all because the amount of gas we use from 00:30 to 05:30 is insignificant as we never have the heating on at those hours. Using a smaller night time setback is going to increase overall consumption - any idea how much?

Here's the half hour consumption for a typical colder winter day: 

I think it makes less difference than you would think, obviously heat loss is higher because of the bigger inside Vs outside differential but against that the hp will be a bit less efficient when working at full pelt to regain the lost temp.

Do you also not heat whilst at work? Our typical day would see about 12kwh for an hour or so first thing then flat usage throughout the day, peak day would be about 7.2kw constant reflecting a heat loss of about 6.6kw accounting for boiler efficiency - our boiler would modulate down to about 2kw so it would be a constant burn until the heat load was lower than this.

We don't tend to have the CH on for most of the daytime, partly it's being frugal, partly it's the house is reasonably well insulated. Not amazingly but I have done quite a bit to improve it and it's noticeable that our gas consumption has declined and comfort increased. One thing I've noticed is that pre-insulation I couldn't grow moth orchids (the type you can buy in supermarkets etc) because the overnight cold would cause them to get leaf spots and go into decline in winter, but now they do fine even though our heating settings are the same so clearly heat loss overnight has decreased. 

Also since getting the bigger battery I tend to run a desicant dehumidifier in winter which keeps the upstairs fairly warm during the day. 

I expect switching to a heat pump would increase our heat consumption and comfort levels.

Martyn1981

Hiya Ed, not sure if our info will help, as we have 2 A2A units, not a full ASHP wet system, but our worst month this last winter was January with ~1,000kWh of leccy import, about 100kWh of gas (DHW and oven), and about 110kWh of PV gen. [Total also includes BEV charging.]

We got through fine with 20kWh of storage, only having to import day rate on a couple of days, and even then, only a few hours at ~500W in the late evening. Total bill, leccy and gas was just over £100 (Dec & Feb both about £90).

We pre-heated the house on cheap rate each night, turning the A2A temps down each morning. When we get rid of gas, the leccy oven will add a small amount to daytime battery demand, but the DHW will be from night rate.

Obviously for us, the A2A units can work fine of the battery, they are both 3.5kW units, and can pull ~1kW when getting up to temp or de-icing, but for the daytime batt use, they will be pulling less. A full ASHP will require, I assume, a bigger battery inverter than our 3.68kW, to manage peaks? We did tend to pull a bit of import at tea time, simply because it's easy for induction hob, kettle, micro-wave etc to exceed 3.68kW at times too. Plus the pulsing demand of some of these items can be tricky for the battery inverter to match.

Economically, I'd guess at 15kWh of batts being 'ideal' for us. We'd have imported more day rate, on more days but balanced against lower battery cost. But from a fun / challenge side, the 20kWh is near perfact, and batts are getting cheap.


Not sure if this will make sense, but I did look back over about 5 years, and the highest monthly gas demand I could find was 2,500kWh in a January. I don't think this January was exceptionally cold, but there were periods of several consecutive days of very cold weather, and the heating was holding fine - by that I mean the house wasn't getting steadily colder. So I'm pretty confident that our setup and battery capacity is adequate for a long and protracted cold month, just that the monthly total energy consumption will be higher.

But going back to finding the right balance, if on the very worst days you have to import a bit of day rate, that may make more financial sense than building out to meet all scenarios, IYSWIM.

ed110220

Martyn1981 said:

Hiya Ed, not sure if our info will help, as we have 2 A2A units, not a full ASHP wet system, but our worst month this last winter was January with ~1,000kWh of leccy import, about 100kWh of gas (DHW and oven), and about 110kWh of PV gen. [Total also includes BEV charging.]

We got through fine with 20kWh of storage, only having to import day rate on a couple of days, and even then, only a few hours at ~500W in the late evening. Total bill, leccy and gas was just over £100 (Dec & Feb both about £90).

We pre-heated the house on cheap rate each night, turning the A2A temps down each morning. When we get rid of gas, the leccy oven will add a small amount to daytime battery demand, but the DHW will be from night rate.

Obviously for us, the A2A units can work fine of the battery, they are both 3.5kW units, and can pull ~1kW when getting up to temp or de-icing, but for the daytime batt use, they will be pulling less. A full ASHP will require, I assume, a bigger battery inverter than our 3.68kW, to manage peaks? We did tend to pull a bit of import at tea time, simply because it's easy for induction hob, kettle, micro-wave etc to exceed 3.68kW at times too. Plus the pulsing demand of some of these items can be tricky for the battery inverter to match.

Economically, I'd guess at 15kWh of batts being 'ideal' for us. We'd have imported more day rate, on more days but balanced against lower battery cost. But from a fun / challenge side, the 20kWh is near perfact, and batts are getting cheap.


Not sure if this will make sense, but I did look back over about 5 years, and the highest monthly gas demand I could find was 2,500kWh in a January. I don't think this January was exceptionally cold, but there were periods of several consecutive days of very cold weather, and the heating was holding fine - by that I mean the house wasn't getting steadily colder. So I'm pretty confident that our setup and battery capacity is adequate for a long and protracted cold month, just that the monthly total energy consumption will be higher.

But going back to finding the right balance, if on the very worst days you have to import a bit of day rate, that may make more financial sense than building out to meet all scenarios, IYSWIM.

Thanks Martyn, actually when I say ASHP it might not be air to water, it could be air to air (split air conditioners). It really depends on cost and whether we have a suitable place for a HW cylinder. I'm thinking that A2A might offer most of the benefits for a lot less, but it would be good to get rid of the gas entirely. 

Martyn1981

ed110220 said:

Martyn1981 said:

Hiya Ed, not sure if our info will help, as we have 2 A2A units, not a full ASHP wet system, but our worst month this last winter was January with ~1,000kWh of leccy import, about 100kWh of gas (DHW and oven), and about 110kWh of PV gen. [Total also includes BEV charging.]

We got through fine with 20kWh of storage, only having to import day rate on a couple of days, and even then, only a few hours at ~500W in the late evening. Total bill, leccy and gas was just over £100 (Dec & Feb both about £90).

We pre-heated the house on cheap rate each night, turning the A2A temps down each morning. When we get rid of gas, the leccy oven will add a small amount to daytime battery demand, but the DHW will be from night rate.

Obviously for us, the A2A units can work fine of the battery, they are both 3.5kW units, and can pull ~1kW when getting up to temp or de-icing, but for the daytime batt use, they will be pulling less. A full ASHP will require, I assume, a bigger battery inverter than our 3.68kW, to manage peaks? We did tend to pull a bit of import at tea time, simply because it's easy for induction hob, kettle, micro-wave etc to exceed 3.68kW at times too. Plus the pulsing demand of some of these items can be tricky for the battery inverter to match.

Economically, I'd guess at 15kWh of batts being 'ideal' for us. We'd have imported more day rate, on more days but balanced against lower battery cost. But from a fun / challenge side, the 20kWh is near perfact, and batts are getting cheap.


Not sure if this will make sense, but I did look back over about 5 years, and the highest monthly gas demand I could find was 2,500kWh in a January. I don't think this January was exceptionally cold, but there were periods of several consecutive days of very cold weather, and the heating was holding fine - by that I mean the house wasn't getting steadily colder. So I'm pretty confident that our setup and battery capacity is adequate for a long and protracted cold month, just that the monthly total energy consumption will be higher.

But going back to finding the right balance, if on the very worst days you have to import a bit of day rate, that may make more financial sense than building out to meet all scenarios, IYSWIM.

Thanks Martyn, actually when I say ASHP it might not be air to water, it could be air to air (split air conditioners). It really depends on cost and whether we have a suitable place for a HW cylinder. I'm thinking that A2A might offer most of the benefits for a lot less, but it would be good to get rid of the gas entirely. 

Also depends on your starting point, as we'd already installed the two A2A units (2017 and 2020 I think) to make use of excess PV in the shoulder months, to heat the house and reduce or remove GCH use in those months.

At that point I got a quote for ASHP of nearly £7k (after the subsidy), as our property design and EPC suggest we should need a massive system, despite that not being the case at all.

So for us, it was £7k v's a HW tank install and some other bits and pieces, so maybe £1.5k all in, but crucially we'd already spent ~£3k in total on the A2A units.

Maybe just man maths, and justification after the act, but I'm also pleased that we can remove the rads, and the risk of water leaks (though that is small), and have two separate sources of heat (and cooling), if one was to ever fail.

michaels

A2A vs A2W may depend on your hot water usage, ours is about 600kwh of immersion electricity a month (so probably at least 650kwh of gas) so moving this to heat pump, even at a cop of 2.5, would give a worth while saving, 100kwh per month and direct electric water heating (immersion or heat as you go shower and taps) might make more sense allowing A2A for space heating.

Martyn1981

michaels said:

A2A vs A2W may depend on your hot water usage, ours is about 600kwh of immersion electricity a month (so probably at least 650kwh of gas) so moving this to heat pump, even at a cop of 2.5, would give a worth while saving, 100kwh per month and direct electric water heating (immersion or heat as you go shower and taps) might make more sense allowing A2A for space heating.

You can do A2A but still HPHW tank if you want, here's a medium sized one for £1k, or large size that can even run a small underfloor heating or CH circuit too.

ed110220

Martyn1981 said:

michaels said:

A2A vs A2W may depend on your hot water usage, ours is about 600kwh of immersion electricity a month (so probably at least 650kwh of gas) so moving this to heat pump, even at a cop of 2.5, would give a worth while saving, 100kwh per month and direct electric water heating (immersion or heat as you go shower and taps) might make more sense allowing A2A for space heating.

You can do A2A but still HPHW tank if you want, here's a medium sized one for £1k, or large size that can even run a small underfloor heating or CH circuit too.

Thanks, that's pretty interesting. I hadn't decided on A2W or A2A yet. Our radiators are probably undersized for lower flow temps so that might swing me towards A2A. Three rooms have hydronic plinth heaters (kitchen, bedroom and spare room - I thought it would be a good idea to save space by replacing radiators with them in the base of built in wardrobes, but I don't think they work well with lower temps?) I'm thinking high level A2A units would probably fit better than larger radiators.

Martyn1981

ed110220 said:

Martyn1981 said:

michaels said:

A2A vs A2W may depend on your hot water usage, ours is about 600kwh of immersion electricity a month (so probably at least 650kwh of gas) so moving this to heat pump, even at a cop of 2.5, would give a worth while saving, 100kwh per month and direct electric water heating (immersion or heat as you go shower and taps) might make more sense allowing A2A for space heating.

You can do A2A but still HPHW tank if you want, here's a medium sized one for £1k, or large size that can even run a small underfloor heating or CH circuit too.

Thanks, that's pretty interesting. I hadn't decided on A2W or A2A yet. Our radiators are probably undersized for lower flow temps so that might swing me towards A2A. Three rooms have hydronic plinth heaters (kitchen, bedroom and spare room - I thought it would be a good idea to save space by replacing radiators with them in the base of built in wardrobes, but I don't think they work well with lower temps?) I'm thinking high level A2A units would probably fit better than larger radiators.

Hi Ed, sorry, I just realised I buried the lede. That model is one that can draw air in from outside, and also vent to outside. There are quite a few that simply take the air from the surrounding area, so ideal in basements, garages etc, which will get cooled, but also have lots of air circulation.

For a unit in the house, maybe a bedroom etc, you ideally want one that does/can vent to outside. Perhaps in the summer, you would let it cool the room instead.