Business case for a battery

[RobLe]

22 hours ago, Bramco said:

The Sunsynk inverter above, may not be man enough for many homes, ideally you want one that can supply enough power for a couple of big white goods needs, so something closer to 6kW. 

We have that 3.6kW sunsynk; it’s enough power for us.  Attached is our Octopus days use for last Saturday, which is the heaviest use it’s ever had in the year we’ve had it, as Mrs Robl cooked thanksgiving turkey dinner for 18 people!  We have an all elec house(car, gshp,etc) - in the morning the tesla and home batt were charging.  By 8pm the 15kWh home batt was flat 🤬 - but this was a very unusual day, I think it goes ‘flat’ 5 times a year roughly.  You can see that even with loads of heavy cooking - and Mrs robl makes no attempt to stagger elec devices when cooking - there’s hardly any elec drawn during the peak period (Oct intelligent go):

[Bramco]

3 hours ago, RobLe said:

We have that 3.6kW sunsynk; it’s enough power for us.

 

Rob,  I need to go back and edit my post - I was thinking about this yesterday and actually, I'm pretty sure, the real reason for having a larger inverter is if you have more PV than the 3.7kWp (or whatever the number is that you can install without having to talk to the DNO), then you need a larger inverter because they top out at their maximum rating. The usage thing isn't actually that important - especially if you can get the other half to work serially rather than having everything on at once.

 

We have a 5kW Sunsynk and a 6.5kW ground mounted array - in the summer, the Sunsynk tops out at around 5.4kW, so we lose some PV. Not a lot in our case, as the array is at 45deg, to give us lower summer output and higher winter output. We had a standard inverter on the array before we installed the batteries and this was larger and didn't top out.

 

So I should have written above that it depends on your PV array - apologies to anyone I've misled on that - I'll go back and edit the previous post.

 

[Bramco]

1 minute ago, Bramco said:

So I should have written above that it depends on your PV array - apologies to anyone I've misled on that - I'll go back and edit the previous post.

Too late to do an edit......  

 

@Archer - you were the OP I think - and I know you said my post was useful, so please take note of the above second thoughts......

[jack]

1 hour ago, Bramco said:

Too late to do an edit......  

 

@Archer - you were the OP I think - and I know you said my post was useful, so please take note of the above second thoughts......

Let me know which post and what you want edited and I'll do it for you.

[S2D2]

1 hour ago, Bramco said:

 

Rob,  I need to go back and edit my post - I was thinking about this yesterday and actually, I'm pretty sure, the real reason for having a larger inverter is if you have more PV than the 3.7kWp (or whatever the number is that you can install without having to talk to the DNO), then you need a larger inverter because they top out at their maximum rating. The usage thing isn't actually that important - especially if you can get the other half to work serially rather than having everything on at once.

 

We have a 5kW Sunsynk and a 6.5kW ground mounted array - in the summer, the Sunsynk tops out at around 5.4kW, so we lose some PV. Not a lot in our case, as the array is at 45deg, to give us lower summer output and higher winter output. We had a standard inverter on the array before we installed the batteries and this was larger and didn't top out.

 

So I should have written above that it depends on your PV array - apologies to anyone I've misled on that - I'll go back and edit the previous post.

 

I have 5.2kWp on a 3.68kW hybrid inverter because it can still divert PV to the battery even when capped at 3.68kW AC output. It does take some intelligent prediction to hold back the right amount of battery to make this work but there's no clipping as a result.

 

All that said, I would have fitted a larger inverter if it wasn't so much hassle/cost with the DNO, the actual material cost difference is negligible.

[RobLe]

We have a 3.6kW sunsynk hybrid inverter, with 3.6kW of PV on one string, which works fine.  
I intend to add another 5kW onto its other string next year😁.  The sunsynk can output 3.6kW to mains whilst charging a battery at 3.6kW - which seems perfect - the difficulty is finding a way to ensure that the battery has enough unused capacity so that it’s got space during the peak few hours of the day.  

Has anyone got this inverter with over 3.6kW of PV, and got a good way of managing summer battery use so that peak daytime PV isn’t wasted ?

[Archer]

22 hours ago, Bramco said:

Too late to do an edit......  

 

@Archer - you were the OP I think - and I know you said my post was useful, so please take note of the above second thoughts......

I'm still trying to get my head around trying to size the inverter. Our ASHP (12.5kw) says it's power output is 4kw or 19amps. I'm assuming that would be the max if it's working flat out for a period? And then we'd want to allow for an electric oven/ induction hob (3kw?!) and maybe some spare capacity for adhoc things... 

 

But an 8-10kw inverter seems much bigger than almost everyone has so I'm assuming that I'm misunderstanding and most appliances only draw the peak for a very short time, is that right? 

 

I'm getting some expert advice now, but it seems to be that 5-8kw inverter is probably the sweet spot for our usage. Does that sound about right?

Edited December 3, 2024 by Archer

[G and J]

I’m assuming that the network is the balancing item/buffer in the system.  
 

Sizing for max need may incur a lot greater cost than is economically sensible.  

[Bramco]

4 hours ago, Archer said:

I'm still trying to get my head around trying to size the inverter. Our ASHP (12.5kw) says it's power output is 4kw or 19amps. I'm assuming that would be the max if it's working flat out for a period? And then we'd want to allow for an electric oven/ induction hob (3kw?!) and maybe some spare capacity for adhoc things... 

 

It depends - as with many things....
 

Your base load will be between 250W and 500W depending on the time of day - more lights on more base load. There are many people on the forum that have their base load down to very small levels by disconnecting all wall warts etc.  But let's say you're a typical family, so between 250W and 500W.

 

The ASHP will draw it's 4kW continuously unless it's doing a defrost, when for a short period, it will draw a lot less.

 

Your oven will draw about 3kW until it's up to heat - 5 to 10 mins? Then will turn on and off at 3kW when it needs more heat. Induction hobs tend to turn on and off repeatedly - more often on a higher setting.

So assuming everything is on, that's at least 500W + 4kW + 3kW + 3kW as a maximum - 10.5kW but only 4.5kW is continuous. 2 x 3kW is on and off.  If you have let's say a 4.5kW inverter, then each of those 3kW has to come from the grid, so at the expensive rate as it's during the day.  And we've not taken into account the dishwasher, washing machine and EV charging....   

 

4 hours ago, Archer said:

But an 8-10kw inverter seems much bigger than almost everyone has so I'm assuming that I'm misunderstanding and most appliances only draw the peak for a very short time, is that right?

 

If this is the case and I'm not sure it is, it could be because the maximum PV you can install without contacting the DNO is just less than 4kW, so a smaller inverter would match that.   However, if you have a battery system sized for your typical consumption, in our case 15kW, then a larger inverter makes sense and means we almost never take peak priced energy, it's all at the 7p Oct Int Go rate, because we load the batteries at night and our solar does the rest. 

 

As for most appliances drawing a peak for a short time, this is true for washing machines and dishwashers but not for ovens and induction hobs and EV chargers and ASHPs draw at the peak rate all the time.  And even dishwashers and washing machines can draw 3kW for 5 or 10 minutes as they heat the water.

 

4 hours ago, Archer said:

I'm getting some expert advice now, but it seems to be that 5-8kw inverter is probably the sweet spot for our usage. Does that sound about right?

 

Hopefully your experts will be expert....   What's right will depend on the appliances you have, how often they are used, what your base load is and also, what you want to get out of this. Do you effectively want to only use cheap rate electricity, or is the plan to reduce as much as you can your use of higher rate at a reasonable rate of return for the investment.

 

EDIT PS - the power output of your ASHP is the higher figure, the electricity usage is the lower figure

Edited December 3, 2024 by Bramco

[Bramco]

4 hours ago, G and J said:

Sizing for max need may incur a lot greater cost than is economically sensible.  

 

True - and you'd need a helluva'n inverter for this but it's worth knowing what typical usage would be, so adding up the ASHP, base load etc.  and also getting into the habit, if you can, of only using high usage appliances serially rather than all together. That way, it will only occasionally mean that you are drawing power from the grid at the expensive rates.

[SteamyTea]

1 hour ago, Bramco said:

It depends - as with many things....
 

Your base load will be between 250W and 500W depending on the time of day - more lights on more base load. There are many people on the forum that have their base load down to very small levels by disconnecting all wall warts etc.  But let's say you're a typical family, so between 250W and 500W.

It does.

There is a statistical quirt when you eliminate parasitic loads in that you get two mean figures.  One that includes times of zero power draw, the other when you exclude them.

Because of that, an inverter size may be different.

 

This is my last 6 years usage as an example.

 

 

Date	Week Day	Mean Zero Power 54%	Mean Power 0.45kW	Mean Power when >0W 0.56kW	Mean Max Power 7.66kW	Mean Energy 3781.7kWh
01/01/2019	Tuesday	54	0.50	0.57	8.91	4362.6
01/01/2020	Wednesday	46	0.53	0.59	9.21	4567.7
01/01/2021	Friday	56	0.55	0.73	13.16	4649.1
01/01/2022	Saturday	63	0.36	0.46	12.99	3133.6
01/01/2023	Sunday	62	0.38	0.50	8.13	3260.8
01/01/2024	Monday	45	0.35	0.47	8.90	2716.4
01/01/2025	Wednesday	#DIV/0!	#N/A	#N/A	0.00	#N/A

[RobLe]

2 hours ago, Bramco said:

 

True - and you'd need a helluva'n inverter for this but it's worth knowing what typical usage would be, so adding up the ASHP, base load etc.  and also getting into the habit, if you can, of only using high usage appliances serially rather than all together. That way, it will only occasionally mean that you are drawing power from the grid at the expensive rates.

We’ve never really bothered staggering loads to avoid peaks.  I have however trimmed the inverter ‘zero’, and that makes a lot of difference.  That is, the CT clamp feeding the inverter to tell it what the power draw is, that the inverter is trying to null, and gives a small signal that is easy to swamp with mains hum giving an offset.  With most inverters this can be tweaked out such that the smart meter shows 0W or maybe slightly -ve (exporting) rather than slightly importing all the time - this simple change makes a significant difference to the daily peak import.  I think I optimised this offset mid April, the pic below shows the peak consumption happening every day before that. There’s still sometimes a high daily peak import - I think that’s mostly the Ohme charging the car at odd times of the day - octo-aid doesn’t understand it’s actually octopus giving cheap hours.

[Dillsue]

So a year on from this chat and I wonder if things have changed significantly from an environmental and £ROI perspective??

 

Having just had our Octopus Go day rates go up to 32p/unit I've been looking at getting batteries to charge with off peak rates and use during peak rate times. Having put in an ASHP in the autumn our usage has jumped up significantly and Nov and Dec total usage averaged around 30kwh/day. If we could shift 20kwh/day of that and most of our usage for the rest of the year to off peak rates that would be quite a saving.

 

Other than the Octopus rates changing and charge/discharge losses what else needs factoring in??

[SteamyTea]

38 minutes ago, Dillsue said:

If we could shift 20kwh/day of that and most of our usage for the rest of the year to off peak rates that would be quite a saving

20 kWh.day^-1 is quite a lot. I use about 1.5 kWh.day^-1 during peak times.

Can you reduce that usage significantly, or get PV to come ntribute more to it.

40 minutes ago, Dillsue said:

Octopus Go day rates go up to 32p/unit

What was it before? 

This is the problem with ToU tariffs, they can change them all too easily making any capital expenditure redundant.

[Iceverge]

Can you shift your demand times or change to a supplier with TOU tarriffs that match your demand. 

[Dillsue]

3 hours ago, SteamyTea said:

20 kWh.day^-1 is quite a lot. I use about 1.5 kWh.day^-1 during peak times.

Can you reduce that usage significantly, or get PV to come ntribute more to it.

What was it before? 

This is the problem with ToU tariffs, they can change them all too easily making any capital expenditure redundant.

The 20kwh/day is battery capacity charged at 8.5p off peak and discharged to offset 32p peak use.....daily usage can be way more than that. ASHP has gobbled an average of 40kwh/day on its own over the last week in freezing temps.

 

We've already got PV but it doesn't do much in the winter

[JohnMo]

4 hours ago, Dillsue said:

So a year on from this chat and I wonder if things have changed significantly from an environmental and £ROI perspective?

All I know is from my own perspective, I almost never pay more than low rate Cosy even when its -5 outside. That's with a 13kWh battery and cosy tariff. (So 15p per kWh). 

[Dillsue]

3 hours ago, Iceverge said:

Can you shift your demand times or change to a supplier with TOU tarriffs that match your demand. 

Demand is 24/7 from the HP which is the biggest user. EV gets charged in the 8.5p off peak period every few days. The rest is consumption that can't really be shifted in any meaningful way.....asking the missus to cook overnight isn't an option.

 

The majority of heating  emitters are radiators so I don't have the thermal capacity to boost heat into a big slab using a Cosy like tariff. Ramping the HP up and down to track a Cosy like tariff would likely give us swings in temperature which I'd like to avoid.....we've just had 3 months of a steady 21 degrees with the HP running WC 24/7

 

 

[Roger440]

15 hours ago, SteamyTea said:

 

This is the problem with ToU tariffs, they can change them all too easily making any capital expenditure redundant.

This ^^^^^

 

Basing significant capital investment on TOU tarrifs is more akin to gambling.

[Bonner]

I think it’s a bit more predictable than gambling. TOU tariffs are based around market prices which are linked to supply and demand. We know there will be oversupply on windy nights and sunny days and, more predictably, large steam turbines which have to keep spinning all night.

[Roger440]

7 minutes ago, Bonner said:

I think it’s a bit more predictable than gambling. TOU tariffs are based around market prices which are linked to supply and demand. We know there will be oversupply on windy nights and sunny days and, more predictably, large steam turbines which have to keep spinning all night.

 

And as more people use them, the price will go up.

 

I guess there will always be some variability based on time, so in that regard, not entirely gambling, but certainly not sensible to base your battery investment on 7.5p per kWh. For example.

 

If you use a sensible number, payback extends to far.

 

[JohnMo]

35 minutes ago, Roger440 said:

This ^^^^^

 

Basing significant capital investment on TOU tarrifs is more akin to gambling.

ToU tariffs have existed since the 70s (E7 etc) so no idea why they will just disappear. Format may change, but that's life. Have managed the battery in simple E7 and now Cosy.

[S2D2]

11 hours ago, Dillsue said:

Demand is 24/7 from the HP which is the biggest user. EV gets charged in the 8.5p off peak period every few days. The rest is consumption that can't really be shifted in any meaningful way.....asking the missus to cook overnight isn't an option.

 

The majority of heating  emitters are radiators so I don't have the thermal capacity to boost heat into a big slab using a Cosy like tariff. Ramping the HP up and down to track a Cosy like tariff would likely give us swings in temperature which I'd like to avoid.....we've just had 3 months of a steady 21 degrees with the HP running WC 24/7

 

 

I still advocate a smaller battery and the Cosy tariff in winter as you get three charges a day. Yes the cheap rate is more than Go but it de-risks the battery payback uncertaintly.

 

ASHP still runs 24/7 from the battery and ignore the really cold days where you end up importing at standard rate, they are expensive but there are very few of these days a year. 60% of my annual bill has been the last 60 days of usage.