How to calculate 2nd PW value?

[kuruma]

I'm not great at maths and wondered if the hive mind could help us calculate whether it's worth our while installing a second PW. Here's some info:

• We have had PW2 since December 2019. In that time, we've generated 18.8MWh of solar energy from our 5.2kW array. 30% of that has gone into the PW.
• We've put 2.6MWh (15%) of our solar into the grid during that time and our typical annual income from FiT payments is £250
• We're on the 'intelligent' octopus tariff so cheap energy for 5 hours a night and expensive otherwise.
• The latest quote for supply and install of a second PW is £6,950
• In the last six months, changes in our family routine have meant that we are now driving about 4-500 miles a week. That's probably about 3-400 miles more than we would have been. That's unlikely to change for the next decade. IOW, we have fairly high energy needs for the car. Even if we didn't have a Tesla, we'd have some other EV. We currently have a SR Model 3 and that's unlikely to change anytime soon either. So, each day, we're using 40-60% of our capacity to do all the running around we now need to do.

I'm getting confused about what of this info is relevant to calculating the benefit to us of a second PW. Can anyone suggest the best way of achieving this so that we can make a decision.

 

[Pink Duck]

4.34 MWh of annual solar production with 15% export, 30% storage and 55% to home.

So doubling storage only matters if charge/discharge rate doubles to boost reward from time-of-use tariffs such as Flux, or if more solar production will happen. Also a doubling of any future Saving Session returns. Usually the threshold for second PW is having a second EV. The benefits of Flux are questionable given your annual mileage (23,400 mi).

All needs to be considered versus £7,000 in a 5% savings account.

 

[WllXM]

I'm not great at maths and wondered if the hive mind could help us calculate whether it's worth our while installing a second PW. Here's some info:

• We have had PW2 since December 2019. In that time, we've generated 18.8MWh of solar energy from our 5.2kW array. 30% of that has gone into the PW.
• We've put 2.6MWh (15%) of our solar into the grid during that time and our typical annual income from FiT payments is £250
• We're on the 'intelligent' octopus tariff so cheap energy for 5 hours a night and expensive otherwise.
• The latest quote for supply and install of a second PW is £6,950
• In the last six months, changes in our family routine have meant that we are now driving about 4-500 miles a week. That's probably about 3-400 miles more than we would have been. That's unlikely to change for the next decade. IOW, we have fairly high energy needs for the car. Even if we didn't have a Tesla, we'd have some other EV. We currently have a SR Model 3 and that's unlikely to change anytime soon either. So, each day, we're using 40-60% of our capacity to do all the running around we now need to do.

I'm getting confused about what of this info is relevant to calculating the benefit to us of a second PW. Can anyone suggest the best way of achieving this so that we can make a decision.

You're not including any of your household consumption figures
What is your grid set up? 3-phase? Single?

We're missing a bit of data here and I think there are several other independent factors to consider.

I am assuming you are not constrained by any capacity issues to charge at IO cheap rate up to 23kWh (single phase 100A)
That means overnight charging of the Tesla + charging the two PWs.
And your 4500 kWh yearly solar generation.

The main question is: are you still finding yourself importing at peak IO rate, or are you already fully capable to live off the existing PW during the day and topping up off-peak?
If it's already the case, then I understand the additional PW only purpose will be for the export tarif, i.e: to charge at 7.5p and discharge at 15p, hence, a potential gain of 7.5p/kWh for you. That's a profit of less than £1 per day, hence quite a long break-even to justify the £7k costs.
If it's not and the second PW can allow you to stop importing at 30p+, then you may have a better investment theory already...

 

[kuruma]

thanks for those insights. Exactly what I was looking for.

I have no idea what our grid setup is. It's a house connected to the mains

The house consumption figures are now muddied by the fact that from 2024, the app now splits off power going to the vehicle whereas before that component was included in house consumption. But overall, we only very rarely ever need to import at peak rate. Usually that's because the PW is on time-based and it hasn't done what it should to give us enough charge to get to 2330 that day. That usually happens in spring/autumn as the weather varies a lot and the forecast can be off from time to time. We can force a reserve though if we don't trust what the PW is doing when we wake up at 2am.

@Pink Duck - we don't qualify for any Saving Sessions. We are only very rarely (see above) ever drawing from the grid in the evening. Therefore our average peak use is effectively zero and that is impossible to beat.

Seems wiser to possibly put the £7K towards upgrading our SR to an LR which effectively gives us more storage where we need it i.e. in the car, not in the PW

 

[Pink Duck]

This year Saving Sessions were calculated based on net import minus export. That’s where extra export power could boost return, if not usually discharging between 17:00 and 18:30.

 

[kuruma]

Sorry if I appear a bit thick here but I'm not sure what that means, poss because I don't understand how saving sessions work. Basically, every evening, we're using the PW to run the house. Are you saying that they can take whatever we're not using from the PW to the grid?

 

[Pink Duck]

As you said your evening peak grid usage is normally 0 kW, but you could be exporting at 4.97 kW or 9.96 kW instead. They are calculated from a baseline of 10 previous weekday half-hours and that of the event day itself and at least £1.75/kWh (up to £4/kWh for live events) below usual.

It’s not a huge return to factor in, I managed £600 with one PW this time around - say £1,200 per annum with two.

There’s likely greater return from topping up the second PW overnight at discount 7.5 p/kWh to export at 15 p/kWh+ during peak.

 

[WllXM]

Sorry if I appear a bit thick here but I'm not sure what that means, poss because I don't understand how saving sessions work. Basically, every evening, we're using the PW to run the house. Are you saying that they can take whatever we're not using from the PW to the grid?

Yes, paid at between £1.75/kWh to £2.25/kWh !
But the season is over now, and no one can know for sure if or how long it may continue at this (too good to be true?) rate. Which makes it rather difficult to project a ROI on that.

 

[kuruma]

ah right. OK. well we pretty much never have enough surplus to make much of that arrangement when we're on time based as the PW generally does a very good job of assessing what our load is going to be over the next peak period.

And if my terrible maths is right, at £1.75/kWh it would take over 300 full PW discharges to pay for the second PW. That's a long time in saving sessions, right?

 

[Dilly]

I went through the same arguments with myself.
I guess the bottom line is that financially it’s probably not worth it. However, having two PW’s allows a 10kWh draw rather than 3.6 or 5.
This means that you can cook, tumble dry, boil a kettle and hoover without using peak grid draw during the daytime. You can also put full bore in the car if needed.
You can also set a higher reserve and have longer backup in a power cut
I use 2MWh from the grid over a year of which just 60kWh is peak rate. Total house/car use is 10MWh, 8MWh comes from Solar.
I can run the house off grid for a day on one PW. Having two is a useful bonus and I don’t regret the outlay one bit!

 

[KennethS]

At the risk of taking a very simplistic view, if you are using very little peak electric with your current setup then adding more battery storage is unlikely to be economically beneficial. If you are in an area where power outages are a concern then having extra storage could be a consideration for longer house running during outages.

 

[kuruma]

Thanks @Dilly I can see why you like the advantages it brings and giving me a couple of use-case scenarios makes me realise that there aren't any we encounter on a regular basis. We aren't in an area where power outages are an issue either @KennethS so not a necessity from that POV either.

 

[WannabeOwner]

changes in our family routine have meant that we are now driving about 4-500 miles a week.

Does that mean the car won't be at home (during peak solar hours)?

There are (fairly significant loses) doing PV->PowerWall and then PowerWall->Car ... but if you have been exporting a fair bit, in Summer (and/or previously charged car during the day in Summer and no longer will be doing that) then you could consider putting that "into PowerWall" and then from PowerWall into car overnight (ready for tomorrow's drive)

 

[kuruma]

A good consideration, but the car will be at home all day most days.

 

[WannabeOwner]

the car will be at home all day most days.

Then I think my consideration would be:

In Summer will my single PowerWall cover from end of Off Peak to "PV Sunup" (when PV generates enough to match house consumption)
and (assuming 100% charged from PV during the day) from "PV Sundown" to start of Off Peak

Then in Winter what proportion of days does charging PowerWall to 100% on Off Peak last (or "not last" ) from end of Off Peak to start of Off Peak following night. If you had a heat pump (assuming that is a likely future purpose) would that additional consumption benefit from additional PowerWall?

Data for PowerWall can be exported from APP - so if you have a year's worth that would be visible.

Within that will be car charging. If you have something like TeslaFi then the actual Car Charging (times / AMPs) could be extracted. I do that, and I take the view that any car charging during the day is from PV (I have occasions when a mate shows up, and charges his car from grid during the day, but few enough that I don't bother to identify them - they are not in my TeslaFi data of course; might be in data from Wall Charger if that is available)

Extra PowerWall would help in shoulder months, and also a cloudy day following a sunny one (IME in mid summer even a cloudy day will fill my PWs to 100%) - but I doubt that would make a cost justification.

If car is at home during the day then could the money be spent on more PV with the aim of charging the car, from excess PV, more often? (Depends on whether you have any roof! but, for example, North face of roof would be a consideration ... or ground-mounted maybe.

 

[kuruma]

Thanks for the detailed description. That's really helpful. We've maxed out our PV roof area and unless they introduce vertical panels and our conservation area allows them to be installed, I think we're set with all the solar we can generate for the next 10 years. All south facing so we get really good coverage from now until late Sep. No one else ever charges here.

Never considered the impact of a heat pump.... that's unlikely though in the next decade but our boiler is only 5 years old and has a 10 year guarantee and is likely to do 20 as it's a top of the range model. But prices of PW and equivalent will drop at such a point as a heat pump was ever needed so prob best to install it then if the calculations make it worthwhile.

 

[WannabeOwner]

We've maxed out our PV roof area .... All south facing

No (unused) North facing roof?

North will generate 70-80% of South (in summer, and provided that the pitch is not steep) and will also give you earlier-start and later-finish (when you are likely to be home before-work and after-work) - in high-summer (only)

 

[kuruma]

interesting... we're in a conservation area so possible that council don't allow panels on the front roof. We don't work much and if we do it's mostly from home anyway. I'll look into that.

 

[WannabeOwner]

I'll look into that.

In you are not familiar with it: PVWatts will give you an approximate "output" value

 

[MrBadger]

North will generate 70-80% of South (in summer, and provided that the pitch is not steep) and will also give you earlier-start and later-finish (when you are likely to be home before-work and after-work) - in high-summer (only)

Worth reiterating that South facing only is a bit of a fallacy. Whilst it may give potential for peak power, mixing other aspects give opportunities for peak energy, especially peak usable energy in many scenarios.