Adding battery to existing solar PV system - any thoughts/advice please.

QrizB

JKenH said:

QrizB said:

JKenH have you still got the issue of the faulty panels in your current FIT-earning array?

Might this also be an opportunity to do something about those?

(Sorry for introducing yet another variable to the equation!)

The defective panels are on two elevations of a two storey roof and the cost of scaffolding plus electrician to lift each panel to get to the underside plus maybe 3 replacement panels would, I suspect, be in the region of £1,500 so that’s around £2k per effective kWp.

Just thinking about this, but do you have vehicular access to both aspects? You can hire a cherry picker for a lot less than the cost of scaffolding.

Something like this:

https://www.highdrive.uk/

I suspect sourcing replacement panels might be the biggest problem, short of repanelling one aspect and then using those as spares for the other.

Screwdriva

JKenH said:

How much control over charging and discharging do you have?

Not much at all. It does it all on its own. 

QrizB said:

I suspect sourcing replacement panels might be the biggest problem, short of repanelling one aspect and then using those as spares for the other.

I had also thought of consolidation the panels to one aspect but I suspect only 3-5 panels have failed in total (out of 28).

In theory, OP will have 8.4kWh (existing arrays) + 5kWh (new array) = 13.4 kWP

Less 1.5kW of failed panel deletion (5 X 300W panels) = 11.9kWP

The PW3 can't export more than 11.04kWh. Current DNO allowance is 7.3kWh.

IMO, deletion of the failed panels and the addition of a new 5kW array at this juncture are the most cost effective ways to optimize system performance. 

NedS

JKenH said:

NedS said:

Yes, for my region, IOF day rates (22.36p) are ~50% higher than SEG (15p) and almost 100% higher at peak times (29.82p). Last month we averaged 26.3p on our export, 1.75 times the SEG rate.

Having a battery may also allow you to manage/buffer your export. Octopus will commonly discharge the battery down to 20% at peak times, and not recharge until the following afternoon, meaning the battery has the capacity to soak up excess solar above your export limit during the peak part of the day when you would otherwise be clipping, and then export that electricity at peak rate, so it's a win-win as generation that may have been lost to clipping and/or export limitation is now exported at ~30p/kWh.

The battery doesn't have to be huge to buffer generation above your 6kW export limit.

Thats interesting to note. I have now placed an order for a PW3 plus a further 5kW of panels. It remains to be seen what the DNO has to say. If necessary I may have to scale back the additional solar. We only generate a maximum of around 4.5kW from our existing ENE/WSW panels so we have almost 3kW of spare capacity on our existing system.  

If the PW3 does leave charging until the afternoon that would be great for me as on a sunny day it could mop up any generation in excess of our export limit which is likely to peak around lunchtime.

The new panels are going on our SSE facing garage roof which this time of year starts going into shade at 4pm and is fully shaded by 5.15pm. The PW3 apparently isn’t compatible with optimisers so the 10 panels will be wired to the three DC inputs on the PW3 on a 3/3/4 split to deal with the shading.

Does Octopus ever cycle the battery more than once - say to meet the breakfast time surge as well as the 4-7pm slot? And does it cycle fully everyday? How much control over charging and discharging do you have?

We have 7.2kWp, south facing which can generate ~7.5kW and sometimes spikes to 8kW. We have a 5kW export limitation. I have found IOF to manage this pretty well this summer (we only installed in July, but it's generally worked very well on IOF)

Octopus only cycle the battery once per day, discharging between 4-7pm. Depending on the battery's capacity and your export rate, they may discharge for the whole 3 hours (if you have a 15kW battery and a 5kW export limit), or they may discharge for less, holding back until 5-7pm when it's needed most. They will discharge down to 20% but you can manually override this on a Tesla PW so I sometimes reduce my reserve to 5% to squeeze a little more out of the battery at peak rate.

On the vast majority of days the battery will sit at 20% all night and recharge the following day. Sometimes they charge to 50% or 80% overnight, but not often. I guess this happens if there is very cheap energy overnight (excess wind maybe), but also tends to coincide with days which aren't overly sunny. Octopus only ever cycle once per day, I've never seen them discharge in the morning peak as the battery is typically empty (at it's 20% reserve) then anyway.

How much control do you have? Simple answer is some, but IMHO not enough.

When on Octopus IOF, you have limited control because Octopus control the battery. There is a slider for the battery reserve level that goes from 0% to 100% As I already mentioned, by default the reserve is set to 20% (so you always have a bit left for an outage), but when Octopus are discharging you can override the 20% down to a lower value to squeeze a little more export out of the battery. I do not know if you can increase the reserve during discharge - never tried.

You can also manually force the battery to charge by moving the slider to a higher value than the current SoC. In this mode, the battery will generally charge at 3.3kW (or up to 5kW if there is more than 3.3kW of solar available). When charging on a schedule, it will charge at it's maximum 5kW rate. At a rate of 3.3kW, my battery takes around 3.5h to charge, so I often manually start it charging on sunny days around midday so that I'm charging throughout the peak of the day and preventing any clipping if I'm generating more than I can export, and I also know the battery will be fully charged by 4pm (plus I'm sat at home all day with nothing better to do than monitor my PW!). So you have some control over charging, and generally IOF works well for me in summer (see more below)

If you are NOT on IOF, but are on any other non-intelligent tariff where you (or rather the Tesla App) is in control of your battery, then the following applies.

There are two modes of operation in the Tesla App:

1. Self Powered

This is for people who want to maximise independence from the grid and minimise import, running purely on solar. The battery will prioritise storing and self consuming all solar before resorting to exporting any excess, and will prioritise powering the house from battery, only drawing from the grid if the battery gets down to its reserve level and there is no solar. Great for the Greens who want grid independence, or maybe have no permission to export so better to prioritise self consumption.

2. Time-Based Control

If you are on a ToU tariff then this is for you. You enter the times/rates of your tariff into the app, and the app then uses it's own AI algorithm to maximise your savings, deciding when to import, export, charge and discharge. I use this mode with Cosy in winter and it works OK. What you don't really have is much control over what it does or when it does it - it does what it thinks is best, even if you'd prefer something different. As above, you can manually force it to charge (at 3.3kW) by moving the slider to 100% but there is no way to automate these actions without 3rd party software (see NetZero, which now charges a monthly fee). There is no easy way to manually force the battery to discharge/export. People end up exaggerating their tariff rates to try to force the algorithm to do what they want. Those on EV tariffs seem to struggle most, waking up to find the battery has decided it's not worth fully charging overnight on the cheap rate because it thinks it will be sunny the next day so you'll want to use all that solar, whereas what would be most cost effective would be to fully charge the battery at 7p overnight and then export all the solar at 15p the next day. The lack of fine control can get frustrating, especially if you are trying to do something more complex. In summer, if you decide not to go on IOF, trying to maintain sufficient space in the battery to allow for midday clipping may be challenging, unless you manipulate the tariff rates to make it think it's more lucrative to export everything at 7am so the battery is empty to take the excess solar. You probably could do it, but I have found IOF much easier and more lucrative in summer. My main issue is that I'd feel like I was constantly fighting the system which thinks it knows best, so I've found IOF to be a far better solution for me. In winter it's not really an issue as you're generating far less from solar and much more likely to be consuming everything you generate and/or can easily export the excess without hitting your export limits. 

What you don't have is a simple interface that says do this at this time, and that at that time. You put it in one of the two modes of operation above, put your tariff times and rates in for ToU tariffs, and let it get on with it. It's supposed to be set and forget for the 99% of people that don't care and just want it to work and save them some money. As you can probably tell from my rantings, I'd prefer much more fine grain control but I accept that it works well for the majority of people, the majority of the time.

debitcardmayhem

@NedS have you looked at Home Assistant integration with PW , it may help, I can use it to manage my Givenergy system , it’s a bit of a learning curve but you can run it on a Pi or as I do in a virtual machine on windows. Having said that I still am learning … that’s a good thing yes?🥴

JKenH

NedS said:

JKenH said:

NedS said:

Yes, for my region, IOF day rates (22.36p) are ~50% higher than SEG (15p) and almost 100% higher at peak times (29.82p). Last month we averaged 26.3p on our export, 1.75 times the SEG rate.

Having a battery may also allow you to manage/buffer your export. Octopus will commonly discharge the battery down to 20% at peak times, and not recharge until the following afternoon, meaning the battery has the capacity to soak up excess solar above your export limit during the peak part of the day when you would otherwise be clipping, and then export that electricity at peak rate, so it's a win-win as generation that may have been lost to clipping and/or export limitation is now exported at ~30p/kWh.

The battery doesn't have to be huge to buffer generation above your 6kW export limit.

Thats interesting to note. I have now placed an order for a PW3 plus a further 5kW of panels. It remains to be seen what the DNO has to say. If necessary I may have to scale back the additional solar. We only generate a maximum of around 4.5kW from our existing ENE/WSW panels so we have almost 3kW of spare capacity on our existing system.  

If the PW3 does leave charging until the afternoon that would be great for me as on a sunny day it could mop up any generation in excess of our export limit which is likely to peak around lunchtime.

The new panels are going on our SSE facing garage roof which this time of year starts going into shade at 4pm and is fully shaded by 5.15pm. The PW3 apparently isn’t compatible with optimisers so the 10 panels will be wired to the three DC inputs on the PW3 on a 3/3/4 split to deal with the shading.

Does Octopus ever cycle the battery more than once - say to meet the breakfast time surge as well as the 4-7pm slot? And does it cycle fully everyday? How much control over charging and discharging do you have?

We have 7.2kWp, south facing which can generate ~7.5kW and sometimes spikes to 8kW. We have a 5kW export limitation. I have found IOF to manage this pretty well this summer (we only installed in July, but it's generally worked very well on IOF)

Octopus only cycle the battery once per day, discharging between 4-7pm. Depending on the battery's capacity and your export rate, they may discharge for the whole 3 hours (if you have a 15kW battery and a 5kW export limit), or they may discharge for less, holding back until 5-7pm when it's needed most. They will discharge down to 20% but you can manually override this on a Tesla PW so I sometimes reduce my reserve to 5% to squeeze a little more out of the battery at peak rate.

On the vast majority of days the battery will sit at 20% all night and recharge the following day. Sometimes they charge to 50% or 80% overnight, but not often. I guess this happens if there is very cheap energy overnight (excess wind maybe), but also tends to coincide with days which aren't overly sunny. Octopus only ever cycle once per day, I've never seen them discharge in the morning peak as the battery is typically empty (at it's 20% reserve) then anyway.

How much control do you have? Simple answer is some, but IMHO not enough.

When on Octopus IOF, you have limited control because Octopus control the battery. There is a slider for the battery reserve level that goes from 0% to 100% As I already mentioned, by default the reserve is set to 20% (so you always have a bit left for an outage), but when Octopus are discharging you can override the 20% down to a lower value to squeeze a little more export out of the battery. I do not know if you can increase the reserve during discharge - never tried.

You can also manually force the battery to charge by moving the slider to a higher value than the current SoC. In this mode, the battery will generally charge at 3.3kW (or up to 5kW if there is more than 3.3kW of solar available). When charging on a schedule, it will charge at it's maximum 5kW rate. At a rate of 3.3kW, my battery takes around 3.5h to charge, so I often manually start it charging on sunny days around midday so that I'm charging throughout the peak of the day and preventing any clipping if I'm generating more than I can export, and I also know the battery will be fully charged by 4pm (plus I'm sat at home all day with nothing better to do than monitor my PW!). So you have some control over charging, and generally IOF works well for me in summer (see more below)

If you are NOT on IOF, but are on any other non-intelligent tariff where you (or rather the Tesla App) is in control of your battery, then the following applies.

There are two modes of operation in the Tesla App:

1. Self Powered

This is for people who want to maximise independence from the grid and minimise import, running purely on solar. The battery will prioritise storing and self consuming all solar before resorting to exporting any excess, and will prioritise powering the house from battery, only drawing from the grid if the battery gets down to its reserve level and there is no solar. Great for the Greens who want grid independence, or maybe have no permission to export so better to prioritise self consumption.

2. Time-Based Control

If you are on a ToU tariff then this is for you. You enter the times/rates of your tariff into the app, and the app then uses it's own AI algorithm to maximise your savings, deciding when to import, export, charge and discharge. I use this mode with Cosy in winter and it works OK. What you don't really have is much control over what it does or when it does it - it does what it thinks is best, even if you'd prefer something different. As above, you can manually force it to charge (at 3.3kW) by moving the slider to 100% but there is no way to automate these actions without 3rd party software (see NetZero, which now charges a monthly fee). There is no easy way to manually force the battery to discharge/export. People end up exaggerating their tariff rates to try to force the algorithm to do what they want. Those on EV tariffs seem to struggle most, waking up to find the battery has decided it's not worth fully charging overnight on the cheap rate because it thinks it will be sunny the next day so you'll want to use all that solar, whereas what would be most cost effective would be to fully charge the battery at 7p overnight and then export all the solar at 15p the next day. The lack of fine control can get frustrating, especially if you are trying to do something more complex. In summer, if you decide not to go on IOF, trying to maintain sufficient space in the battery to allow for midday clipping may be challenging, unless you manipulate the tariff rates to make it think it's more lucrative to export everything at 7am so the battery is empty to take the excess solar. You probably could do it, but I have found IOF much easier and more lucrative in summer. My main issue is that I'd feel like I was constantly fighting the system which thinks it knows best, so I've found IOF to be a far better solution for me. In winter it's not really an issue as you're generating far less from solar and much more likely to be consuming everything you generate and/or can easily export the excess without hitting your export limits. 

What you don't have is a simple interface that says do this at this time, and that at that time. You put it in one of the two modes of operation above, put your tariff times and rates in for ToU tariffs, and let it get on with it. It's supposed to be set and forget for the 99% of people that don't care and just want it to work and save them some money. As you can probably tell from my rantings, I'd prefer much more fine grain control but I accept that it works well for the majority of people, the majority of the time.

Wow, that’s a lot of useful detailed advice. I am thinking (assuming the DNO doesn’t throw a spanner in the works and say No to everything, that IOF will be the best tariff for the summer to maximise solar export (having limited import to worry about for those 6 months) but initially I am thinking if seeing how Agile import/fixed (15p) export goes. The margin on export over import rates isn’t great on IOF and when I do import for consumption at around 21p, that maybe more than my average on Agile which for the last 6 months has been under 15p. With a battery I can focus on filling the battery for domestic consumption at the cheap, sometimes negative rates. I know there were some 99p import slots last winter but even on those days I think there were some cheap slots around 20p. 

If I have understood you correctly, you are IOF in the summer and Cosy in the winter. Did you look at Agile import and decide it didn’t work so well. Do you think the battery would be too difficult to manage manually to make optimal use of Agile or does it see the Agile half hour tariffs and optimise charging and discharging for around them using the AI algorithm you refer to? I know you think the AI isn’t perfect so I would assume it would need tweaking. I am assuming that if you aren’t in IOF then the battery prioritises domestic consumption over export to suit Octopus.

JKenH

Screwdriva said:

JKenH said:

How much control over charging and discharging do you have?

Not much at all. It does it all on its own. 

QrizB said:

I suspect sourcing replacement panels might be the biggest problem, short of repanelling one aspect and then using those as spares for the other.

I had also thought of consolidation the panels to one aspect but I suspect only 3-5 panels have failed in total (out of 28).

In theory, OP will have 8.4kWh (existing arrays) + 5kWh (new array) = 13.4 kWP

Less 1.5kW of failed panel deletion (5 X 300W panels) = 11.9kWP

The PW3 can't export more than 11.04kWh. Current DNO allowance is 7.3kWh.

IMO, deletion of the failed panels and the addition of a new 5kW array at this juncture are the most cost effective ways to optimize system performance. 

That’s the conclusion I reached. 

Just a small correction to the figures quoted. I have 12 panels on one roof and 14 on the other (26 in total) both on 3.68kW inverters so with the new 5kW array total kWp will be 12.8 (7.8 +5.0). Underperforming existing panels are knocking off approximately 1kWp so a net 11.8 kWp (effective). In practice being split ENE/WSW max peak output (ignoring spikes) of existing arrays is only 4.5kW plus a potential 5kW of new panels = 9.5kW. That’s 2.2kW over the current DNO limit. If the existing panels continue to deteriorate then it won’t be the end of the world if I am stuck with a 7.3 kW export limit as there aren’t that many good days in the year when there will be a huge loss of generation.

NedS

JKenH said:

Wow, that’s a lot of useful detailed advice. I am thinking (assuming the DNO doesn’t throw a spanner in the works and say No to everything, that IOF will be the best tariff for the summer to maximise solar export (having limited import to worry about for those 6 months) but initially I am thinking if seeing how Agile import/fixed (15p) export goes. The margin on export over import rates isn’t great on IOF and when I do import for consumption at around 21p, that maybe more than my average on Agile which for the last 6 months has been under 15p. With a battery I can focus on filling the battery for domestic consumption at the cheap, sometimes negative rates. I know there were some 99p import slots last winter but even on those days I think there were some cheap slots around 20p. 

If I have understood you correctly, you are IOF in the summer and Cosy in the winter. Did you look at Agile import and decide it didn’t work so well. Do you think the battery would be too difficult to manage manually to make optimal use of Agile or does it see the Agile half hour tariffs and optimise charging and discharging for around them using the AI algorithm you refer to? I know you think the AI isn’t perfect so I would assume it would need tweaking. I am assuming that if you aren’t in IOF then the battery prioritises domestic consumption over export to suit Octopus.

@JKenH

IOF is a tariff for summer when you are a net exporter, so that's pretty much over for this year (but hopefully great or next summer).

I think Agile import will be difficult to manage on a Powerwall unless you have some way to automate when to charge and when to export. You could just crudely set it up as ToU and say generally Agile is cheap in these periods (10:30opm to 5am, 11am to 4pm, cheap rate, with a bit of fine tuning) and expensive (peak rate) at all other times or get as creative as you like with those slots, and leave the battery to average down the cost, but full automation would be a better option and that requires an additional system to fully integrate with Agile.

How much is your daily usage? Will a PW (13.5kWh, maybe 12kWh usable) cover you, topped up with some solar as required? If so, then those 99p peak rates are irrelevant as you'll be running on battery having charged up during the afternoon.

If not on IOF, then the battery either prioritises self consumption of solar in Self-Powered mode, or prioritises monetary savings if in Time-Based mode for ToU tariffs.

NedS

debitcardmayhem said:

@NedS have you looked at Home Assistant integration with PW , it may help, I can use it to manage my Givenergy system , it’s a bit of a learning curve but you can run it on a Pi or as I do in a virtual machine on windows. Having said that I still am learning … that’s a good thing yes?🥴

Thanks for the suggestion. At the moment the Tesla App is pretty much doing what I need so we will see how that goes. I was very concerned how I would manage the battery over summer with my export limitation, but IOF has effectively removed that concern and does it for me as a side effect of the way it works.

JKenH

NedS said:

JKenH said:

Wow, that’s a lot of useful detailed advice. I am thinking (assuming the DNO doesn’t throw a spanner in the works and say No to everything, that IOF will be the best tariff for the summer to maximise solar export (having limited import to worry about for those 6 months) but initially I am thinking if seeing how Agile import/fixed (15p) export goes. The margin on export over import rates isn’t great on IOF and when I do import for consumption at around 21p, that maybe more than my average on Agile which for the last 6 months has been under 15p. With a battery I can focus on filling the battery for domestic consumption at the cheap, sometimes negative rates. I know there were some 99p import slots last winter but even on those days I think there were some cheap slots around 20p. 

If I have understood you correctly, you are IOF in the summer and Cosy in the winter. Did you look at Agile import and decide it didn’t work so well. Do you think the battery would be too difficult to manage manually to make optimal use of Agile or does it see the Agile half hour tariffs and optimise charging and discharging for around them using the AI algorithm you refer to? I know you think the AI isn’t perfect so I would assume it would need tweaking. I am assuming that if you aren’t in IOF then the battery prioritises domestic consumption over export to suit Octopus.

@JKenH

IOF is a tariff for summer when you are a net exporter, so that's pretty much over for this year (but hopefully great or next summer).

I think Agile import will be difficult to manage on a Powerwall unless you have some way to automate when to charge and when to export. You could just crudely set it up as ToU and say generally Agile is cheap in these periods (10:30opm to 5am, 11am to 4pm, cheap rate, with a bit of fine tuning) and expensive (peak rate) at all other times or get as creative as you like with those slots, and leave the battery to average down the cost, but full automation would be a better option and that requires an additional system to fully integrate with Agile.

How much is your daily usage? Will a PW (13.5kWh, maybe 12kWh usable) cover you, topped up with some solar as required? If so, then those 99p peak rates are irrelevant as you'll be running on battery having charged up during the afternoon.

If not on IOF, then the battery either prioritises self consumption of solar in Self-Powered mode, or prioritises monetary savings if in Time-Based mode for ToU tariffs.

Thanks for that clarification. I had been hoping the PW3’s AI would (like me) at 4pm look at the following day’s Agile rates and pick and choose the best slots for charging. For me it will be a year of two halves, with IOF maximising the return from my own generation export in the summer and banking that to cover the winter where I will be looking for the PW3 to minimise my import cost. There will of course be a transition period twice a year which will need some thought. Part of the reason for getting a battery is that extra interaction it gives with the electricity market - in other words fun.

For the last 180 days my Octopus Watch app tells me I have averaged 6.5 kWh a day import and 19 kWh export, most of that time on Agile. Prior to that I had been on standard Flux. Over the last 365 days my average import has been 11.2 kWh at an average cost of 20.1p/kWh and average export 11.1kWh at 14.8p/kWh.  That works out at a total import of 4088 kWh and total export of 4052. My autumn and winter import will therefore have been 2918kWh or 15.8kWh and my export for Autumn/winter of 632 kWh or 3.4kWh/day. Last year I generated 4779 kWh so I exported 84.8% of my generation - more than my 80% estimate. My electricity import (excluding SC) over the past year cost me £822 and that was offset by export earnings of £600. 

My import is so high because one of our hot water cylinders is heated purely by electricity and the other one is heated by electricity in summer and oil boiler in winter. I could use oil all year round for that bathroom but it isn’t economical to fire the boiler up to displace a couple of kWh electricity. In practice when I have cheap Agile rates I bang a load of heat in and try to avoid heating it when it’s expensive. I could and probably will do that in winter as well. 

Screwdriva

JKenH said:

My import is so high because one of our hot water cylinders is heated purely by electricity and the other one is heated by electricity in summer and oil boiler in winter. I could use oil all year round for that bathroom but it isn’t economical to fire the boiler up to displace a couple of kWh electricity. In practice when I have cheap Agile rates I bang a load of heat in and try to avoid heating it when it’s expensive. I could and probably will do that in winter as well. 

Have you considered?

1) An EV (even a cheap one) to gain access to the Intelligent Go EV tariff? 7p from 11:30pm to 5:30am every day. This tariff works well with the PW3 and you'll still have access to their 15p export rate. 

and/ or

2) A heat pump hot water cylinder like the Mixergy iHp?  No mains gas or oil needed. 

You could always manually program in the cheapest Agile rates on a daily basis but that may become tedious. Another alternative is the Cosy tariff but it's not as cheap as Intelligent Go.