Posted by: @davesoa

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I’ve had a PowerWall for 3 weeks now, solar panels on the FIT schemes (and no, it doesn’t invalidate your FIT payments) and an EV. I’ve been signed up to Go since the battery was installed. Octopus did not check that I had an EV. Our PV output is around 2500 pa and we typically use around 2000 kWh per year excluding the EV which I think will add a further 1700 kWH charging at the Go rate, as is the battery. So far so good. The payback on the battery as it stands looks to be around 8 years.

This is where I share your concerns. I’m having a HP fitted in April. Throughout the summer my bills will be low so I’ll stay on Go. The battery will clearly not cope with the demand in the winter months so I anticipate I’ll need to switch to Cozy from Oct to end March and use the battery to tide me over the daily high peak rate. I can’t find a way to sensibly forecast my likely bills as I have no idea what the actual HP usage will be. I’m with @batalto on this. 

What is your heat loss calculation and what size of heat pump is being installed? What is your DHW consumption?

From the above it may be possible to make a rough estimate of your likely consumption.

@derek-m The demand is 19000, the HP is a 12kWh unit and the projected annual usage is 6058 kWh. The difficulty is knowing what the likely winter monthly usage will be and therefore the most appropriate tariff. The PowerWall only delivers around 11kwh on one charge so, if as one of the other posters stated they are using 60kWh some days, that’s a large deficit. On the current Go day rate of 0.4164 that’s £25 a day. Good job prices are predicted to fall later this year. 

@davesoa I have FiT and a battery. No issues

Posted by: @davesoa

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@derek-m The demand is 19000, the HP is a 12kWh unit and the projected annual usage is 6058 kWh. The difficulty is knowing what the likely winter monthly usage will be and therefore the most appropriate tariff. The PowerWall only delivers around 11kwh on one charge so, if as one of the other posters stated they are using 60kWh some days, that’s a large deficit. On the current Go day rate of 0.4164 that’s £25 a day. Good job prices are predicted to fall later this year. 

Do you have the detailed, room by room, heat loss calculations, on which the size of your heat pump should have been based?

What kind of heating do you have now, and how much energy does it consume?

Your heating demand will vary daily, dependent upon the weather conditions. As a very rough guide, if your estimated annual heat energy consumption is 19000 kWh, then the electrical energy supply to your heat pump should be approximately 1/3 of that.

@derek-m I have all that detail comprehensively produced for the quote so am content that it’s been calculated correctly - or, like car manufacturers mpg figures, to a standard, even if that doesn’t match the real world. I was simply wondering about the most effective (i.e. cheapest) tariff for the winter months when the battery will be least effective/useful. I cannot, yet, connect to Octopus IO because my EV isn’t compliant but the rumour is Zappi chargers will be integrated later this year. I’ve got all summer to think about an answer. A lot can change between now and then.

One other aspect to chuck in the mix is the inverter. Check the max you can put into the batteries .. or draw out. e.g. my Gen 1 GivEnergy hybrid 5.0 can only do 2.6kw. Which is fine as far as it goes.

But with 3 x 8.2kw batteries they ideally need much longer at cheap rates to fill them. Cosy, Flux tariffs not ideal. With our large ASHP we ideally would be able to draw more. 

Gen 2 is 3.6kw I think - so a useful improvement. I may consider an upgrade especially if they have a Gen 3 at say 5kw (no idea if planned!). Maybe there will be a trade in market...

For now I'm on Tracker (daily rates) tariff with Agile Outgoing.

May move to Flux in May - Nov as long as I can swap back to Tracker + Agile Outgoing in Nov.

Posted by: @tim441

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But with 3 x 8.2kw batteries they ideally need much longer at cheap rates to fill them.

Consider also the demand you are making through your Service Fuse whilst importing electricity to charge the batteries!

The most common size of single-phase inverter is 5kW due to the components available to the manufacturers.
A 5kW hybrid inverter will be drawing 20Amps if you want to charge batteries as fast as possible during a cheap-rate window.

To run a heat-pump from storage batteries you'd need at least two such inverters because a HP compressor will require around 30A each time it starts a cycle.

I'm just installing my 3rd 5kW hybrid inverter.
Although I don't (yet) have a HP, they will allow the entire house to run off-grid from the attached batteries.

So in theory I could be clocking 60A through my Service Fuse and Smart Meter between 00:30 and 04:30 using Octopus Go. 😲

We also have an EV, which gets charged at the same time.
That's another 32A on max charge-rate.

Houses generally have Service Fuses rated at 100A.
Some houses have 60A fuses, particularly if the feed-cable is 16mm2 (or its older imperial equivalent).

I won't be taking 60A of mains power to charge batteries during the cheap-rate time.

That decision is based on

• common sense!
• knowing that I'm the only house on my sub-station with LCT approval from the DNO, and I'm therefore causing massive phase-imbalance 🤔 
• I have a storage micro-controller which reduces my grid-import in line with the solar forecast for the following day
• the required energy-demand is spread over as long a period as possible (rather than as fast as possible)
• the DNO would forbid that level of import if there was an Application Process for hybrid off-grid equipment (which there isn't yet)

Most consumers will be unaware of the adverse conditions which they impart to the Distribution Grid when they install EV Chargers, Heat-pumps, Storage Batteries, Solar PV etc.

It's important that we discuss these issues here and make everyone else aware of how to use these devices whilst still being kind to our regional DNO!

@transparent

I fully agree.

Whilst it is important that people can use such systems to help reduce their electricity bill, and hence recoup the original costs, they should also be aware that how they operate such a system is equally important.

Keeping the cost of the bill down is not necessarily the most environmentally friendly course of action.

@batalto Great. That’s the most helpful consumption chart I’ve seen. Thank you. 

Have you looked at the Cosy Octopus tariff for the winter? They’ve supposed to have designed it for running ASHP with two cheap time slots a day. Edited: sorry, just spotted someone already mention Cosy.

Posted by: @si-fillo

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However, I believe the Octopus Go tariff where you can access even cheaper overnight rates only apply if you own an EV (I don't). 

When I switched there were no checks. Now they say it is intended for EV households. I have have just switched to Flux having reached the point it is difficult to use the peak solar generation. I will probably switch back to GO in November/December.

Phil

Posted by: @tim441

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One other aspect to chuck in the mix is the inverter. Check the max you can put into the batteries .. or draw out. e.g. my Gen 1 GivEnergy hybrid 5.0 can only do 2.6kw. Which is fine as far as it goes.

But with 3 x 8.2kw batteries they ideally need much longer at cheap rates to fill them. Cosy, Flux tariffs not ideal. With our large ASHP we ideally would be able to draw more. 

Gen 2 is 3.6kw I think - so a useful improvement. I may consider an upgrade especially if they have a Gen 3 at say 5kw (no idea if planned!). Maybe there will be a trade in market...

For now I'm on Tracker (daily rates) tariff with Agile Outgoing.

May move to Flux in May - Nov as long as I can swap back to Tracker + Agile Outgoing in Nov.

 

 

I checked with Octopus and will be staying on Tracker all year and have moved to Standard Outgoing Fixed (15p). My thinking:

EXPORT
1. even though Outgoing Agile rates are higher at times it is not possible for me to maximise as in summer my solar panels produce say 5 or 6kwh but max I can take into batteries is 2.6kwh (due to inverter max capacity) and with low summer household daytime usage (say 500w or less) I am obliged to export at the going rate. Agile in the daytime is typically 10p or less - often far less.

2. Even if I empty batteries at peak rates and fill during day the 2.6kw cap makes it impossible to benefit sufficiently from the higher rates 

IMPORT
1. I would ideally be on Tracker for winter and Flux for summer. But Octopus confirmed if I come off Tracker they cant guarantee if/when I go back on. Tracker currently has up to 6 months waiting list.

2. So i will stay on Tracker. Ave since Jan 1 2023 is around 23p saving 30% vs EPG. Mon 10/04/23 Tracker import was just 16.38p/kWh.

3. With an ASHP I cant draw enough at cheaper rates to make enough use of Cozy, Flux etc - even with benefit of batteries. As the max rate I can fill the batteries during cheap periods is again capped by the inverter capacity