question

johnone avatar image
johnone asked

UK newbie 'store at night, use in the day' query

Help please,

Given energy costs, in the UK lots of retailers are now selling domestic 'on-grid storage and self-consumption' kits "for storing cheap night-time energy (off-peak tariffs e.g. Octopus) for use during the day", e.g. a kit with Multiplus II 48/5000/70-50 and 14kWh of batteries, "more battery modules can be added at any time", and PV at later date - https://www.bimblesolar.com/ongrid/self-consumption/5kVA-On-grid-Victron-ESS-14kwh

With 14kWh of 6000 cycle, 95% DOD batteries I'm expecting to be able to store all my elec need at night (@ 7p/kWh) and not use any grid elec in the day (@ 47p/kWh). When we add solar PV, 5kWp will mean we are mostly self-sufficient for 5 months of the year (according to https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html ). But, with a 4kw inverter the above Multiplus seems underrated for 14kWh of batteries. We typically use 7Kwh night and 7kWh day but with occasional peak demand of 8kW (cooking, kettle etc). The expectation was to have all loads via the inverter - 'critical' on AC out 1 - otherwise the system doesn't pay for itself. Also we're rural and have frequent, short duration power outages and thus need the UPS functionality. Am I misunderstanding something here? I'm assuming that, in the day, all power needs can be supplied by the batteries and thus via the inverter and thus the 4kW inverter seems wrongly matched with the 14kWh batteries?

Thanks

Multiplus-IIinverter current draw
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5 Answers
wkirby avatar image
wkirby answered ·

4x Pylontech US3000C batteries are a very good match for a 5kVA inverter. The batteries can supply a maximum of 148A in that configuration and the inverter would be drawing about 100A at full chat.
Also, that MultiPlus can charge from the network at 70A battery current (~3.5kW) which means that you can pretty much get 14kWh of energy in the magical 4 hours that Octopus Go gives you.

These systems are usually specified for grid parallel operation meaning that the network is always connected even if no energy is being taken from it. This means that if your AC loads are higher than the inverter capacity then the ramining power is supplimented by the network. This MultiPlus has a 50A passthrough capacity, so you can have loads up to ~12kW if the network is available. This does mean that you will be using peak rate electricity for short periods of time.
If the network is not available then you have to make concessions to keep your loads within the inverter's capacity. This, of course, would be a nuisance if your network is unreliable.

If you don't want to run a grid parallel setup, only connecting to the network to charge at night and you really don't want to use any peak rate electricity at all, then you'll need a higher capacity MultiPlus to cover your peak loads.
An 8kVA MultiPlus would also need at least 5x US3000C batteries so as not to overload them.

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sharpener avatar image
sharpener answered ·

Looks OK to me at a first glance. The 4kW from this inverter will be supplemented from the grid when necessary (assuming it is available), as I have just seen @WKirby has also mentioned there is a 50 amp pass-through capabililty for this which will give you a total of 12kW.

We have one and it is enough for the whole house, obviously if/when the grid is down we have to be sparing with the use of the cooker and if you want to use nothing but battery power during the peak rate period you would have to do the same. I think however there would be little economic justification for a bigger inverter and if you are in a rural area you might have more difficulty getting DNO approval for it.

The 14kWh battery meets the requirements in this link, it will give you >3 hrs endurance at full output in a power cut. We manage to get away with only 7kWh (but we have PV as well) so I don't see you would have any problem with 14, you could start with less and add batteries later. If you are planning to add 5kW of PV anyway you might find they are not needed.

If you are happy to use a PC for controlling the system you might find the Multi II-GX version with integral computer is cheaper/more convenient and you would not then need the

Victron MK3-USB interface for Configuring Multiplus


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semlohnhoj avatar image
semlohnhoj answered ·

@Johnone just to further the above comments. I'm also moving onto the Octopus Go tariff with the same aim. I'm using the Multi RS Solar which is a 6KVA inverter with a built in MPPT charger so everything is in the same box (apart from the Cerbo GX). There are caveats though using this model:

1) it doesn't currently support any grid feed in as it doesn't have approval for that in the UK (which in my case is a bonus because I don't want to give the grid my energy for almost no return).

2) it doesn't support scheduling the charging of the batteries out of the box at present. You can either disconnect the grid feed with a relay and timer during peak but that means you will only have 6kva inverter capacity, or you can do what I did and use a PC to change the settings via the victron connect app using Macro Scheduler or similar app. It's not a clean solution but until they offer better support via firmware then that is what I do.


I suspect the Multi RS Solar is cheaper than buying a MultiPlus-II and a separate MPPT Controller.


Oh, by the way, you are supposed to have an electric car to use the Octopus Go tariff as well.

3 comments
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wkirby avatar image wkirby ♦♦ commented ·
I also have a Multi RS and I use it in much the same way as you do.

I too have less than zero interest in feeding in to the grid, so the lack of feed-in certification does not bother me in any way.

The 6kVA capacity has been fine for me. I did get 7kVA out of mine once, some vistors were over who have no understanding of an "off grid" house and they switched the kettle on whilst the immersion heater and the oven were on at the same time.

I use the Auxiliary input to toggle the "AC-IN connect" which basically opens or closes the AC input relays. I do the scheduling using Homeassistant.

I also have Octopus Go 5p/15p but they are going to take it off me in a few weeks time because I don't have an EV.

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awalkerinuk avatar image awalkerinuk wkirby ♦♦ commented ·
Look at standard ECO 7 Tariff with Octopus 14p Kwh for 7 hours at night 440 Kwh all other times. Octopus Go rates have just gone up to 12p Kwh so not that different.
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wkirby avatar image wkirby ♦♦ awalkerinuk commented ·

Ah, wonderful. ECO 7 would suit me just fine. 7 hours also means that I can charge at a lower power.

Thank you.

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johnone avatar image
johnone answered ·

Thanks all for thoughtful responses. I can now see how it's about a cost balance between how much 'top up' you'll need vs the cost of bigger inverter and more batteries. I'm now satisfied with what Bimble specify. On the tariff, I'm not on Octopus - the rates mentioned are the new '2 year fix' rates from another supplier.

Thanks again

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sharpener avatar image
sharpener answered ·

Maybe @johnone you would be good enough to share the details?

Here in Devon our Scottish Power E7 offpeak rate has just gone down from 15 to 11p, peak is up from 35 to 45.

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johnone avatar image johnone commented ·

EDF E7 in East Midlands, from Oct 1st, day up from 35 to 47p, night down from 13 to 7p. I've shared this on other forums and it seems E7 rates vary widely. Octopus provide useful tables of the from-October price caps per region and payment method. With E7 there's a new 'blended average' mechanism: "This means that suppliers can alter day and night price levels so long as together, their average price does not exceed the Energy Price Guarantee cap." https://octopus.energy/blog/how-the-energy-price-guarantee-works/

It seems entirely up to the supplier to decide what proportion day/night they want to assume and whether or not to 'reward' greater proportion of night usage. I've no idea if they set the E7 rates per customer and will change them when, for example, my ESS is set up!

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awalkerinuk avatar image awalkerinuk johnone commented ·
ESS works very well with E7, have only used it for a few days.



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