question

Keep in mind that there are some loses of energy when you do this (charging/discharging the battery and converting from AC to DC and back to AC).

The day/night price difference from your electric power company must be high enough, otherwise you end up losing money.

Thank you both for those answers. Could I ask, do you have any idea of what the round trip losses would be? I knew there'd be some, though I was only factoring in around 10% in my calculation.

The ratio on night/day is 7.5p to 31.1p per kWh, so it's over 4x cheaper at night.

Thanks again.

Apologies, I think I found the answer here for this question:

https://community.victronenergy.com/questions/56351/multiplus-485000-efficiency-curve.html

I also have Octopus Go (5p / 16p) and this setup works well for me.
I have a 3kVA MultiPlus, so I only charge at 1.7kW during the four hour period at night which is not enough to completely fill the battery (also 4x 3.5kWh Pylontech) from empty.
You have got 3.5kW charging capacity, so this is plenty to fill up during the cheap hours. Just make sure you have leave space for solar power to fill the battery during the day. Perhaps limit the night time charge level in the charging schedule. Summer is coming,so I am starting to reduce my night time charge level to allow space for solar power.

When it is stormy (like it is now), then I set the system to keep batteries charged and if there is a network outage my battery is full.

For me, the round trip losses are small next to the price differences of the tariff.

@wkirby, thank you for your answer. What you're talking about is the type of set up we have in our office. Grid charging at night and then in summer I play a bit of a guessing game and reduce how much charge at night to let the solar fill the batteries and run the office in the day. (Though the intention of this system was more as a UPS as we kept having power outages.)

What I am looking at now is having only batteries and no solar for the moment, as I'm not sure how long we'll be staying in our current home. It's a lot easier to take the batteries and inverter if we were to move than getting solar off the roof!

It's a sort of mitigation strategy against the huge price rise we're about to face. (With the added bonus that we'd be using, on average, greener night time energy and reducing dirty fossil fuel energy use in the day.)

I have a Multiplus II in my campervan with a 7kWh self built LiFePO4 battery.

I'm currently doing exactly what you want to do with it. I charges at night on the 5p Octopus Go rate and discharges into the house during the day. My average cost per kWh for the whole day is ~7p.

I set the target pull from the grid to be 20W and the Multiplus tracks the load from the house via a ET112 meter with Zigbee connection. You can set the max the Multiplus will output too. Mine's at 1000W. The battery isn't big enough for the total house usage in a day so it's just taking the baseload and smaller usage during the day but it won't be powering 100% of the kettle for example. No need to stress the components in the Multiplus if I'm not going to benefit from it.

One plus point is the 600W of solar on my van also charges the same battery and hence my house! :)

This is a super helpful thread, I'm thinking of getting a Victron with similar objectives in the Octopus tariff, being in the UK I realise our "solar season" isn't the longest.

While it's pretty clear you can make the payoffs work if you can buy power at 7.5p/kwh, this really is a loss leader for Octopus. Even if you look at only the wholesale prices (https://www.nordpoolgroup.com/en/Market-data1/GB/Auction-prices/UK/Hourly/?view=chart), it's very rare that power is below £75/mwh, even putting aside any grid and other charges.

The agile tariff sometimes has big price plunges, but I'd be surprised if they're often enough, or if anyone yet has developed the software that's smart enough to properly take advantage of them.

I guess this points toward opting for a smaller battery and (if possible) charging direct from solar DC to minimise efficiency losses.

Out of interest, does anyone know if battery life runs contrary to the above? I kind of suspect that batteries would much prefer a deep charge every night to sporadic daytime charging.