Partial off-grid plan

[Radian]

So I've identified a zone in the middle of the house that could do with an off-grid battery backup. The zone is served by four 13A radials that run back to a services cupboard (adjacent to an airing cupboard) that was originally used for a home-brew DMX lighting setup I built over 20 years ago. The idea is to install a modest 24V 100AH LFP battery and Victron 800W Multiplus intverter/charger in there. The zone has around 300W of 'essential' appliances: Sky Q, Oled TV, HT Amp, Broadband router and PS4. Enough to keep us entertained when the lights go out - except for three table lamps also on the unused outlets. Oh, and in the airing cupboard is the CH system (pumps valves) with a reversible route down to the boiler. That lot adds about 40W max. and could well do with being 'uninterruptable'. Maximum draw is going to be 400W so at least 5 to 6 hours of battery life.

 

The Victron Multiplus is an elegant design that syncs incoming and outgoing AC with bidirectional transformer coupled-battery switching. According to the spec Switchover to inverter happens in 20ms so the unit operates as a UPS. I'm thinking of this as serving multiple purposes: By integrating the incoming AC with my solar divert to immersion I would have the option to keep the connected appliances (mostly used for a few hours at night) running off grid from excess solar. With electricity at a higher premium than gas, the HW can be given a lower priority. The cost of this system is coming out at around £1300 so considerably less than a whole house ESS.

 

Economically speaking a whole house ESS doesn't care what appliances gobble up the battery banked power, sticking the electric oven on for an hour or running the rest of the house for 10 hours is all the same in terms of making use of whatever PV has been stored. But ring-fencing essentials is a different matter and the plan I'm considering is quite different to the usual applications I've seen.

[TonyT]

I use several second hand UPS.

we get lots of dips in our supply and a few power cuts a year..

I have a UPS on the BT Router.

another on the kids Xbox

a larger on the others PC.

that solves the power dips..

 

i have a generator changeover switch for the longer power outages wired to the garage where an inverter generator sits or some batteries and inverter for playing with.

 

I like the victron stuff, installed one a while ago with a pylon tech battery for  an essential fan supply.

[Radian]

I will need to add some more UPS as well. The main broadband modem/router is in a different part of the house so a smallish UPS should take care of that. One thing I'd like to know before I proceed is just how noisy the Victron multiplus is. Inverters can kick out a significant amount of noise from fans and transformers. If this is going to be used for energy time-shift then it has to be reasonably quiet. Unfortunatley this thread is beginning to put me off: https://community.victronenergy.com/questions/108663/victron-multiplus-ii-noise-level-what-is-your-expe.html

[markocosic]

The standby usage also looks a little careless unless I'm misreading data sheets.

 

Who else make equivalents?

[markocosic]

This chap is fun for blueprints:

 

https://www.mobile-solarpower.com/

 

Shame that those "camping kits" can't be made with just a little more intelligence to sit in the corner pretending to be a little enphase "hotel load shifter and backup" unit!

[Radian]

On 09/09/2022 at 03:20, markocosic said:

The standby usage also looks a little careless unless I'm misreading data sheets.

I think this is mostly from losses in the Mosfet switching when the inverter is actually running off-grid, hence the reduced power when in 'search mode'

This video shows the power saving modes at around 16m in:

 

 

On 09/09/2022 at 03:20, markocosic said:

Who else make equivalents?

 

Good question. I'm not sure there are any inverters with the same kind of fully symmetric topology. But then Victron seem a lot more open about the design details (as per above video) so I might be missing some Chinese equivalent.

 

As a UPS it looks perfect for my needs. I like that the 800VA model is only charging the battery at 16A (around 500W max from AC side) as it can be left to charge in parallel with my PV immersion diverter and replenish the approx. 1kWh I would be cycling through the battery every day, in a few hours on most days (it might be a simple as putting it on a timer to coincide with peak PV).

 

My only reservations at the moment are having a noisy unit and a LiFePO4 battery indoors. The noise I can probably do something about with an acoustic treatment in the cupboard but the battery still makes me nervous.