Building a UPS for a house with an "off grid solar inverter"

[Fflint]

I've been a happy user of on-grid solar for quite a few years now, but every time we had a power cut (and they always happen at the most inconvenient time - once we were with no power for a week in winter) I would assemble an ad-hoc system of a cheap Chinese 4kW 12V inverter. 400ah of lead acid batteries, a modified 1000W computer psu as a 80A charger and a gas/petrol generator.

My house is on 3-phase power and all loads are spread out evenly. So I'd cut off utility power, short all phases together and supply power through my "ad-hoc" system. The biggest benefit was I only had to run the generator to top up the battery,or when I'd need to switch on any loads over 1kW(due to crappy inverter), not 24/7. The biggest disadvantages are, the time to switch everything to the off grid system and back as well as not using my 2.2kw of solar.

So having recently helped to build a small off grid solar system for a friend I decided to use a so called "hybrid inverter" as a sort of ups.

I've ordered a 7kW (14kW surge) 48v battery, 55v-450v solar "hybrid" inverter. What's hybrid about it you may ask? I think it's the fact it can take utility power and simply bypass itself supplying utility power to the loads. Charging batteries from solar or utility (or a generator). Then when there is a power cut it starts supplying power in 10ms as a ups would from the battery. The solar would be manually switched to the off grid inverter when there is a power cut.

I decided to get the isolar smh ii 7kw that looks like this internally:



I'm posting it to ask for your opinions. Is it the worst Chinese inverter you've ever seen? Or are there worse ones? My on-grid solar inverter is also Chinese by a company called deye and I had no issues in last 3 years, but this one does look a bit cheaper.

So in addition to the inverter I'm getting 16x100ah 3.2v lifepo4 cells and a 16s 200a (350a surge) 2a balance BMS. All these parts cost me $1150 delivered... (once they are hopefully delivered). This price is insanely low.

I initially planned to have to use an additional electric wire to feed the backup power into my house, but as the inverter simply passes through utility power I may get away with just using one of the phases instead and move all essential loads to it. When utility power is on, the inverter is bypassed so the phase delays all match fine. When the utility goes dark, the inverter supplies power to this one phase.

I'm planning to check how exactly it behaves with an oscilloscope, but I can't think of many disadvantages to this solution. The only 3 phase loads in the house are resistive (water heaters). If anyone can think of any reason why that may be a bad idea let me know please.

I'm not rushing into it. I already have an extra wire coming out of the house for backup power, but it was sized for 4kW. I'd much rather use the main wiring that is sized for 40kW.

I'm also interested in hearing what people use for alternatives. I briefly considered an AC coupled system with a battery powered grid forming inverter like the Victron Multiplus, but I decided against it due to price and greater simplicity of the system. Of course my system will not be fully automatic unless I can figure out a way to automate transferring solar power to the battery inverter, but i can't argue with the price...

[Stevietech]

How did you get on with this?

I'm looking at something similar with a hybrid but feeding it the output of my 4kw solar inverter as its "mains" power and using a pile of recycled 18650s for a battery (looking to cover the background load of freezer, kids tv, etc)

Genius idea using a computer psu as your 12v charging source!


(wife moved out 2 months ago, nobody around to tell me no when i get a dumb project idea.