Recently, I have managed to acquire a used, APC 3000 VA uninterruptible power supply – minus batteries. (APC branded battery packs cost almost as much as a new UPS. Generic sealed lead-acid batteries of the same capacity can be bought from the likes of http://www.tayna.co.uk/ for a fraction of that.) The UPS is basically a self-contained battery pack, charger and inverter. It converts DC from a bank of batteries, which are ordinarily kept charged from the mains, to AC when the mains fails. I plan to use this, in conjunction with a bank of large batteries, to implement a solar energy storage system.
Now I have managed to pick up some batteries. Although they are not the right ones for this UPS, they are "just about" compatible – the right voltage, but the wrong capacity (there would normally be two series chains of four 12 V, 5.5 Ah batteries, in parallel for 48 V / 11 Ah; I have just one series chain of four 12 V, 7 Ah batteries, for 48 V / 7 Ah).
These batteries are rather used, and have a remaining usable capacity somewhat lower than advertised – but they cost nothing, which is always a point in favour.
And of course, having some batteries, we can test out the UPS!
"C20" plug for the UPS output (which can be up to 3000 VA, which is more than the usual "C14" / kettle-type can handle.) The other end goes to .....
I was worried that the microwave would pull down the battery voltage far enough to trip the undervoltage cut-out in the UPS –this was what happened with a 2 kW kettle. Fortunately, the slow-start action of the magnetron filament heating up was enough to allow the UPS batteries to recover.
This is a good sign. And the nice people at http://www.tayna.co.uk/ are very helpful. They certainly know their batteries.