I took an old APC battery back up apart and rigged it to a lawn mower battery and plugged a battery charger into it and a lamp,it ran a 60watt light bulb for almost 2 weeks solid in my shed.
All I had to do was hook a switch to the low voltage kick in Relay and basically just click it on.
Eventually the battery charger could not keep up with the power demand but it was fun to monkey around with.It still works fine but I have to keep a fresh battery hooked to it.
Did you also test to see how long the 60 watt light would burn without the charger hooked up?
And your point is? You used external power to power your; whatever you want to call it!
GP...he powered the charger off the AC converted by his battery backup, not grid power.