Use small UPS to power steam boiler
When temporarily testing things...

Just take box covers off and wire-nut stuff.

My cheater is male on one end, bare wires on the other.  I just go into the box, wire-nut up the load under test, plug the cheater into the Kill-a-Watt or whatever, and do my testing. Easy peasy. 

I test hardwired loads that way all the time, it's no big.  

Don't bottom your UPS

Use the Kill-a-Watt.   Tempting though it may be, don't run a UPS down to dead, except to test the battery.  It has a lead-acid battery, every time you run them dead takes a big chunk out of their life. Lithiums don't have that problem. 

I agree it's likely the furnace doesn't take much power.  

You might also have the conversation with your furnace manufacturer as to whether it can be run on 12V or 24V battery power directly.   It's quite possible that most or all of it runs on 24VAC and doesn't care AC vs DC.