0AlphaOmega:
Going out on a limb here, but if it's in the loft of a house, why not tap into the house wiring? I've done some math, I think it could work.
I was just about to say that ^
If it's in a house then what's the chances of the house being a) abandoned without electricity and b) somebody being in the house to hear the siren?
Seems pretty small to me.