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.
Have a little crank on the outside of the time capsule that turns a generator and charges a battery. Put a sign that reads "Please turn crank".
Seriously, though. It's going in an attic? Has anyone owned an attic for 100 years? Maybe I'm just American, but after 60 years or so, houses tend to get remodeled and/or demolished.