I have a 275 gallon tank and in my 75 year old ~1200 sqft poorly insulated house we can go through that much in a month (ouch!) in the depths of winter.

The boiler has a coil in it for domestic hot water ("tankless"). Hot water consumes negligable fuel - we run all summer with hardly any change in the fuel level.

The water will thermosiphon through the radiators quite well when you manually open the check valve, so I'm thinking about investing in a small inverter, some deep cycle batteries and a trickle charger as a backup power supply for the burner.

On the other hand, a completely indepentant backup heating system like a wood stove would be better...