A lot of older homes used gravity hot water,, but it won't do the job in cold weather.. the flow rate is too slow.. our baseboards and radiators are sized for being at and staying at 160-180*, with the slower flow rate with gravity only, the baseboards will be a lot cooler,, expecially the ones near the end of the zone's circulation..
Remember, the pump only runs when there is a call for heat,, not continously.. I have three full time running circulators with my system, sometimes four.. and three thermostaticly controled circulators.. my electic bill goes up in the summer from the one A/c unit in my Mom's apartment, and from the waterfall's water pump in the Pond..
I certainly wouldn't worry about the electrical use... however,, if you have a good chimney, with plenty of draft, if the power goes out,, with your AA, you can get some heat by proping a pipe against the inspection door, closing it so the chimney pulls some draft through the coal bed, and then opening your flow check, and zoned valves.. you will have some gravity flow.. enough to keep everything from freezing..
I've opened the zone valves and run a Natural gas boiler for over a week during a power outage, this was many years ago..