a 6 egg incubator well insulated shouldn't require 60watts. A 40 egg incubator I built has 60watts of power available but uses probably 30 to 40 watts average during the incubation period.

so what's wattage.

watts is a simple calculation based on voltage x amp = watts

60 watt light bulb at 120v uses about 1/2 amp of power. A 60 watt 12v light bulb uses 5 amps of power.

A battery has a amp/hour rating. Lets say a large deep cell has 50 amp/hour rating. That's equal to 1 amp for 50hrs which in a perfect world would be about 10 hours running a 60watt bulb. As the light turns on and off you might be able to get 24hrs. But nothing in life is free so recharging the battery will require just as much (more in real life) power to recharge the battery thus the electric bill will be the same.

A standard car battery is not deep cycle so after a few days of full discharge/ recharge it will probably quit and die.

So what do you have that's cheap and makes heat. Maybe kerosene, paraffin as these have been used for many years to heat incubators.