The fan in the heater sucks the power. The amount used by the sensors, electronics, and thermostat is negligible. The more the fan runs the more juice it will consume. So no one can really answer the question until the night actually comes. Keep the windows closed and shades pulled to minimize heat loss through the wall and windows, and set the thermostat to a lower temperature. Use an extra blanket, flannel pajamas, and cuddling. Call the dogs to the bed.
We have gone 5 nights with the overnight lows in the 30's without recharging, and the batteries were pretty flat at the end. They were 2 brand new batteries and we used scarcely any electricity during the day (lights, etc.).