Many people using Webasto or Eberspächer fuel heaters have issues with batteries running out. This is just a short video showing the electricity used when the heater is used.
Car alternator can load the battery with 12Amps. At least Toyota seems to use that current.(Current changes depending on the battery charge level. The Eber heater pulls first 2 minutes 13 or so. After that it uses 6amps. 6amps x 12 volts is around 70Watts. So it uses little more than normal 60Watt bulb, (imagine the heat of the 60Watt lamp and you get the idea of the power level).
So using the heater for 20 minutes and driving for 20 should keep the battery charged enough, issue is that if the battery is very cold, it cannot take charge well before it warms up. But usually same time usage vs driving works, but if you heat the car for 30 minutes and drive for 10, the battery is going to run out.(or you need to charge the battery periodically with external charger.
Toyota actually has Eberspächer heater, but a Webasto clock/timer unit.
-JP
James Parus research inc. 