Re: 20 watts=how many amps ?
20 watts at the typical 14 volts DC coming from the running alternator regulator
would be approximately 20/14 or 1.43 amps.
The heat from the unit would be measured in BTU's I imagine.
The electrical power used by the heater would be 20 watts.
If they specified that power at the maximum, as it should be, then that 1.43 amps would be
on the high heat setting of the switch.
The low heat setting would probably be a setting of something more than half of that max.,
as heater elements stop producing heat when the amps get too low.