Yes you would need a 1500W inverter, which is quite large and
expensive.  You would also need special wiring to handle the load the
inverter will put on the 12V system of the van.  If you did get this set up
working, I think it would be pretty limited.  As you say the engine would
have to be running, but then it *would* put out heat right away.  I have a
1500W heater I use when plugged in.  If the van is already cold, it helps,
but can take quite a while to get the temp comfy if you are starting at 32
F and it is windy out.  Once the vehicle heater starts making heat, it puts
out a lot more BTUs than the 1500W electric heater.  So really, the giant
inverter and electric heater will have the most use during those first few
minutes of starting the engine.  I would suggest an alternative would be a
small portable propane heater.  It will make instant heat while the engine
warms up.  I have even used the stove for a little burst of heat in the
morning.  I use a small fan to blow the heat down toward the floor.  It
takes the chill off, and you don't have it on long enough to worry about
the fumes/moisture it will put in the van.
Edward

At 09:33 PM 4/22/2005, you wrote:
>I don't know much about this stuff either.  I was thinking of getting
>an inverter so that I could power up a space heater for those cold
>off-season mornings & evenings.  The heater that I have is 1500W.  Does
>that mean I'd need to get a 1500W inverter?  I wouldn't be using it
>without the van running, but it would be more efficient than the van's
>heater which can take so long to heat the space up.
>
>Any advice would be appreciated.
>
>Thanks,
>-Robyn.