1500W comes out to a little over 5100BTU. It would work in a small to medium size room in an insulated house, but no where near enough to heat a garage.

Personally I use a 60,000BTU propane torpedo heater connected to one of the tanks I have for my gas grill. I get a delta of around 30F in 15 minutes.

 

Reviving an older thread, but just wondering if Robert did go with the OEM solution for this. Is the install as simple as it seems?

Looking at the numbers I realized that I will probably not get to extend my garage space this year and my 450h will spend another winter outside, too many vehicles and not enough space. Looks like you have to take a lot more off to get the heater into the block of a hybrid, but I'm thinking about putting one in over the summer. Best thing, I have a full Insteon home automation system installed in my house so I can just tell the Amazon Echo to start warming up the car when I get up and have everything ready to go when I get out there ,except for clearing off any snow and ice

 

Way back when I had a 1992 Civic EX, Honda sold an engine block heater that screwed into one of the water jacket drain bolts. The heater plug could be accessed thru the grill slots to plug in an extension cord. It was very easy to operate and worked quite well to keep the engine warm overnight.