[oil pan 4]

I was doing some light diesel engine reading:
http://www.man-diesel.com/files/news...5.00pr_low.pdf
(Ship engines)

Found that Man diesel recomends engine pre-heating input be roughly equal to 1% of engine output and to add more if you plan to operate after shutdown in cold climates.
Now they also recomend preheating the coolant to a bare bones minimum of 20'C but our on road vehicle engines aren't that sensitive to deltaT changes, so don't worry about it.
No wonder I always thought a single 600w heater was undersized on my 6.5L and explains why I am so pleased with my two 600w heaters and wish I had went with 1kw and 600w heaters.

So a single 600w block heater on a small 4 cylinder is not unreasonably oversized.

I recomend putting oversized block heaters on to save electrical power and fuel. Use a 600w heater on a small 4 cylinder for an hour or so before driving as opposed to leaving a 200w to 400w heater plugged in all night or using an ineffective 50w or 100w heater and getting almost no result after leaving it plugged in all night.