Actually, the only block/coolant heater available for my 1.6 TD is 550W, and I use it in tandem with a 125W oilpan heating pad.
Using it for 2-3 hours raises the engine temperature by over 30°C.
I can see the advantages of larger heaters (shorter warmup time), but be careful or you might melt something (my heating element fits into a plastic housing on the coolant line).
EDIT: A better alternative to more powerful heaters would be better engine insulation. Not only will it improve warmup time and temperatures, but it will also keep the engine warmer after shutdown, which is good for EOCing and for running short errands.
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread