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Block heater observations
So I thought I'd put a theory to the test, and that is that it's best to plug in around 3 hours before driving. This was done in my Toyota Avalon Hybrid that has the same engine as the Camry Hybrid. It has the OEM block heater from Toyota that is, I believe, 200W rated.
Most of the day was cloudy, snowing a little with no wind and around 30°F/-1°C the whole day. The engine was shut off all night and all day in the conditions until around 7:15PM. At 10:15PM the Scan Guage said the coolant temp was 64°F/18°C.
Then I drove it and warmed it up to 180°F/82°C and shut it off and plugged it in immediately. During the night it was plugged in for 7 hours (10:30PM to 5:30AM) in similar conditions (no wind, some snow) with the ambient temp dropping only to 27° or almost -3°C by 5:30AM. After those 7 hours were up at that time the Scan Gauge registered 76°F/24°C.
Conclusion:
At around freezing ambient temps:
3 hours plugged in cold engine = 34°F/19°C warmer engine
7 hours plugged in after running = 49°F/27C° warmer engine
Difference: 15°F/8°C warmer if plugged in longer and right after shutting off engine.
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