One or both RoboBrew elements during the boil?
The RoboBrew is fitted with two elements that can be manually switched on and off independent of the built-in electronic thermostat.
Generally speaking, you’ll use both elements to first heat up to your mash in temperature — usually around 70°C — and then switch off the larger, 1900W element so that the 500W element can maintain your mash temperature for the duration.
You’ll then want to use both elements to reach your mash out temperature — if you use this step in your brews — or to start the climb to your boil temperature.
Once your boil is underway, you’ll certainly want to keep the 1900W element on. As for the 500W element, it really comes down to how well your RoboBrew maintains a boil, and just how much you want to boil off. Here in Brisbane on a hot day, you can generally get away with switching off the 500W element. I avoid the use of an extension cord to minimise resistance in the circuit. Colder climates, houses with older wiring or a brewing location that requires the use of an extension cord are all common reasons you might need to use both elements.
I generally use the 500W element if I’m behind on my efficiency and want to make it up during the boil. If I’m OK for my efficiency I’ll often switch off the 500W element to take it back to a more gentle boil that’s still adequate… it might mean an extra pint out of the batch at the end of the day.