Find the cost of using a microwave per minute.
The wattage on a microwave's front panel is cooking output, not what it draws from the wall. A magnetron converts roughly 60 to 65 percent of input power into microwave energy, so a 900 watt oven actually pulls around 1400 watts while running. Using the output figure understates running cost by about 40 percent. Even corrected, the cost per use is tiny — a few hundredths of a unit of currency — because the run time is minutes rather than the tens of minutes an oven needs. That short run time, not superior efficiency, is why microwaves are the cheapest way to reheat a single portion.
Actual power drawn
Input watts = rated output watts / efficiency%
Cost
Cost per minute = (input watts / 1000) / 60 x rate per kWh
The label states cooking output — the microwave energy delivered to the food. Generating it is only 60 to 65 percent efficient, and the rest becomes waste heat in the magnetron and cabinet. A 900 watt oven therefore draws roughly 1400 watts, which is what your meter records.
For reheating and small portions, dramatically. A microwave heats the food directly for three minutes with no preheat, while an oven heats a large air-filled cavity for fifteen minutes before the food even starts warming. For large items that need browning the oven is the right tool regardless.