How to calculate electricity consumption (KWH) and what is the formula?

How to calculate electricity consumption (KWH) and what is the formula?

Do you calculate electricity? For example:
The electric cooker is 500W (i.e. 0.5KW), and it takes 30 minutes (i.e. 0.5h) to cook. The electricity consumption is p * t = 0.5 * 0.5 = 0.25kwh, that is, 0.25 kwh. If the charge for one kwh is 1 yuan, the cooking cost is 25 cents

The electricity consumption of a canteen in April is 150 kwh, which is 1 / 5 less than that in April,
How much is the electricity saving in May compared with that in April

Electricity saving = 150 × 1 / 5 × 0.55 = 16.5 yuan