kWh is the written short form of kilowatt-hour (or kilowatt hours) — the unit that measures how much electrical energy you have used, not how strong the supply is at one moment. A kilowatt (kW) is a rate, like speed; a kilowatt-hour is an amount, like distance. If something uses one kilowatt and you leave it on for one hour, that is 1 kWh. That is what an electricity bill is counting. You will see it written kWh, sometimes kW·h or kW-hr. The W is a capital because it comes from the name Watt. Though actually, people mix up kW and kWh all the time, even on boxes in the shop: one is how hard the machine works right now, the other is how much energy you have already spent.