Most electricity meters installed by electricity suppliers are of rotating disk type. You can watch the disk for a while and measure how many seconds one revolution takes to see how much electricity you are consuming.
You can find the meter constant in the label plate of the meter. The constant is given either in rev/kWh or in Kh. Set either value before you start the watch. Measure the rotating period by tapping the start/stop button and you will see the kW value.
You cannot use this app if the meter constant is not available or if you cannot see the rotating disk.