Determine the ratio of the primary to secondary current. For example, a common ratio for electrical meters is 200 for the primary current and 5 for the secondary, producing a ratio of 200 to 5.
Solve the ratio by dividing the primary current by the secondary current. For example, 200 to 5 equals 200 / 5 or 40.
Multiple the resulting number (in this case 40) by any differences in the reading during different times to calculate actual power consumption. The result of the ratio is the multiplying factor for the meter.