https://www.iammeter.com/newsshow/why-voltage-times-current-not-equal-power

In residential solar and energy monitoring, many users wonder why multiplying voltage by current does not always equal the actual power shown on the meter. The explanation lies in the difference between apparent power (S), active power (P), and reactive power (Q).

When you calculate voltage × current, you are getting the apparent power, measured in kVA. This represents the total "capacity" of the electrical system, but not all of it is converted into useful work. Actual energy consumption, which utilities use for billing, is based on active power, measured in kW. Active power reflects the portion of electricity that truly performs work, such as running appliances, lighting, or charging devices.

The gap between apparent power and active power comes from reactive power, which arises when the current and voltage are not perfectly aligned. Inductive loads (like motors, air conditioners, transformers) and capacitive loads (such as capacitor banks) cause phase shifts between voltage and current. This results in some energy oscillating back and forth rather than being consumed, contributing to reactive power (measured in kVAR).

The relationship between these three elements is expressed in the power triangle:

P (kW): active power, the useful energy.

Q (kVAR): reactive power, representing energy exchange but not actual consumption.

S (kVA): apparent power, the product of voltage and current.

Therefore, the power displayed on residential energy meters—like those from IAMMETER—represents active power, since only this part is relevant for electricity billing. Voltage × current shows apparent power, which is always greater than or equal to active power, depending on the power factor.

In short, multiplying voltage and current gives the system’s total power capacity, while energy meters show the real power that households actually pay for.