Wattmeter is a measuring instrument used to determine how much real power in the load that is operating in an electrical system with several load conditions, such as DC load, single phase AC load and three-phase AC load. In operation must pay attention to the instructions in the manual book or table listed on the wattmeter. Similarly, in the case of reading should refer to the existing manual book.

Electricity in the sense can be grouped in two groups according to their power supply, namely DC power and AC power.

**DC electrical power is formulated as:**

P = V.I.

Where :

P = Power (Watts),

While AC electric power there are two kinds, namely: power for one phase and power for three phases, which can be formulated as follows:

**On a single phase system:**

P = V. I.cos φ

Where :

V = Working Voltage (Volt),

I = The current flowing to the load (Ampere)

**On a three phase system:**

P = 3 .V.I.cos φ

Where :

V = Neutral phase voltage (volt),

I = The current flowing to the load (Ampere)

Generally speaking, electrical power contains elements of resistance and reactance or complex impedances so that the absorbed power depends on the nature of the load. This is because the absorbing power is a resistive load, while the reactive load does not absorb power. Thus the multiplication of effective voltage with effective current is the apparent power (S).

**S = VI**

While the real power magnitude (P) is:

**P = V.** **I.** **Cos**

Also the power caused by the reactive load (Q), the magnitude is:

**Q = V.** **I.** Sin

The relationship between the three apparent power, pseudo power and reactive power can be illustrated by the following power triangle:

Figure 45. Power Triangle

*Watch and observe!*

Below is shown the picture of analog wattmeter and the basic circuit construction.

Figure 46. Watt-meters

In the Wattmeter base circuit, the current coil of the Wattmeter is connected in series with the circuit (load), and the voltage coil is connected parallel to the line. If the line current flows past the current coil of the Wattmeter, it will generate a field around the coil. This field strength is proportional to the magnitude of the voltage line current flow of the series mounted wattmeter with the resisitor with a very high resistance value. The goal is to make a series of voltage coils from meters have high accuracy. If the voltage is paired to the coil voltage, the current will be proportional to line voltage.