Several cells can be connected to form a battery. In a series connection, the negative terminal of one cell is connected to the positive terminal of the second cell, the negative terminal of second cell is connected to the positive terminal of the third cell, and so on. The free positive terminal of the first cell and the free negative terminal of the last cell become the terminals of the battery.

The total energy used by any device is obtained by multiplying the power and duration of the time when it is ON. Unit of the energy will be in joules. (or) watt second. Electrical energy is measured in kilowatt hour (kWh). 1 kWh is known as 1 unit of electrical energy. (1 kWh = 1000 Wh = (1000 W) (3600 s), 1 kWh = 3.6 x 10⁶ J)

The electrical power P is the rate at which the electrical potential energy is delivered
\(P=\cfrac { dW }{ dt } =\cfrac { d }{ dt } \left( V.dQ \right) =V\cfrac { dQ }{ dt } \)
Since the electric current \(I=\cfrac { dQ }{ dt } \).
So the equation (1) can be rewritten as P = VI
This expression gives the power delivered by the battery to any electrical system, where I is the current passing through it and V is the potential difference across it. The SI unit of electrical power is a watt.

When several resistances are connected in series, the total or equivalent resistance is the sum of the individual resistances
R₅ = R₁ + R₂ + .........+ Rn.