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When considering a conductor section S, through which there is an ordered motion of charges, as electrical current (expressed in A; Ampere) is defined the amount of electrical charge ΔQ that in the time interval Δt crosses the surface S.

Voltage (or electrical potential difference or an electric tension, measured in V; Volts) is the electric potential difference between two points, or the difference in electric potential energy of a unit test charge transported between two points. Voltage is equal to the work done per unit charge against a static electric field to move the charge between two points.

A characteristic of every photovoltaic element is the power. This value, expressed in W (Watt), shows the quantity of energy J (Joule) that is consumed (or produced) every second. In electrical terms, 1 Watt corresponds to the power produced by 1 Ampere of current that flows under the influence of the potential of 1 Volt (V).

**1 W = 1 J/s = 1 A x 1 V**

The power of the photovoltaic modules that are measured in standard conditions (1000 Wp/m2 incident radiation, module temperature 25°C, AM1.5) is expressed in pW (picowatt). Typically, a mono or multicrystalline has a power peak superior to 100 Wp/m2.

The amount of electricity that is supplied to (or bought by) the distribution company is measured in kWh (kilowatt hour). This energy E=P*t is the product of a power P (in kilowatts), and a time t (in hours), during which the power is delivered.

**1 kWh = 1 kW x 1 h = 1000 W x 1 h = 1000 J/s x 3600 s = 3.6 MJ**

The kWh is used for measuring the active energy whether consumed or produced. To give an idea of this quantity of energy, with 1 kWh you can prepare 40 cups of coffee with an espresso machine, wash a load of laundry at 60°C or watch television for 12 hours. In Switzerland, a household consumes annually 3500kW-4000kWhh (excluding hot water production) on average.