In the International System of Units, energy is expressed in joules (J). A joule is defined as the amount of energy needed to move an object weighing 1 kg a distance of 1 m, at an acceleration of 1 m per second. It's immaterial if that energy is produced by burning fuel, consuming electricity or simply getting one's hands dirty.

It could take hours to produce the energy, it could take seconds. That's why it's important to measure the power, that is to say the speed, with which it's done. To quantify that, you need watts (W). The International System calls the speed with which energy is produced or consumed over time, or joules per second, watts (1 W = 1 J/s).

Using straightforward watts for most domestic and industrial applications is rather awkward, so instead we use a multiple of it. Kilowatts (kW) are used on a small scale, while large plants and power stations rely on megawatts (MW) or gigawatts (GW) (1 million and 1 billion W, respectively).

When you choose an electricity provider for your house, you sign a contract for a certain number of kW. If you sign a contract for domestic use of 3 kW of electricity, that means you can take up to 3 kW of instant power off the grid. If you go over your contractual limit (which usually leaves a little leg room), they'll cut off your contactor, which is the electrical device for restricting it, and you'll be left in the dark. Imagine you have your fridge (400 W of instant power) plugged in, along with your computer (100 W), TV (100 W) and washing machine (1,400 W). You also have lamps on around the house (another 200 W, let us say). We are now at 2,200 W, or 2.2 kW. If you turn on your hair dryer (800 W), you will get to just under 3 kW; no problem there. But if, on the spur of the moment, you decide to bake a cake, in your oven with its 1.4 kW of power, you'll find yourself in the dark, with a raw cake, unless you turn something off to get the contactor working again.

The fixed amount we pay in bills every two months – not including taxes – is based on the maximum power available and expressed in kW. But variable amount depends on the total energy we consume in those two months. And what's it expressed in? Kilojoules? No. That would be far too easy. Just to make things a little more complicated (in truth, just for custom's sake), here we abandon the International System and use kilowatt hours (kWh).

Kilowatt hours are nothing more than the power used in an hour. If your oven has 1.4 kW of power and you use it for an hour, the energy you've consumed is 1.4 kW x 1 h = 1.4 kWh. But if your cake only takes half an hour to bake, you'll consume half: 700 W. When you do a roast for two hours, you consume four times more than that: 2.8 kWh.

The total consumption of the average Italian family over a whole year is 3,000 kWh, or 3 MWh per year. To convert energy expressed in kilowatt hours into the preferred kilojoules (kJ), just multiply it by 3,600 (the number of seconds in an hour).

If you want to install a photovoltaic plant, it will take a system of solar panels with a certain number of kilowatts. A 3 kW plant is recommended for domestic use, as this is the maximum power provided by solar panels in standard optimal conditions, when there are no clouds, the sun is directly above the surface of the panels, solar radiation is 1,000 W/m² and the temperature of the solar cells is 25 °C.

Ideal conditions are pretty rare. The hours of production and the maximum intensity depend on latitude, atmospheric conditions, season and shade, as well as the direction and inclination of the plant's panels. There is historic data on the average amount of sunlight on every point of the earth's surface, which can predict how much energy a given photovoltaic plant will produce at maximum power. In Italy, for example, a solar plant with a nominal power of 1 kW could produce from 1,100 kWh (South Tyrol) to 1,500 kWh (Sicily) of energy a year. Naturally, in summer our plant will produce about double the electricity it does in winter.