Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)

A Brief Introduction to Electricity

Before we can start playing with solar power, we need to talk about electricity. To be more precise, we need to talk about voltage, current, resistance, power and energy.

Having these terms clear in your head will help you to understand your solar system. It will also give you confidence that you are doing the right thing when it comes to designing and installing your system.

Don’t panic

If you have not looked at electrics since you were learning physics at school, some of the principles of electricity can be a bit daunting to start with. Do not worry if you do not fully grasp everything on your first read through.

There are a few calculations that I show on the next few pages, but I am not expecting you to remember them all! Whenever I use these calculations later on in the book, I show all my workings and, of course, you can refer back to this chapter as you gain more knowledge on solar energy.

Furthermore, the website that accompanies this book includes a number of online tools that you can use to work through most of the calculations involved in designing a solar electric system. You will not be spending hours with a slide-rule and reams of paper working all this out by yourself.

A brief introduction to electricity

When you think of electricity, what do you think of? Do you think of a battery that is storing electricity? Do you think of giant overhead pylons transporting electricity? Do you think of power stations that are generating electricity? Or do you think of a device like a kettle or television set or electric motor that is consuming electricity?

The word electricity actually covers a number of different physical effects, all of which are related but distinct from each other. These effects are electric charge, electric current, electric potential and electromagnetism:

·       An electric charge is a build-up of electrical energy. It is measured in coulombs. In nature, you can witness an electric charge in static electricity or in a lightning strike. A battery stores an electric charge

·       An electric current is the flow of an electric charge, such as the flow of electricity through a cable. It is measured in amps

·       An electric potential refers to the potential difference in electrical energy between two points, such as between the positive tip and the negative tip of a battery. It is measured in volts. The greater the electric potential (volts), the greater capacity for work the electricity has

·       Electromagnetism is the relationship between electricity and magnetism, which enables electrical energy to be generated from mechanical energy (such as in a generator) and enables mechanical energy to be generated from electrical energy (such as in an electric motor)

How to measure electricity

Voltage refers to the potential difference between two points. A good example of this is an AA battery: the voltage is the difference between the positive tip and the negative end of the battery. Voltage is measured in volts and has the symbol ‘V’.

Current is the flow of electrons in a circuit. Current is measured in amps (A) and has the symbol ‘I’. If you check a power supply, it will typically show the current on the supply itself.

Resistance is the opposition to an electrical current in the material the current is flowing through. Resistance is measured in ohms and has the symbol ‘R’.

Power measures the rate of energy conversion. It is measured in watts (W) and has the symbol ‘P’. You will see watts advertised when buying a kettle or vacuum cleaner: the higher the wattage, the more power the device consumes and the faster (hopefully) it does its job.

Energy refers to the capacity for work: power multiplied by time. Energy has the symbol ‘E’. Energy is usually measured in joules (a joule equals one watt-second), but electrical energy is usually shown as watt-hours (Wh), or kilowatt-hours (kWh), where 1 kWh = 1,000 Wh.

The relationship between volts, amps, ohms, watts and watt-hours


Current x Resistance = Volts

I x R = V

Voltage is equal to current multiplied by resistance. This calculation is known as Ohm’s Law. As with power calculations, you can express this calculation in different ways. If you know volts and current, you can calculate resistance. If you know volts and resistance, you can calculate current:

Volts ÷ Resistance = Current

V ÷ R = I

Volts ÷ Current = Resistance

V ÷ I = R


Volts x Current = Power

V x I = P

Power is measured in watts. It equals volts times current. A 12-volt circuit with a 4-amp current equals 48 watts of power (12 x 4 = 48).

Based on this calculation, we can also work out voltage if we know power and current, and current if we know voltage and power:

Power ÷ Current = Volts

P ÷ I = V

Example: A 48-watt motor with a 4-amp current is running at 12 volts.

48 watts ÷ 4 amps = 12 volts

Current = Power ÷ Volts

I = P ÷ V

Example: a 48-watt motor with a 12-volt supply requires a 4-amp current.

48 watts ÷ 12 volts = 4 amps

Power (watts) is also equal to the square of the current multiplied by the resistance:

Current² x Resistance = Power

I² x R = P


Energy is a measurement of power over a period of time. It shows how much power is used, or generated, by a device, typically over a period of an hour. In electrical systems, it is measured in watt-hours (Wh) and kilowatt-hours (kWh).

A device that uses 50 watts of power, has an energy demand of 50Wh per hour. A solar panel that can generate 50 watts of power per hour, has an energy creation potential of 50Wh per hour.

However, because solar energy generation is so variable, based on temperature, weather conditions, the time of day and so on, a new figure is now often shown specifically for solar systems: a watt-peak (Wp) rating.

A watt-peak rating shows how much power can be generated by a solar panel at its peak rating. It has been introduced to highlight the fact that the amount of energy a solar panel can generate is variable and to remind consumers that a solar panel rated at 50 watts is not going to be producing 50 watt-hours of energy every single hour of every single day.

A word for non-electricians

Realistically, if you are new to electrical systems, you should not be planning to install a big solar energy system yourself. If you want a low-voltage system to mount to the roof of a boat, garden shed or barn, or if you want to play with the technology and have some fun, then great: this book will tell you everything you need to know. However, if the limit of your electrical knowledge is wiring a plug or replacing a fuse, you should not be thinking of physically wiring and installing a solar energy system yourself without learning more about electrical systems and electrical safety first.

Furthermore, if you are planning to install a solar energy system to the roof of a house, be aware that in many parts of the world you need to have electrical qualifications in order to carry out even simple household wiring.

That does not mean that you cannot specify a solar energy system, calculate the size you need and buy the necessary hardware for a big project. It does mean that you are going to need to employ a specialist to check your design and carry out the installation.

In conclusion

·       Understanding the basic rules of electricity makes it much easier to put together a solar electric system

·       As with many things in life, a bit of theory makes a lot more sense when you start applying it in practice

·       If this is your first introduction to electricity, you may find it useful to run through it a couple of times

·       You may also find it useful to bookmark this section and refer back to it as you read on

·       You will also find that, once you have learned a bit more about solar electric systems, some of the terms and calculations will start to make a bit more sense.

·       If you are not an electrician, be realistic in what you can achieve. Electrics can be dangerous and you do not want to get it wrong. You can do most of the design work yourself, but you are going to need to get a specialist in to check your design and carry out the installation.