Back-up generators

A generator is an essential part of any domestic scale solar system. Although it is theoretically possible to design a system with enough battery storage capacity to see you through a prolonged period of cloudy weather, the cost of such a system is just too high. Typically a battery bank is sized to store enough energy to last the household for 3 days without any sun.

Back-up generators can run on a variety of different fuels such as petrol, diesel and bottled gas. Each fuel type has its advantages and disadvantages and to some extent the generator's output governs the type of fuel used.

Sizing a generator is generally straightforward and involves determining the size of the load that the house will draw from the generator and adding that to the power needed to re-charge the batteries. This total should be no more than 75% of the generator's output to ensure that it is not run too hard and to allow for some extra power now and then if necessary.

More advanced (and therefore expensive!) generators have the ability to be started and stopped by your solar system without any human intervention. These are used on installations where automatic generator back-up is desired/required.

Inverters

Inverters are used to convert the low voltage direct current that is stored in the battery bank, into high voltage alternating current that is used in household wiring systems. The type of electricity that inverters produce is similar to "mains" or "grid" electricity and can actually be of a better quality. The voltage and frequency of the inverters is usually governed by local standards which dictate the kind of electrical equipment available.

In grid connected systems, the inverter harvests the energy produced by the solar panels directly and either feeds this into the house or the grid, depending on the energy demand from the house.

Battery monitors

Arguably one of the most important and yet often forgotten about elements of a solar energy system, the battery monitor keeps a constant check on the amount of energy going in and out of your batteries. The monitor can then inform the user of the time since the batteries last received a full charge, their present state of charge, the last time they received an equalisation charge and many other functions. The monitor is usually installed in an area where it can be easily read and therefore is checked regularly.

Charge controllers

These advanced pieces of electronic equipment take care of your batteries, ensuring that they are charged at the correct voltage and current. They stop the batteries from becoming overcharged and many models can log the charging history of the past few months or weeks, allowing you to keep track of your system's performance.

Batteries

Batteries are your storage medium for the energy that you harvest from the sun. A larger battery bank means you can store more energy which is useful if you have long periods without sunny days, or if you have a lot of high power equipment that is used regularly. Batteries come in different voltages, which are multiples of 2, since all battery cells have a nominal voltage of 2V. So you will find 12V, 6V, 4v and 2v batteries.

Joining batteries or battery cells together in series (positive to negative along a line) adds up the voltage. Joining them in parallel (positive to positive and negative to negative) adds up the storage capacity of the batteries, but the voltage remains the same. Larger systems typically have smaller voltage batteries (2V or 4V) joined together in series to make 24V or 48V systems. This is really due to the manoeuvrability of the individual cells, since a high capacity 24V battery on its own would weigh about 1 tonne, so, the 12 x 2V cells are separated to enable them to be installed without a forklift truck!

PV panels

Also known as solar panels, photovoltaic (PV) panels are used to convert the energy from the sun into electricity. This electricity is then fed into your battery bank through a charge controller, where it is stored until needed. Panels come in many different shapes, sizes and voltages. Smaller off-grid systems often use either 12V or 24V panels, whereas larger systems using a special type of charge controller (MPPT) can use what are known as "grid-tie" panels. This doesn't mean you have to be connected to the grid - it is just that the panels cannot be used without an MPPT controller. For more information about PV panels and MPPT controllers, read more...

Read more: PV panels

Solar energy system overview

The main components of an off-grid solar system are:

  • PV panels also known as solar panels.
  • Charge controllers for regulating the voltage and current of the solar panels.
  • Batteries used to store the energy you generate during the day.
  • Inverters are used to create mains type electricity (230V AC, 50Hz) from the batteries (12V, 24V or 48V DC)
  • Back-up generators for those rare occasions in Spain when the sun doesn't shine!