Energy

Back-up power should be installed for emergencies, particularly with photovoltaic and wind systems, where supply is more likely to be intermittent. Back-up power may be provided by:

• a generator, or
• a UPS (uninterruptable power supply)

Generators

Generators are often used as back-up power supply. They are commonly installed with photovoltaic and wind systems, but not necessarily with micro-hydro systems. Back-up generators give out heat and exhaust gases, are noisy, need maintenance and run on non-renewable fossil fuels (although biodiesel may be available in the near future).

When installing a generator, considerations include:

• fuel type (diesel or petrol) and storage
• whether to have manual or automatic operation
• sound-proofing of the enclosure to mitigate operating noise
• providing protection from the weather
• separation provision from the batteries and inverters
• an effective exhaust and silencer system
• good access for maintenance.

A 3–5 kW (or 6–10 kW for higher demand situations) inverter-type petrol generator will suit a household back-up power requirement. As the frequency is generated by the inverter, only load varies the RPM, so it is quieter, relatively fuel-efficient and gives good quality power to run electronic items. It should be wired through a plug-in connection into a three-position main switch on the distribution board.

When starting the generator if there is power cut, all unnecessary loads should be switched off, and the main switch turned from ‘grid’ to ‘generator’. A grid indication light at the main board will indicate when the grid supply is available again.

UPS systems

UPS units vary in size from a plug-in multi-box that can power a computer for 10–30 minutes to large units that will provide power for several hours.

There are two main types available:

• An online UPS generally offers the best protection as it powers the appliances continuously using grid power to charge the batteries and the inverter that supply power to the appliances. If critical power is required, this is the most appropriate option.
• An interactive UPS has the battery and inverter on standby. As soon as the power fails or goes outside the normal parameters, the inverter will start supplying power to the appliances. Some inverters will switch to battery power within 20 milliseconds and will act as a standby UPS for all the circuits connected to them.