How can I keep electrical loads running when the mains / grid supply is intermittent?
I work in an agricultural training centre in a small town. We have a mains electricity supply but we get power cuts regularly. In the rainy season, the power is only available 4 to 8 hours a day and sometimes goes off for several days at a time. We use electricity for water pumping, lights, fridges for the canteen, computers, TV, kettles and power tools, among other loads.
A common approach is to use a diesel (or for small loads, petrol) generator (‘genset’), but this is expensive and noisy to run. An alternative solution is to use a battery, charger and inverter. When mains is available, the charger fills the battery. When there is a power cut, a changeover switch connects (some of) the loads to an inverter which converts the 12 or 24 V DC (direct current) of the battery to 230 (or 110) V AC (alternating current) like that provided by the grid.
A similar approach can also be used with an off-grid system running on a generator, to reduce the run time to periods of a few hours for high loads such as cookers, washing machines, heavy power tools, air conditioning and pumps. The inverter and batteries can then supply smaller loads such as lighting, computers and TV for the rest of the time, removing the need to run a large genset just to keep a few lights on.
Inverter rating Add up the power rating of all the loads that you think may be required to run at the same time. The inverter rating (in watts or kilowatts) needs to be higher than this value. If you have any large motors, e.g. pump, or compressors e.g. fridge, freezer, these have high starting currents, 3 to 6 times their running value. Good inverters should be able to supply brief peaks above their rated output. Check the specification.
Useful inverter features
Wave form: inverters convert direct current (fixed voltage) to alternating current (voltage varying positive to negative in a sine wave). The most basic devices do this by creating a square wave. A ‘modified sine wave’ is effectively a stepped square wave. The best inverters produce a pure sine wave (or a good approximation). These are the best to use as they work better with inductive loads like motors and fluorescent lights and produce less interference.
Low voltage disconnection: helps to protect the battery from over-discharge by switching off when the battery voltage is too low. Over-current protection: most inverters are protected from connecting the battery the wrong way round, although this might blow a fuse. Similarly, they should disconnect if the load is too large or has a fault (short circuit).
Sleep mode: better inverters have the ability to switch off when there is no load, sending an occasional pulse to detect if something has been connected, in which case the inverter fires up and provides 230 Vac (or 110 V). This reduces the standing losses when idle to a few watts.
Inverter-chargers: some inverters can also ‘work backwards’ as battery chargers and include a transfer switch to automatically change supply from mains to inverter. They may also provide a start signal to an automatic generator. A variant of this type is known as an uninterruptable power supply (UPS) and can switch from mains to battery power fast enough for loads such as computers not to notice the change in supply.
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