An engine driven alternator producing electricity.

Generators are measured by their KVA (kilo volt amps).

To calculate the amps that it can produce divide KVA by the council supply voltage, typically, 230 volts.

To calculate the wattage it can produce, multiply KVA by 80%.

e.g. a 7.5KVA generator single phase:

Wattage = 7500*0.8 = 6000w

Amps = 6000/230 = 26amps

It is important to note that this is the maximum that it can produce but not the rated amount which is normally around 90% of this. In addition to this, all engines are specified at sea level so for a generator working in Johannesburg you would need to multiply by approximately another 82% to compensate for altitude.

Therefore your 7.5 KVA generator in Johannesburg would be produce about 4920 Watts.

A list is attached showing the approximate wattage required by various appliances. It is also shown on the back of most appliances. It is worth noting however that on start-up, electric motors may require up to double their wattage.

The main point to consider is whether your generator is for emergency back-up in which case you don’t need to run all of your appliances e.g. geysers and heaters or whether you want a "seamless" transition from Eskom to generator without turning anything off.

If you want a seamless transition then it’s important that the sizing and installation is done correctly to make sure that the correct circuits are powered by the generator. If money is no object then you will purchase a generator large enough to power the whole house including geysers stoves etc.

A more cost effective solution will be to have an electrician only connect certain of your house circuits to the generator at your distribution board and in the event of a power failure only these circuits will be powered when the generator detects the power failure and start automatically (do you want this happening at 2.00am?).

If you wanted to save a little more money then you could do the above but instead of having an automatic switch over, the electrician would install a manual transfer switch.

When the power fails you need to physically turn this switch which would isolate the council mains and connect the chosen circuits to the generator and start the generator. You would then need to reverse this process when the council mains came back on.

If your house has a 3 phase supply then the transfer switch could be set up to power all 3 phases with electricity even though the generator is a single phase unit. In this case you would need to make sure to turn off any genuine 3 phase devices that you may have eg. Pool pumps, bore hole pumps etc.

An able bodied male can easily "pull start" anything up to and including a 7.5KVa generator however females battle to start them so electric start (key and battery just like a car) generators are suggested for home use.

They do make a lot of noise, you have been warned!

There are however some enclosed diesel units like the WH550DGS listed below which are a lot quieter.

Where should the generator be installed?

As the engines are air-cooled, they need to be outside in a well ventilated area.

As electricity is involved, a suitable cover must be used to prevent the generator becoming wet .

Fumes from the engine are hazardous so the generator must not be run inside including running in a garage.

The petrol generators use between 1.5 litres per hour for the smaller generators up to 3 litres per hour for the 7.5KVA generators.

Fuel consumption is directly related to the load that the generator has to service.

What can you run on a 7.5 KVA generator?

It’s not easy to give a definitive answer. You would need to add up the wattage of the various that you need to power by the generator and make sure that it’s within the rated output of the generator.

As an example the following could all be run at the same time

Deep freeze, 2 fridges, TV, Decoder, Lights, Alarm system, Hi-Fi and one of either kettle, hot plate or microwave.

What happens if I overload i.e. draw too much from the generator?

You would hear the engine of the generator labouring under the load. The alternator will then trip out, the engine will continue running and you will need to re-set the alternator by pushing a button.

An inverter converts DC (from a battery) to AC (what you currently get out of your plugs).

It is a quiet and relatively maintenance free alternative to a generator with low running costs. However from a cost per watt perspective they are more expensive to purchase than generators.

Just the same as with the generators it is possible to get automatic switchover inverter based systems in various sizes for your home or office.

If you run a business or live in a place where it’s not possible to run a generator then this would be a reliable option.

Typical inverters comes with large crocodile clips that you attach to a large battery similar to the one in your car. The inverter has a plug socket built into it where you plug in the device that you want to run. There are also smaller units terminated with a cigarette lighter plug for power your laptop or other small devices from inside your car.

When the power has been off for some time and the inverter has been running off it’s battery the battery will start going flat. This means that when power is restored one must immediately start to re-charge the battery. One would need a suitably large charger that would ensure the battery is properly re-charged with about 12 hours.

So, as one can see, this is a multi-step process.

One then gets inverters with built in chargers. These operate similar to an Uninterruptable Power Supply (UPS). This means that devices that you want to run can remain plugged into the inverter even while you have power and when the power fails the inverter switches over automatically to the attached battery. When power is restored the batteries then automatically start re-charging.

The larger the inverter the more you can run from it, the bigger the battery bank, the longer you can run during a power failure.

Typical domestic inverters range in power from 500w to 3000w however one gets them much larger than this depending on requirements.

Inverters can put out a sine wave (same as the Eskom supply) or a modified sine wave (squarewave).

The cheaper modified sine wave units can run most devices however there are some that would require a sine wave inverter.

These devices include digital clock radios, sewing machines, some fluorescent lights, medical equipment, some fridges, some power tools and others.

In order to achieve longer run time, some inverters will only operate on 2 or more batteries. Make sure you know how many batteries the system you choose will require in order to plan the space to house them.

You will need to add up the maximum load required, add about 25% and purchase the inverter with the closest wattage you have calculated.

Be aware that some TV’s, fridges and other devices do surge on startup and this needs to be taken into account when doing the sizing. For example, a 72cm colour TV will run on a 500w inverter but in startup on may need as much as a 1000w.

You could choose to have a number of smaller inverter/battery combinations at strategic points in the house or alternatively have a larger system wired into your DB board for seamless switch over.

A good starting point would be a deep cycle 100Ah battery. This battery is large and heavy, similar to the one in your car. If you have a dedicated low power application you could consider a smaller battery. If you want long run times with varying loads you would need to link a number of 100Ah batteries together.

A convenient combination

As always, it depends on a number of factors.

The size of the inverter/UPS will not determine the run time, it’s how much you are running off it that will. If you are only running your 52cm TV and satellite decoder from an inverter attached to a 100Ah battery then you will get many hours of runtime.

Add a 100w light, a fan etc. and this run time will start to reduce proportionately to the load.

To get more runtime, reduce the load or increase the capacity of the batteries.