Download How to calculate your generator needs

There is a perception that Eskom is “keeping the lights on” only until the election later this year.
Thereafter, the true nature of the utility’s lack of capacity will become apparent as it will no longer
need to pander to its political master’s requirement to appease the voting masses, and as a result,
power outages in residential areas will become the norm once more.
Whether this is a possibility or is merely a scurrilous conspiracy theory we will find out soon enough
but the fact remains that power failures have long been a part of life for smallholders, and it makes
sense to have a standby generator on your property.
Buying a generator with sufficient capacity is the key and it is suggested that in order to supply the
electricity needs of a smallholding one requires a generator of 15kVA or upwards. This will enable
the family and workers to seamlessly continue their activities. Ideally the unit should be fitted with
an automatic changeover switch, electric start and a battery, with a battery charging mechanism
attached to the mains. The automatic changeover switch is also called and AMF (automatic mains
fail) and it senses when the mains power fails, and sends a signal to the generator to start up,
simultaneously disconnecting the mains supply and allowing the generator to feed current into the
house. When it senses the mains supply is live again, it reverses the process.
Installation must be done by a licensed electrician.
That’s the Rolls-Royce version, which is often beyond the budget of the average smallholder. That
doesn’t mean, however, that you need to be left in the dark when the power fails. A smaller
generator, and a manual changeover switch, can still be an option that will enable you to keep most
appliances running without breaking the bank.
The most common small portable petrol generators now available for standby use are Chinese-made
petrol units, with electric start, providing an advertised 6kVA. These, fitted with a manual
changeover switch, are the entry level units to consider which will enable you to keep the lights on,
and with judicious load management run a kettle, a microwave, refrigerator, freezer, and a
television.
The Chinese units, however, although they say they are fitted with “AVRs” (automatic voltage
regulators) deliver such “dirty” current that they often won’t run a computer’s UPS.
You should shop around for the size that best suits you. From 6kVA, you can find 9kVA, 10kVA,
10,5kVA, 13kVA, 15 kVA and upwards, some (the smaller ones) petrol driven.
Some are normally-aspirated, others silent and others still ultra silent. The difference between the
silent and ultra-silent machines is usually in the engine speed, with the ultra units running slower,
and driving larger alternators with different wiring configurations to achieve the same current.
The ultra-silent units also usually have lower fuel consumption because of the slower engines, which
becomes a factor in long power failures, not only because of the cost, but also because of
inconvenience of more frequent refuelling.
In residential settings silent and ultra-silent units are necessary, if only not to drive the neighbours
crazy. On a smallholding you might get away with a non-sound-attenuated machine, particularly if
you can site it away from the house (although the cost of cabling and trenching over any great
distance will become a factor in this decision).
If you are not prepared to buy a generator the size of your mains supply, you need to ask yourself
what it is, in terms of electricity, that you absolutely need, and what you can do without.
If you acknowledge that anything that heats uses the most current, you can forego power to your
geysers and oven for half a day or more.
Similarly there will be appliances, rooms and outbuildings where you can do without power for a
while. In most cases these will be contained in individual plug or light circuits which are easily
identifiable on your distribution board. So you can identify those circuits which must be connected
to the emergency supply and those which can be left off.
One or two lighting circuits (eg, the bedrooms, kitchen and bathroom) and a plug circuit for the
kitchen microwave and kettle, as well as a circuit for the refrigerator and deepfreeze will be your
basic choice.
Is your water supply dependent on a borehole pump or booster pumps?
Then you also need to decide if you want to include security lighting and power to electric fences,
gate motors and security systems.
But how do you work out what size of machine is suitable for your circumstances? Here’s the
dummy’s guide.
First you need to establish whether you have single or three-phase supply. You can check which your
property has by looking at the council or Eskom meter box. If you have one meter it’s single phase.
Many smallholdings have three-phase.
If you have a three-phase supply it doesn’t mean that you need a three-phase generator as a
standby, unless you wish to run three-phase appliances ~ typically a borehole pump ~ during the
outage. A single-phase 220V generator of adequate size will be suitable for all single-phase
appliances and circuits.
Power is supplied by local authorities in standard capacities, typically 60A or 80A.
You now have to walk through your home or office or workshop and write down every item that is
connected to an electrical source, noting the number of watts it consumes.
Total up the number of watts consumed, divide by 220, and that gives the number of amps your
house consumes when everything is running at once.
Assuming you’re on three-phase power, dividing this number by three gives you a rough idea of the
load per phase. Now you might argue that this is a meaningless figure because it is unlikely that
every single appliance in the home will be on at the same time.
However, it will give you an indication of the minimum size of the generator that you will need to
run your home with no bother. You could of course buy a generator of the same capacity as the
mains supply, but this might result in your spending more money than necessary on a larger
generator than is necessary.
You also need to decide whether you want a generator that is designed to run for extended periods,
which will be more robust than one designed for light work and short periods of operation.
With municipalities struggling to maintain power supplies with their run-down, outdated and poorlymaintained equipment, protracted power failures, as opposed to Eskom’s capacity outages, are
becoming more common, so a light-duty machine may not last.
Here’s a little technical stuff to help you refine your calculations.
You need to think about cyclic versus resistive load. Most electrical appliance draw much more
power on start-up than they do in operation. If you’ve ever listened to the sound of a generator
running when a motor-driven appliance is switched on – the generator’s engine immediately takes
strain, but after a few seconds it settles down to an easier running sound. This kind of load is called
cyclic load.
Resistive load, on the other hand, is the kind produced by heating elements, incandescent lights, etc.
A generator’s output is measured in volt amps (VA) rather than watts. To know whether a generator
is suitable, work out your power requirement in Amps (watts divided by 220). Include in this number
a suitable factor for cyclic load on start-up (20% extra or more) to prevent strain on your generator
and possible damage to your appliances.
To work out how many amps a generator produces multiply its kVA rating by 1,5 (which is the
conversion factor to use at this altitude).
So let’s do the calculation on a full house load. If your total wattage in your three-phase house
comes to, say 25 000W (eg, a smallholding house), add 20% (=30 000W) and divide by 220 (=136.4
Amps) and divide again by 3 (=45.5 Amps per phase).
Often the loads on the three different phases are unbalanced and it is advisable to get in a licensed
electrician to balance the loads on each of the phases to minimise the size of the generator required.
In this case the smallest generator you should be looking at is a 30kVA three-phase unit. However, a
generator run at close to maximum capacity for any length of time will not last. To cater for 45,5A
you should be looking at a unit that can deliver around 55A, which will be a unit with a kVA rating of
37kVA. However this is an in-between number that you are unlikely to find, so you will have to
choose between a 35kVA machine (which might be a little underpowered for your needs) and a
40kVA machine, which should deliver perfectly and even allow for expansion at a later stage when
you buy a bigger television or build on a granny flat.
Another important consideration is how you are going to refuel the machine. Generators have diesel
tanks which can take upwards of 100 litres. So how you transport such a large amount of fuel should
be considered.
You can test whether your system will work in an emergency by simulating a power failure: simply
throw the switches of the circuit breakers in the meter box where the power comes into your
property from your municipality. (Throwing the mains switch on your distribution board will not
have same effect.)