Make your own 220 Volt backup power supply
Dr Garth Cambray
South Africans, as elsewhere in Africa are experiencing almost daily -
"load-shedding". The country cannot meet the demand for electricity
resulting in daily scheduled power-outages nationwide. In this article we
suggest some pro-active solutions.
With Eskom in South Africa constantly shedding its load all over our economy, it is important
for anybody who requires a reliable power supply to understand how to build
their own 220V backup power supply, so as not to be ripped off by an ever
increasing number of pseudoscientific power salespeople.
In this article we look at how you can cobble together a 220 Volt power
supply using components you can buy at your local automotive store. But first it
is important to understand a few basics about batteries, and how to convert 12 Volt
DC battery supplied electricity into 220 Volt AC current that your
household appliances can use.
A twelve Volt battery is a storage device that, when charged, stores
electricity in chemical structures that can reverse and release electricity if
required. Batteries are typically rated in Amp Hours (AH) - ie how many Amps for
how many hours the battery will perform. An inverter is a device which can take
the DC current from a battery and convert it into 220V AC current for your
To calculate how many batteries you need, and how big an inverter is quite
simple. Investigate the specification plates on the back of each appliance. In
the case of my laptop, the power supply is a 90W unit.
For an inverter one can work on a rule of thumb that to produce 100W will
require approximately 10 Amps, hence in reality the lap top requires about 10 AH
of battery life per hour. With a 90 AH battery one can therefore expect about 9
hours of power before the battery provides too little current to keep the
inverter going. But it is not this simple.
Hence to power a more intensive system, one would add up the power
requirements of each item:
Hence the system will require 500W divided by 10 Amps = 50 Amp Hours
So to run this system for 4 hours (during one of the many power outtages in
the country) would require about 200 Amp Hours of
Again, when drawing current from a battery it is important to understand that
the AH rating of a battery is calculated based on the battery being discharged
over a 20 hour period. In other words for a 40AH battery, 2 AH per hour. If one
draws more current from the battery a reduced output is achieved. This can mean
that if the battery is rapidly discharged as little as half the expected AH can
The table below shows how if one discharges a 100AH battery over 20 hours one
gets the rated AH capacity, but for example if you discharge it over 5 hours you
only get 80% of the capacity.
|| Hours of Discharge
Hence, in the above situation, we are drawing 200 AH over 4 hours, or
50 AH per hour. A 100AH battery ideally provides 5 AH per hour to achieve maximum
life, one would require 10 100 AH batteries to run this system with little damage
to the batteries and maximum efficiency. In reality this is very expensive. If
one were to draw 50 AH from one 100AH battery however, it would, due to the heavy
usage be only 50% efficient and discharge after an hour, as opposed to the two
hours one would predict. So, ideally one needs to reach a compromise. In this
case choosing to run the batteries at 80% efficiency would require 200 AH divided
by 0.8 (80% capacity) giving us a requirement in reality for 250 AH of battery
capacity. Standard deep cycle batteries in South Africa often come as 105 AH
batteries, hence three of these batteries would be adequate to power this
system, and, as the batteries decline with age a small amount of additional AH
are available to cover this shortfall.
In the next section, we show how to build a battery bank and inverter with
off the shelf components from an auto parts store.
You will need and inverter (in this case an 800W model), a plug strip set,
batteries (ideally deep cycle batteries), insulation tape, a plug, a battery
charger, a screwdriver and some scissors, or if you have one, a cable stripper.
Cut the two prong plug off the charger and the three prong plug off the plug
Attach the two prong plug to the plug strip and tape the cable up neatly. Attach
the three prong plug to the severed battery charger. This makes for a more
reliable battery charger as two prong plugs often don't make good contact. Plug
the two prong plug into the inverter output socket and connect the inverter to
the battery/s. Turn inverter on. It should power and an indicator light
somewhere will come on.
Attach battery charger to battery.
Connect to equipment to be powered. Note - in this photo only one battery is
shown - to operate this system effectively for 5 hours one would require 5 100AH
batteries. In this photograph, Dr Janice Limson, editor of ScienceinAfrica
magazine, and an electrochemist based at Rhodes University powers some of her
research equipment by the
inverter system depicted in this series.
Dr Garth Cambray is a biotechnologist and developer of African mead products
in South Africa. He also consults in the alternative energy sector. More
Also visit: www.iqhilika.co.za
Other articles by Garth Cambray:
Diesel goes one step better
Fermenting waste fruit to fuel ethanol
Off the grid for 5 years
Separating your alcohol from your fermented fruit
Making your own reflux column from a piece of old irrigation pipe or some tin cans
The hidden danger in home biodiesel production