The Basics

So if you've got no clue about these sort of things, but just want some power during load shedding, here are your options.

There are two numbers to look out for: Capacity in Watt hours, and Power in Watts or Volt Amps (VA).

Power capacity, or storage, is measured in watt-hours (Wh). A really cheapo UPS will hold about 100Wh, A lightbulb might draw 25W, so 100Wh divided by 25W gives 4h of power.

There is also an upper limit to how much maximum power the UPS can deliver at once. This will be a figure in Watts or Volt Amps. A small fridge, TV and a few lights would add up to about 500W.

Pap and Gravy: Hundreds of rands
These sort of UPS units will power a computer for about between 10 minutes and an hour, and cost about R500 up to R2000 or so.

Big downside: These units are designed for emergency backup so they usually beep annoyingly when the power is out. So you'll need to do a bit of surgery to disable the buzzer.

A super cheapo UPS will hold about 50Wh of usable charge, and be able to deliver up to 500W at once. This means you can draw 25W (or one energy-saving lightbulb's worth) over a two hour load shedding period.

These units usually have 'kettle' (IEC60320) type connectors so you'll need to get adapters for your plugs.

Remember that the batteries in low-end UPSes don't last long if it gets used a lot. You can expect it to give out in a year or two.

Streetwise Two: Thousands of rands

Some enterprising souls on gumtree are selling small inverter kits with a medium sized battery and inverter.
You can expect to pay upwards of about R5000 to R10000 for this sort of thing.

A 1000W inverter with a 500Wh of usable power in the battery would be able to power your TV and decoder (250W) for two hours.

They usually come with a plug socket so you can run an extension lead to your TV or fridge etc.

Col-o-nel burger: Tens of thousands
Here's where you start wiring the inverter into the house's electricity distribution board. The system automatically kicks in when the power fails so you don't need to mess around with extension leads etc.

My system here cost about R25000, and can supply 4000w and has 2000Wh of usable storage, so I can run the TV, fridge, computer, lights etc for 2 hours.

Big solar setup

Fully loaded box meal: Hundreds of thousands
We're out of DIY territory now, but this sort of thing would make Eskom irrelevant (if you have a few spare kidneys to sell.)

Invert-huh?

So what's an inverter anyway?

Your appliances run off 220V Alternating Current (AC), whereas a battery might store Direct Current (DC) at 12V.
So the inverter's job is to pump up the battery voltage to a couple hundred volts, then switch it back and forth really quickly to generate Alternating Current.

There are two types of inverters: square wave and sine wave. Square wave inverters kindof suck, so the marketing guys had a wank session and decided to call them 'modified sine wave' inverters.

What's the problem with square wave inverters?

The short answer is that, like a cyclist's legs, smooth is better.
Every time you have a quick change in voltage, a large amount of electrons will flow quickly to make the change happen.
Large amounts of current flowing then suddenly turning off is bad because it creates sudden surges. Things with motors in them especially don't like this because of things*. Computers and small electronics don't mind as much since they are converting the Alternating Current back to Direct Current anyway.

*Disclaimer: I'm dumbing this down severely, so rather learn about things like reactance, harmonics and filtering from someone better at explaining!

A sine wave is the smoothest sort of wave, but actually making a sine wave from DC is quite tricky because the electronic components that generate the wave can either switch on or off. This means they have to switch so fast that the resulting voltage averages out into a sine wave without anyone noticing. (I should probably put in another disclaimer here too.)

So long story short, pure-sine inverters cost more because they won't burn out your fridge motor.

Choosing an inverter

Choosing an inverter:

There are a bunch of different options to choose between when deciding on which inverter to buy. Here are some of the things to look out for.

Quality:
Name brand units like Victron or Outback will come with a warranty of many years, but obviously you will have to pay for it, typically they will cost twice what a cheap Chinese inverter of the same rating will go for.

One usually doesn't think of electronics as being able to wear out, but the capacitors used to smooth out the generated sine wave will wear out eventually. Cheap inverters will contain cheap capacitors which will burn out faster, especially if you run your inverter at its maximum power rating, since this will cause the inverter to run hotter, and heat kills capacitors. The switching components in very cheap inverters also might not be protected properly so these can also pop more easily due to surges.

If you go with a cheap inverter, make sure you leave a good margin spare: Don't run it at more than half of it's rated capacity for extended periods of time.

DC Voltage:
How many batteries can you string together? The more batteries you connect in series, the more efficient the system becomes.
If you have the choice, try go with the higher voltage option because this will be the lower current option for the same amount of power.
High current would mean thicker battery cables and/or more losses to heat.

Expansion:
Some inverters let you chain up more units in parallel in case you need to add more capacity. Unfortunately it's typically only larger units that let you do this in the first place.

Integration:
The inverter is only half the story- you will also need to charge your batteries. Many units have a built in charger (This makes it a UPS) and some will also accept input from solar panels.
It's probably better to keep things separate in case one breaks, but all-in-one units can be way cheaper. I paid R13000 for my 4KW inverter, 50A AC charger and 50A MPPT solar charger unit (Voltronic Axpert MKS 5K).

Monitoring:
Most larger inverters have data ports so you can fiddle with settings and monitor usage. You might want to investigate your monitoring options before choosing. You will want to be able to keep an eye on how much power you are using so you don't run into any nasty surprises.

The inverter typically won't be able to tell you very accurately how much power is left in the battery, usually only a high/medium/low sort of thing. If you want a percentage readout you'll need a separate battery monitor that counts charge flowing in and out of the battery.

Stock:
One of the main reasons I bought the inverter that I did was that I found stock. Unfortunately Eskom has caused massive shortages of Inverters and Deep Cycle batteries in South Africa. I think it's getting better though.