Now that you know how to build a battery backup system, it's time to delve into the theory and science behind it. It's not enough to just know how to build it. It pays to understand the underlining principles, the physics and chemistry of it. By the time you're done reading, you should almost be thinking like an electrical engineer.
Hopefully, this this would be an easy read, rather than come across as a technical manual.
My goals are as follows:
1. Shed more light on Inverters, Chargers, and Batteries.
2. Explain what to look for when making purchase decisions.
3. Understand how to upgrade an existing backup system.
4. Graduate from this backup system, into a full blown solar generator.
5. Save power by switching to power saving devices.
My plan is to write something each night, till I cover everything.
Without further ado, let's get started.
Take a look at this illustration:
In a nutshell, when power is on from PHCN, the Charger would charge the battery.
An Inverter is connected to the battery, which converts the 12 volts DC current, to 240 volts AC.
A power strip can then be connected to the Inverter, and electrical appliances connected to the strip.
During a blackout, this system will be able run most electrical appliances, with the exception of high power appliances like refrigerators, air conditioners, etc.
If you look at the picture again, you notice that we only show one battery. A single battery like this would be drained quickly if used to run electrical devices. So we need multiple batteries, to extend the time you can run things for. For example, if one battery lasts for 90 minutes, two batteries would last 3 hours, and four batteries would last 6 hours. More batteries = longer run time.
Now, let us see why more batteries equates to longer run time.
Pay attention to what I'm about to present. It's the theory behind the system.
Battery power is measured in AmpHours (abbreviated to Ah). This is a measure of the amount of power the battery can store. If you look on a battery, you will find two important values written on it: The Voltage, and the AmpHours.
So, assuming a battery is rated at 80 Ah. This means that you could theoretically pull from it:
80 amps for 1 hour
40 amps for 2 hours
16 amps for 5 hours
8 amps for 10 hours,
etc.
Since electrical appliances consume power measured in Watts, we're really only intereted in how many Watts the battery produces, not Amps. So how many Watts can this 80 Ah battery produce?
The formula for calculating Wattage is:
Watt = Amperes x Volt.
If you're interested, the formula is known as Ohm's Law.
DC car batteries are rated at 6 volts, 12 volts, 24 volts, etc.
So, assuming our sample battery above is rated at 12V, 80Ah, we can calculate the Watts output as follows:
12V x 80Ah = 960 Watts for 1 hour.
480 Watts for 2 hours
192 Watts for 5 hours
96 Watts for 10 hours,
etc.
Now that we know what AmpHours is, and how to use it to calculate power in Watts, let's look at typical home applicances we could run on the battery above.
As a rule of thumb, a home run on batteries should have energy efficient electrical appliances.
Here are some examples: instead of tungsten incandescent lamps, you should use fluorescent or LCD bulbs. Instead of CRT TV, you should get LCD TV and computer monitors. And so forth. We will elaborate on this topic later.
Fluorescent bulbs consume less power than incandescent light bulbs. Take a look at this bulb:
http://www.greenelectricalsupply.com/3 ... white.aspx
They are 3 watts each. If you run 20 of those around the house, that is a total of 60 watts being consumed.
In other words, 20 of those bulbs would consume the same amount of power as a single 60 Watt incandescent bulb.


3 Watts 
60 Watts 
Let's do some calculations of power consumption at home.
Let's say we're running 10 of those 3 watt bulbs = 30 watts.
Next, assuming the TV is rated 100 watts,
and the computer needs 60 watts,
and our fan is a 45 watt fan. (Let's say we have 3 fans. 3 x 45 = 135 watts).
So far we have: 30 + 100 + 60 + 135 = 325 watts.
We have already determined that our battery above can supply 960 watts for one hour.
So, how long can this battery last, if we ran all of the listed appliances above?
960 / 325 = 2.9 hours.
As you can see, 2.9 hours is not very much time.
In order to increase time, we can add another battery.
So, let's say we do go ahead and get one more battery. How do we need to connect the second battery in order to increase the total AmpHours?
There are two ways by which we can connect a second battery  in Series, and in Parallel.
If we connected the two batteries in series, we would increase the battery Voltage.
If we connected them in parallel, we would increase the AmpHours.
Therefore, since our goal is to increase the Ah, we need to connect the second battery in parallel.
Connecting in parallel means connecting the negative terminal of one battery, to the negative terminal of the other other battery. And the positive of one to the positive of the other.
As we assumed before that the battery is 12V@80Ah, connecting a second battery in parallel would give us 80 x 2 = 12V@160Ah.
To calculate the Watts that the two batteries would produce together,
again with our formula: Watt = Amperes x Volt.
We calculated the power output of a single battery to be: 12V x 80Ah = 960 watts for 1 hour.
For two batteries,
160 Ah x 12 volts = 1920 watthour
In other words, the two batteries would produce 1920 Watts for 1 hour.
We have effectively doubled our power output.
Back to the question we asked before: how long can the two batteries last, if we ran all of the electrical appliances we listed earlier (i.e. bulbs, tv, fans, etc)? Go ahead and answer that question on your own.
If we continue the calculation for more batteries, you would find that each successive battery we add effectively increases the length of time the batteries can run for.
In summary, when you want your batteries to run longer, you add more batteries in parallel.
Here is an example of a large number of batteries connected together to form a powerful battery bank.
That's it for today. The writeup continues tomorrow.