HOME UPS/INVERTER BUYING GUIDE
An inverter is basically a device that converts DC (Direct
current) into AC (Alternating current). The inverter is brought into play
during outages and other power emergencies. The inverter is used for
running various types of household appliances, such as microwave ovens, computers,
printers, televisions, power tools, and even cars, among many other gadgets.
The inverter is powered with a 12 Volt or a 24 Volt battery.
Sometimes more than one battery is wired in a
Parallel configuration. The AV appliance is connected to
the inverter for getting the power. Inverter batteries need to be
recharged after a period of time. Recharging a battery is done in various
ways. The inverter batteries can be recharged either by using a gas generator,
activating solar panels, or using an automobile engine. The batteries
cannot be recharged independently. Using an extension cord going into the
house, electrical devices can connected
Enough of power cuts in the
region I live and it became inevitable to buy an inverter/home UPS system. So I
went ahead and did some very basic research before deciding upon buying the
same. I am sharing the same over here in anticipation that it would help
someone 
With loads of power
backup options available in the market, it becomes quite a bit of an exercise
to get to the right product. More so because of the aggressive marketing of the
products, which may be misleading. Before even getting into the options, one
needs to figure out the needs. For example, in my case I wanted a solution
which would give a back-up of 2-3 hours for the following:
What to look for in an inverter?
Finding the correct inverter for your requirements can
be made easy by following some of the parameters that need to be
considered when buying an inverter.
1. Inverter type:
Power inverters are available in two main types - Pure Sine
Wave inverters and Modified
Sine Wave inverters. These two types of inverters can be
differentiated from each other
by the properties of the waveform generated when the
inverter converts DC in0to AC.
Pure sine inverters along with a high-end switching
circuitry make for better accuracy
When providing power to the various household devices.
Depending on your specific needs, you should choose the
inverter most applicable to
You. Pure sine wave inverters can be used with appliances
such as heaters or toasters.
These inverters can also be used with medical
equipment, electronic timing devices switch mode battery
chargers, or induction motors. Pure sine wave inverters are also used for sensitive
scientific research apparatuses. Other applications of sine wave
inverters include digital clocks, laser printers, variable
speed motors and gadgets that
work on timers. Modified Sine Wave inverters are usually
more cost effective as compared to pure sine wave inverters. These types
of inverters are great for devices that have in-built power adapters such
as chargers and laptop computers.
Square wave Inverter or Sine
wave Inverter?
The AC (alternating current) power
supply we use every day is pure sine wave (like A in picture). But, because of
the solid state (means no moving parts) devices they use, inverters naturally
produce square wave AC.
To produce pure sine wave (well,
nearly pure) AC, in a domestic solid state power inverter, it must have more
sophisticated circuitry, hence costs much more. As a thumb rule, pure sine wave
inverters are usually at least 25 % costlier than their equivalent square wave
inverter.
For ordinary heating or lighting
equipments, like heaters, geysers, incandescent bulbs, square wave or sine wave
doesn't make a difference. However, for sophisticated electronic equipments,
like computer, refrigerator, TV etc. square wave AC which has jagged edges, can
be really harmful.
Hence if you have and care for
your sophisticated electronic gadgets, go for pure sine wave inverter.
2. Inverter size:
The choice of the inverter size depends on the wattage of
all the electrical devices that
need to be connected to the power inverter as well as the
maximum number of devices
that needs to be connected to the inverter at any point of
time. The combined wattage of
all the devices would need to be supported by the inverter.
Another parameter to be considered is the
continuous rating needed for the inverter.
Suppose if the combined wattage required by all the devices
connected to the inverter is
around 1400 to 16000 watts, acquiring a
2000 watt continuous or 4000 watt peak
performance inverter would be ideal.
3. Inverter rating:
Inverter wattage rating is generally of two types - peak
load rating or top continuous
load rating. Peak rating is the maximum wattage offered for
the time required to start an
appliance. Top continuous rating is the top wattage
available for a prolonged duration of
time. Generally, an inverter offering Peak load
rating is preferred as these kinds of
inverters are known to have a long life and are also more
consistent in performance.
What should be my inverter's power rating (in VA)?
Inverters are usually rated in terms of VA (Volt-Ampere), as
opposed to Watts. But most of your appliances are rated in
Watts. The formula is: Watt (W) = Volt-Ampere (VA ) x Power-factor (p) (of your
equipment). For example, assuming your fan's power rating is 7 5 W, and
power-factor is 0 .6, the VA requirement for your fan is 1 25 VA . Some
appliances like incandescent bulbs have higher power factors (close to 1 ), and
some others like TV have even lower power factors (close to 0 .5 ).
Leaving all those complicated formulae for electrical
engineers, we can safely assume an average power factor of 0 .6 for our total
domestic load. This means if we buy a 80 0 VA inverter, we can
"run" appliances upto a
maximum of 480 W (=80 0 x 0 .6).
Now there is a big difference between
"running" wattage, and
"starting" wattage. For
example, when you switch on your 7 5 W of fan, it starts consuming around 1 5 0
W to 20 0 W of power. But, as the fan speed picks up, the wattage requirement
slowly settles
down to 7 5 W. Similarly , for your 1 5 0 W refrigerator,
when the motor starts, the wattage requirement shoots to as high as 40 0 W,but
settles down very quickly to 1 5 0 W. Even your large screen TV of 1 5 0 W,
when switched on (with the physical switch, not with remote from stand-by ),
starts sucking in almost 30 0 W of power,
but quickly settles down to 1 5 0 W in 2 or 3 seconds.
Understood, but again, what should be my inverter's VA
rating? Before we calculate that, we have to answer one more question. How do
we connect and use our inverter?
1. Connection
- I want EVERY
THING in my house to be connected to my inverter
- I want a
subset of what I have, connected to the inverter (This would mean
additional wiring, but is well worth the cost)
2. Usage
- I want to
keep my inverter always switched on (ready mode, battery power kicks in as
soon as power goes off)
- I want to
keep my inverter always switched off. Once power goes off, I want to
manually start the inverter.
- I want to
keep my inverter always switched off, EXCEPT at night. At night, before
going to bed, I want to switch on the inverter. At that time, only my fans
would be running
4. Battery:
It is the battery that plays an important role in
determining the performance of the
inverter. Therefore, it is advisable to check the battery
specifications too when deciding
on the inverter to purchase. Generally,
the size of the battery is determined by the
wattage load and the projected run time. Using a
battery that is not strong enough can
cause the inverter not to power up and
may lead to discharging issues that could
permanently damage the battery.
The formula mentioned below can
be used to calculate the battery size:
Inverter rating in watts ÷ input voltage x usage time (hrs)
= Battery size.
e.g. 300W ÷ 12V x 5 hours = 125 Amp
Some other useful formulas that could come in handy when
purchasing the inverter are
mentioned below:
To Convert AMPS to WATTS:
Multiply: AMPS X 120 (AC voltage) = WATTS
The result is a ballpark figure of wattage required for
continuous load of device.
To calculate approximate Start
up Load:
Multiply: WATTS X 2 = Starting Load
The result is a ballpark figure of wattage
required for starting load of devices,
nonetheless few devices might require more starting load.
For Example: Induction motors such as air conditioners, refrigerators,
freezers and
pumps may have a start up surge of 3 to 7 times the
continuous rating.
Formula to convert AC Watts to
DC Amps:
AC Watts divided by 12 x 1.1 = DC Amps
Continuous rating:
Maximum combined wattage + 20%.
| S. No. | Equipment | No. of Units | Approx. Wattage / Unit | Total Wattage |
| 1. | Fan | 3 | 70 | 70*3 = 210 |
| 2. | CFL | 4 | 25 | 25*4 = 100 |
| 3.. | Notebook | 2 | 75 | 75*2 = 150 |
| 4. | TV | 1 | 120 | 120*1 = 120 |
| 5. | Miscellaneous (Modem, Router etc.) | NA | 50 | 50*1 = 50 |
| Total | 630 W |
So, I need a solution
which can give me 630 W of power for 3 hours at a stretch (assuming I run all of the above
for 3 hours). Now let’s do some high school physics
calculations:
P (Power in Watts) = V (Voltage in Volts) * I
(Current in Amperes)
Before we move ahead
into the calculations, let’s clarify a couple of points:
What is the difference between
an inverter and a UPS?
Well some think that
these two are competing concepts, however the bottom line is that an ‘inverter’ is an equipment to convert Direct Current
(D.C.) into Alternating Current (A.C.) where as an UPS (Uninterrupted Power
Supply) is a circuitry which allows an instantaneous switch to the backup power
source in case of a power failure thereby ensuring an uninterrupted power
supply to sensitive equipments like a computer.
Now the only thing
which needs an uninterrupted power supply in my list of equipments is the
notebook, but that is anyway ensured by the notebook battery. So do I need a
UPS? Well yes, I would want an uninterrupted internet connection in case of a
power failure so I need the modem and router running in continuum.
Now that I know I need
an inverter with UPS, do I need to look into anything else? Well yes, there are
different types of inverters available in the market:
Square Wave Inverters: Popularly known as ‘Digital Inverters’ produce a Square Wave AC
output which is not so great to run appliances as all the appliances are
designed to run on a Sine Wave Alternating Current Pattern. Also, you would
notice a humming sound in some of the appliances. Though electrical appliances
would bear this, running electronic appliances over Square Wave is not at all
recommended.
Sine Wave Inverters: These inverters produce the right wave
pattern (Sine Wave) for which the appliances are designed. One can safely run
most of the appliances on such current output.
Quasi Sine Wave Inverters: These fall somewhere in-between the
above. I am not too sure about the internals. Also, didn’t find them in the
market. But apparently they offer a low cost solution to run PCs and other
electronic equipments on inverters.
Now what remains is
the power storage medium, which, of course, is the battery. Again loads of
options available which only confuse you. I didn’t think a lot over this and
decided to go with an Exide Tubular Battery. Let’s have a real quick glance at the main
options available relevant to inverters:
Standard Batteries: Loads of them available in the market.
But they need maintenance i.e. putting in the distilled water on an ongoing
basis.
Maintenance Free Batteries: While some claim they don’t need
maintenance throughout their life-time, most of the maintenance free batteries
need maintenance once a year or so.
Tubular Batteries: These batteries are superior in
technology, construction and the quality of material used within. While you can
figure out some nitty gritty here and , they offer
the following advantage:
- They are maintenance free.
- Long life (5+ years)
- Faster Charging
- More efficient
Though tubular
batteries are a bit expensive, but considering their advantages I concluded
that in the long-run, they actually turn out cheaper.
To wrap this up, let’s
quickly get back to the calculations. So I needed a solution which could
provide me with 630 Watts of power for 3 hours. Inverters available in the market are generally
rated in VA/KVA.
Since, V*A=P, I need a 630 VA inverter. A
very important point to keep in mind is the power factor. You
would never get the rating mentioned in the inverter specifications. Considering
a power factor of 0.8 (again figured out with some research) I would need an
inverter with the following rating:
xVA * 0.8 = 630 VA
=> x= 630/0.8 = 787 VA
Luckily, for me we
have 800 VA inverters available in the market, which perfectly fit to my needs.
Note that this is a limiting factor w.r.t. the total wattage of appliances you
can use. For instance I can’t run a 1000 watt appliance
on an 800 VA inverter!
Now the battery.
Inverter batteries are usually available in 12 V and are rated
in Ampere Hours (AH). Since P=V*I and I need a
backup for 3 hours,
630 W * 3 Hours = 12V * x (Ampere Hours)
=> x = (630 * 3)/12 = 157.5 AH
Again, luckily I
discovered that we have batteries rated 165 AH in the
market. So I decided to go with it.
Bingo! I have the
details now. I need an 800VA inverter and
a 165 AH battery for my needs. I just need to decide
upon a brand based on the reviews.
Also, note that the
above calculations are indicative. To quickly figure out how much back-up you
would get while running a subset of the wattage considered at the time of
buying, use the following:
Backup Time (Hours) = (Battery Voltage *
Rating (in AH))/ Wattage required.
So if I just run 3
fans, i.e. 210 Watts, I would get a backup time of (on a fully charged
battery):
12*165/210 = 9.4 hours
Power Requirements of different
appliances:
| Equipment | Running (W) | Starting (W) |
| Fan (70 W) | 70 | 140 |
| CFL (15W) | 15 | 23 |
| Tube light (36 W) | 36 | 40 |
| Bulb (Incandescent) (40 W) | 40 | 45 |
| Desktop PC (CRT monitor) | 180 | 270 |
| Desktop PC (LCD monitor) | 120 | 140 |
| Laptop | 70 | 100 |
| Washing Machine (with Heater) | 2000 | 3200 |
| Washing Machine (no Heater) | 1200 | 2400 |
| Refrigerator (150 W) | 150 | 300 |
| Air Cooler (900 W) | 900 | 1800 |
| Air Conditioner (1.5 Ton) | 2250 | 4000 |
| Water Pump (270 W) | 270 | 550 |
- all the above figures are for indicative purpose only, actual figures will vary from time to time, manufacturer and model, and depending upon the use of the product.
RATINGS OF COMMANLY AVILIABLE RESIDENTIAL INVERTERS
| NOMINAL VOLTAGE | WATTAGE | NO. OF 12V BATTERIES NEEDED | BATTERY SPECIFICATION | INVERTER COST | BATTERY COST | TOTAL COST |
| 600 VA | 355 | 1 | 12V 100-125 AH | INR 3400 | INR6500 | INR 9900 |
| 650 VA | 365 | 1 | 12V 100-135 AH | INR 3600 | INR7200 | INR10800 |
| 800 VA | 460 | 1 | 12V 125-150 AH | INR 5400 | INR9500 | INR14900 |
| 850 VA | 475 | 1 | 12V 135-150 AH | INR 5700 | INR10000 | INR15700 |
| 875 VA | 490 | 1 | 12V 135-180 AH | INR 6100 | INR12000 | INR18100 |
| 1200 VA | 710 | 2 (series) | 2*12V135-180 AH | INR 7200 | INR24000 | INR31200 |
| 1400 VA | 830 | 2 (series) | 2*12V135-180 AH | INR 10190 | INR24000 | INR34190 |
| 2400 VA | 1120 | 4 (series) | 4*12V150-200AH | INR 17200 | INR48000 | INR65200 |
| 3200 VA | 1940 | 4 (series) | 4*12V150-200AH | INR 23100 | INR48000 | INR71100 |
- All the above figures are for indicative
purpose only, actual figures will vary from time to time, manufacturer and
model, and depending upon the use of the product.
Anything else to keep in mind while buying?
Lots
of things to keep in mind. I'll try to mention a few important ones below.
1. Tubular batteries generally come with 3
yrs manufacturers Warranty. However if proper care is taken they should last beyond
5 years. Keep the batteries in a well ventilated place, preferably not inside
your bed room. Make sure that the batteries and the inverter do not get exposed
to water or direct sunlight. Keep them away from wash basin, and outside
balcony . Top off the batteries every 6 months, with distilled water. Charge
the batteries every fortnight, even if inverter was not used.
2. Get a good electrician to do the
additional wiring in your house for your inverter. The electricians from the
shop where you purchased your inverter, will most likely be an inexperienced
part-timer, and would do a shoddy job, and would definitely over-charge you.
3. Use an MCB (Miniature Circuit Breaker,
also commonly called "trip") at the output of your inverter, in
addition to the built-in fuse. MCBs breaks the circuit much faster than the
fuse, in case of an overload, and hence better protects your inverter.
Max current for an xVA inverter, is =X/ 220 .
I use a 4A MCB, for my 80 0 VA inverter, because 80 0 / 220
= 3.6, which is less than 4.
4. Please check the manufacturer's warranty
period of your inverter. Typical warranty periods are 1 yr, and 2yrs. Many manufacturers
sell machines with similar specifications, but different warranty periods, at
different price points. Try to go for the machine with higher warranty period –
less headache and more value for money .
5 . The tubular batteries are usually very
heavy (more than 50 kgs), and hence difficult to handle yourself. Also , it contains
extremely strong sulphuric acid, which can easily "leak" through the porous ceramic
"caps" and burn your skin.
Make sure the battery is properly handled and installed, by the delivery folks.
Once installed, avoid moving or pushing the battery yourself.
6. Many of the pure-sine-wave inverters
supports "UPS mode" in addition to normal mode. In UPS mode, the
inverter's "switch on" time in
case of a power cut is significantly lower (less than 35 milli-second), than in
the normal mode. This ensure, your PC doesn't get reset if there is a power
cut. This saves you the cost of buying UPS for your PC.
