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.