Inverter Buying Guide

Table 1: Power Requirements of different appliances

Equipment	Running (W)	Starting (W)
Fan (75W)	75	150
CFL (18W)	18	25
Tube light (40W)	60	120
Bulb (Incandescent) (40W)	40	45
Desktop PC (CRT monitor)	250	350
Desktop PC (LCD monitor)	170	200
Laptop	70	100
Washing Machine (with Heater)	2000	3200
Washing Machine (no Heater)	1200	2400
Refrigerator (150W)	150	300
Air Cooler (1000W)	1000	2000
Air Conditioner (1.5 Ton)	2200	4000
Water Pump (250W)	250	500

Table 2: VA Ratings of Commonly Available Domestic Inverters

Nominal VA	Wattage	Number of 12V Batteries Needed	Inverter Cost	Battery Cost	Total Cost
600VA	360W	1	Rs. 3500	Rs. 11000	Rs. 14500
800VA	480W	1	Rs. 5500	Rs. 11000	Rs. 16500
1200VA	720W	2	Rs. 7000	Rs. 22000	Rs. 29000
1400VA	840W	2	Rs. 9000	Rs. 22000	Rs. 31000
2400VA	1440W	4	Rs. 20000	Rs. 44000	Rs. 64000
3200VA	1920W	4	Rs. 26000	Rs. 44000	Rs. 70000

* All the above figures are indicative only. Actual figures vary from time to time, from manufacturer to manufacturer, also from model to model.

Table 3: Comparison of Inverter Batteries of Different Capacities

Battery Capacity (Ampere-Hour)	Price (in Rs)	Approx. Backup Time 800VA (80% load)	Full Recharge Time
135 AH	9500	2hr.	4hr.
150 AH	11000	2hr. 30min.	5hr.
180 AH	14000	3hr.	6hr.

* All the above figures are indicative only. Actual figures vary from time to time, from manufacturer to manufacturer, also from model to model.

What is an Inverter?

An inverter is a machine which converts DC to AC (direct current to alternating current). It is the inverse of rectification (and hence the name), which means converting AC to DC.

A domestic inverter does quite a bit more than just doing inversion (DC to AC).

1. When the AC power supply is on, the inverter charges its batteries, from the power supply (essentially converts AC power to DC and stores into the batteries).
2. Also, inverter "forwards" the AC power directly to its output, when the AC power supply is on. This means, whatever appliances we have connected to the output of the inverter, runs on AC power, when the power is on.
   
3. When there is a power cut, the inverter automatically starts drawing power from the batteries, and supplies "generated" AC power, to the appliances connected to its output. Hence, the appliances connected to the output of the inverter, enjoys "seamless" electric power. It must be noted that "Seamless" is not the same as "un-interrupted". That means there is always a delay (switch-on delay), however small, before the inverter starts supplying power, and after the main AC supply goes off. This delay is usually of the order of 50 milli-seconds or around 1/20th of a second. This delay is often large enough to restart your computer and other sensitive equipments.
   
4. When the power comes back, inverter stops drawing power from the batteries, starts the AC power "forward" function, and then starts charging the batteries by drawing power from the supply. Even here there is a small delay (switch-off time of the inverter), but usually this delay is much smaller than switch-on delay, usually of the order of 20 milli-seconds or 1/50th of a second.
   

Square wave Inverter or Sine wave Inverter?

The AC (alternating current) power supply we use everyday is pure sine wave (like A in picture). But, because of the solid state (means no moving parts) devices they use, domestic power 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.

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 75W, and power-factor is 0.6, the VA requirement for your fan is 125 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 800VA inverter, we can "run" appliances upto a maximum of 480W (=800 x 0.6).

Now there is a big difference between "running" wattage, and "starting" wattage. For example, when you switch on your 75W of fan, it starts consuming around 150W to 200W of power. But, as the fan speed picks up, the wattage requirement slowly settles down to 75W. Similarly, for your 150W refrigerator, when the motor starts, the wattage requirement shoots to as high as 400W, but settles down very quickly to 150W. Even your large screen TV of 150W, when switched on (with the physical switch, not with remote from stand-by), starts sucking in almost 300W of power, but quickly settles down to 150W in 2 or 3 seconds.

Let us now look at the power requirements of different appliances in Table 1.

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
1. I want EVERYTHING in my house to be connected to my inverter
2. 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
1. I want to keep my inverter always switched on (ready mode, battery power kicks in as soon as power goes off)
2. I want to keep my inverter always switched off. Once power goes off, I want to manually start the inverter.
3. 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 (IMPORTANT: Because if your refrigerator or AC is running at night, this is probably as bad as 2.1).

The chart below explains, the VA requirements of your inverter, for different combinations of your connection preference and usage style.

Connection	Usage	VA Requirement	Comment
1 (Everything connected)	1 (Always ON)	Equal to the sum-total of all the equipments' "starting wattage"	Not Recommended
1 (Everything connected)	2 (Always OFF)	Equal to the sum-total of the "starting wattage"s of ONLY the equipments you want to use DURING the power cut	Not Recommended, may be used, to avoid additional wiring costs. But, at night, if there is a power cut, fan goes off, until you get up and manually switch the inverter on - your choice.
1 (Everything connected)	3 (Always OFF, except fans at night)	Equal to the sum-total of the "starting wattage"s of ONLY the equipments you want to use DURING the power cut, OR equal to the sum-total of the "running wattage"s of all bedroom fans, whichever is higher	Not Recommended, may be used, to avoid additional wiring costs. But, at night, you may need to switch off your refrigerator also.
2 (Subset connected)	1 (Always ON)	Equal to the sum-total of all the "connected" equipments' "running wattage"	Highly Recommended
2 (Subset connected)	2 & 3	(As per connection mode 1, wattage only for "connected" equipments	Not needed, if additional wiring is already done, then better keep the inverter switched on.

I highly recommend connection 2 and usage 1.

What I have done in my house is, in each bed room, living room, and bathroom, I have connected the mostly used fan and light to the inverter. In the drawing room, I have connected the TV also to the inverter. In my study/work room, I have connected my desktop PC to the inverter. I have connected 3 plug-points (scattered across my house) to the inverter as well. I have left out the ACs, refrigerator, microwave, the bed-room TVs, Geyser, water pumps and pretty much everything else, and kept them connected to main power line, not to the inverter.

I have bought a 800VA inverter, and this is how the wattage works out.

800 VA = 800 x 0.6 = 480W

• 6 CFLs (at any time only 4 would be in use, each 23W = 92W)
• 2 tube lights (at any point only 1 would be used = 60W)
• 4 ceiling fans (at any time only 2 would be in use, each 75W = 150W)
• 1 desktop PC (with LCD monitor = 170W).

All total = 472 W

The wattage calculation might suggest that there is not enough buffer space, but actually that is not the case. Because

• Almost never the fans run at full speed, rather almost always they run at 50% speed
• Light requirement is high only during the night, when it is colder, and hence fan requirement is lower. And during the day, it is the other way round.

What are the VA ratings usually available?

Table 2 gives you the commonly available VA ratings for domestic inverters.

What battery do I need?

Almost all domestic inverters run on 12V rechargeable batteries. But normal cheap rechargeable 12V batteries, technically known as "shallow discharge cycle" batteries, such as the car batteries, won't be suitable. Because these are only good if you discharge them only upto 50%/60%, and recharge them back again. If you discharge these batteries beyond 60%/70%, they suffer permanent damage, and their life and capacity gets drastically shortened. The other problem with these batteries, is the amount of electrical energy needed to recharge, is far greater than the amount of electrical energy used during charge, meaning these batteries are "inefficient". The efficiency for car batteries are usually as low as 70%.

Hence for power inverter, you would need high capacity, high efficiency, "deep discharge cycle" batteries. Tubular lead-acid rechargeable batteries are very commonly used for power inverters.

Battery capacities are specified in Ampere-Hours (AH). The higher the number, longer is its capacity, and hence higher is its backup time. The amount of backup time needed, would depend on the duration and frequency of power cuts in your area. Table 3 compares the commonly available tubular batteries in the market.

Many small local battery manufacturers claim that their batteries are "as good as" the branded ones, but are substantially cheap. Please don't fall into their trap. Always go for branded batteries, insist on VAT receipt, and fully filled in warranty card.

Exide Inva Tubular range of batteries are market leaders. You would find plenty of Exide dealers in any city in India, and their after sale service is good as well. Other good battery manufacturers are Amaron, and Standard Furukawa. I would strongly recommend to restrict yourself within these manufacturers.

DO NOT buy batteries manufactured by the inverter company itself, even if you get a killer bundle offer. Companies who manufacture good inverters, seldom manufacture good batteries. And good battery manufacturers seldom make inverters.

Who are the good inverter manufacturers?

Su-Kam is the market leader in domestic power inverter sector. APC (American Power Corporation) is also very good, and their technology is the best of breed. Luminous and Hyundai spend a fortune for marketing, however I am doubtful about their product quality and quality of service. Microtek is also very popular, but they usually serve the low-end square-wave inverter segment, not very popular in pure-sine-wave segment.

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 manufacturer's 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 X VA inverter, is =X/220. I use a 4A MCB, for my 800VA inverter, because 800/220 = 3.6, which is less than 4.
4. Please check the manufacturer's warranty period of your inverter. Typical warranty periods are 1yr, 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 50kgs), 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.