How to Choose an Inverter or UPS for Your Home

For a long time, I postponed purchasing a power inverter even as Bangalore reeled under severe power cuts, lasting from as little as two seconds to all night sometimes. Why should I pay out of my pocket for the Government’s inefficiency, I thought? But it was only a matter of time before frustration triumphed over principle. So finally, last year I decided to purchase a power inverter for my home.

Having done a good amount of research before zeroing in on an inverter system, I decided to share with all of you some points you must keep in mind while purchasing a power inverter for your home.

The following sections explain how an inverter works and what are the components of a typical power inverter system. Also mentioned are points to consider while purchasing each individual component.

How does an Inverter work?

An inverter takes DC power from a battery and converts it to AC power. It does this by using electronic circuits to first convert DC battery voltage to AC and then amplifying it to grid voltage of 220-240 V.

APC inverter

In the event of a power cut, the inverter near-instantly detects the loss of mains power and starts generating backup AC power from the battery. This allows your electrical and electronic equipment to function as normal during the power cut.

When the grid power is restored, the inverter stops generating AC power from the battery and switches back to the mains supply. It also starts recharging the battery from the mains supply.

Inverter vs. UPS

Inverters are primarily meant to power electrical devices, whereas an Uninterrupted Power Supply (UPS) are meant mainly for electronic equipment that is less tolerant to power fluctuations.

A UPS has more stringent operational requirements than an inverter. It must provide:

  • Instantaneous backup during power failure (inverters take a few milliseconds to switch to backup power)
  • Stable output voltage, even when input voltage fluctuates or output load varies (inverters only kick in when power fails, not when it fluctuates).

The above two features are extremely important for electronic equipment as delays in switching to backup or fluctuation in voltage can lead to malfunctions and/or loss of data.

The flip side of this is that a UPS is more expensive than an inverter. Moreover, because a UPS is less tolerant to voltage fluctuations, it will switch to backup power more often if such fluctuations are common, and thus wear out the battery faster.

Electrical appliances like lights, fans and TVs do not need stable voltage or instantaneous backup, so unless you intend to run computers, modems, etc. on backup power, buying a UPS is not necessary and an inverter will suffice.

That said, some newer inverter models come with a built-in UPS mode which can be turned on or off manually based on the equipment connected, thereby offering the best of both words to the consumer.

Components of a Home Inverter System

1. Inverter Unit

The inverter unit is the brains of the system. It has components like oscillators, DSPs, microprocessors, etc. to do the DC-to-AC conversion, and transformers for amplification to grid voltage of 220-240 V AC. It also has sensors to monitor the input voltage and switch between mains and backup power as needed.

Inverter units are rated in terms of Volt-Amperes (VA). The VA rating of an inverter determines how many electrical appliances it can run simultaneously. Inverters are available in various capacities ranging from 600 VA to 4000 VA and more, with the higher capacity units capable of powering even air conditioners and refrigerators!

If your requirements are beyond these limits, you are better off purchasing a generator instead.

2. Battery

Batteries provide the backup DC power to the inverter unit. Most home inverters are designed to work with 12 V DC input voltage, although higher capacity units (2 kVA and more) require 24 V or 48 V input voltage, which would require a battery bank (multiple 12 V batteries connected in series).

The capacity of a battery is measured in Ampere-Hours (AH). The AH rating of a battery determines how much power it can supply and for how long.

Theoretically speaking, a 120 AH battery should be able to supply 1 Ampere current for 120 hours or 120 Amperes for 1 hour. In practice, however, a battery can only supply current up to a limit without lowering its AH capacity. This limit depends on its discharge rating or Coulomb (C) rating, specified as C10, C20, etc. A lower C rating implies a higher current supplying capacity, as per the following formula:

Maximum current capacity (A) = Battery capacity (AH) / Discharge rating (C)

So a 120 AH C10 rated battery can supply up to (120 / 10) = 12 A of current, but a 120 AH C20 rated battery can only supply up to (120 / 20) = 6 A of current without reducing its effective AH capacity. If discharged at 12 A, the effective AH capacity of the C20 battery goes down as per the below formula:

Effective battery capacity (AH) @C10 = 0.8 x Rated battery capacity (AH) @C20

So, if a 120 AH C10 and C20 battery are both made to supply 12 A of current, the effective capacity of the C20 battery would come down to (0.8 x 120 AH) = 96 AH, which means it wouldn’t last for 10 hours but instead would drain in 8 hours (96 AH / 12 A = 8 H). By contrast, the C10 battery would last the full 10 hour duration.

For this reason, C10 rated batteries are costlier than C20 rated batteries.

For most home inverter applications, C20 batteries are adequate as appliances like tubelights, fans, etc. do not draw too much current. However, C10 batteries can be considered if heavier loads are such as ACs and refrigerators are being used.

3. Trolley

A trolley is used to house the inverter unit and battery and to move them around. Trolleys come in different sizes for different types of batteries. Tubular batteries, for example, need taller trolleys.

Types of Inverters

Depending on the type of AC waveform generated by the inverter, they are classified as Square Wave, Quasi-Sine Wave and Pure Sine Wave.

1. Pure Sine Wave

A sine wave is the purest waveform that an inverter can generate. The mains power supply is also sine wave and all electrical appliances are designed to work with sine waves, so using a pure sine wave inverter ensures the longevity, reliability and safety of your electrical equipment. It also reduces power wastage when running on backup.

In reality though, different manufacturers use different techniques to generate the sine wave signal, so its purity can differ from product to product based on the technology and quality of components used.

2. Digital Sine Wave (also called Quasi-Sine Wave)

A quasi-sine wave is not a pure sine wave but a digital approximation of it. This means that the output of the inverter will be a stepped sine-like waveform instead of a smooth sinusoid.

While this may not pose a problem for electronic equipment, electrical appliances such as lights, fans, TV, etc. may not work very well with it. Tubelights may flicker and fans may produce a slight humming sound.

If you can’t afford a pure sine wave inverter, this is the next best option.

3. Square Wave

The earliest inverters were all square wave. As the name suggests, the waveform generated here is a pure square wave. As a result, it is highly inefficient and not well suited for electrical equipment. Apart from wasting a lot of power, it will also reduce the life of electrical appliances.

Square wave inverters are pretty much obsolete now.

Types of Batteries

There are several types of batteries available for inverters in the market. The following are some popular options:

1. Tubular Batteries

Tubular batteries are the newest entrant in the inverter battery market. These batteries are designed specifically for inverter applications and have several advantages over other types of inverter batteries, such as:

  • Longer life (typically 5 years) compared to lead acid batteries
  • Can undergo several rounds of deep discharge (i.e. complete drainage) without loss of capacity
  • Low on maintenance (no need for frequent recharge of distilled water)
  • Faster recharging
  • Emit less fumes compared to other lead acid batteries

On the flip side, tubular batteries are more expensive than any of the other batteries available.

2. Sealed Maintenance Free (SMF) Batteries

Traditionally used in cars, in recent years specially designed SMF batteries have emerged for inverters as well. These batteries have thicker electrode plates to provide high current for longer duration, unlike car SMF batteries that have thin plates.

As their name suggests, the primary advantage of SMF batteries is that they are sealed and maintenance free. This means that they neither emit any fumes nor require any kind of maintenance (recharge of distilled water). This makes them suitable for indoor usage with inverters.

On the flip side, they have a shorter life than tubular batteries (typically 3 years) which means they need to be replaced more frequently.



3. Flat Plate Batteries

These are the traditional lead acid batteries that are in the market since a long time. They are designed for inverters, but are not as efficient as tubular batteries. This means they are not as long lasting as tubular batteries, require more frequent maintenance (top-ups) and cannot undergo as many deep discharges as tubular batteries. They also emit more fumes as compared to tubular batteries.

Can I use a Car Battery for my Inverter?

Yes, but you shouldn’t. A car battery is designed to provide very high current for short bursts of time. Its primary job is to crank the car’s starter motor (which requires a large current) until the car starts, which only takes a few seconds. By contrast, an inverter battery needs to supply a steady amount of current for longer periods of time, ranging from a few minutes to several hours.

And unlike a car battery which is not designed to be discharged fully, an inverter battery can be drained completely if power cuts are of long duration. So using a car battery with an inverter may result in the battery dying out quickly and pushing up your maintenance bill.

What Inverter and Battery capacity is right for me?

While buying an inverter system, you need to calculate what capacity inverter and battery you need. To do so, follow these steps:

1. Calculate your power usage

Evaluate how many electrical appliances you are likely to run at a time during a power cut. Then add the wattage of all these devices to arrive at the total load.

The following are tentative power ratings of some common electrical appliances:

Tubelight - 40W
Fan - 60W
TV (32" LCD) - 120W
Air Conditioner (1 Ton) - 1500 W
Refrigerator (200 litres) - 500 W

Let’s assume the following two usage scenarios and calculate the total load in each case:

Scenario 1:

3 tubelights + 3 fans + 1 TV running simultaneously

Total load = (3 x 40 W) + (3 x 60 W) + (1 x 120 W) = 420 W

Scenario 2:

5 tubelights + 3 fans + 1 TV + 1 AC + 1 refrigerator running simultaneously

Total load = (5 x 40 W) + (3 x 60 W) + (1 x 120 W) + (1 x 1500 W) + (1 x 500 W) = 2500 W = 2.5 kW

2. Calculate inverter capacity required

Next, we need to calculate inverter capacity required to handle the measured load. The formula is:

Required inverter capacity (VA) = Load (W) / Power Factor

In our examples above, assuming a typical Power Factor (PF) of 0.8 the required inverter rating would be:

Scenario 1:

Load calculated = 420 W

Required inverter VA rating = (420 / 0.8) = 525 VA

The nearest higher rating inverter available is 600 VA (12 V input).

Scenario 2:

Load calculated = 2.5 kW

Required inverter VA rating = (2500 / 0.8) = 3125 VA = 3.125 kVA

The nearest higher rating inverter available is 3.5 kVA (48 V input).

3. Calculate battery capacity required

The battery capacity determines how long the calculated load can run on backup power. While calculating battery capacity required, it is necessary to know the load as well as the duration for which backup is required.

Following is the formula to calculate battery capacity:

Required battery capacity (AH) = (Load (W) / Input voltage) x Backup duration

Assuming load shedding of 3 hours everyday, battery capacity required would be as follows for our two scenarios:

Scenario 1:

Load calculated = 420 W

Inverter input voltage = 12V

Battery capacity required = (420 / 12) x 3 = 105 AH

The nearest higher battery capacity available is 110 AH. A C20 battery of this capacity would be adequate for our requirement, given the low current requirements.

Scenario 2:

Load calculated = 2.5 kW

Inverter input voltage = 48 V

Battery capacity required = (2500 / 48) x 3 = 156.25 AH

The nearest higher battery capacity available is 160 AH. While a C20 battery of this capacity would do, given the higher current requirements, a C10 battery is recommended. You will need to install four such 12 V batteries in series to meet the inverter’s input voltage requirement of 48 V.

Maintenance



Periodic inspection of the water level in the battery is necessary to ensure a long life and avoid deterioration in performance. Use only distilled water for refilling and not RO water.

Other factors

Apart from type of inverter and battery, other important factors must be kept in mind while selecting a particular brand of inverter and battery, such as reliability of the brand, market feedback of the specific product, quality of after-sales service, etc. This will ensure peace of mind after the purchase is made.

Conclusion

Buying an inverter involves two decisions: how many appliances you want to run simultaneously, and how long you want to run them on backup power. Understanding these two requirements properly will help you choose the right equipment.

The best inverter today would consist of a pure sine wave inverter unit coupled with a tubular battery (although SMF batteries are more suitable for indoor use as they don’t emit any fumes).


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123 Comments

  1. Yogesh KumarYogesh Kumar11-18-2016

    Thanks for sharing information post. I have electric load 3 Fan, 5 Tube light, 1 TV, 1 Laptop and some lighter gadgets. Suggest me which option and capacity home ups is enough for me.

    • Vijay PadiyarVijay Padiyar11-18-2016

      All relevant formulas and information are given here sir, you can easily calculate yourself.

  2. Yogesh KumarYogesh Kumar11-17-2016

    Thanks for the post.

  3. VinayVinay10-19-2016

    Thank you for the Info Vijay. The info really helped me decide and choose the right UPS/Inverter.

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