Power supply situation continues to be bad in India with power failures ranging from 2 to 4 hours in cities and 8 hours or more in small towns and villages. This makes it difficult to manage daily life and thus installing power backups is inevitable.
Although there are many solutions to this problem but the most common solution that most people apply in India; is to go for an Inverter-Battery system. However when somebody plans to purchase an inverter, the first question that comes to his/her mind is “which inverter to purchase?” and how one knows what suits best for its requirements. When you think of buying a new inverter, it is important to escape from the marketing gimmicks, and understand some basic related facts, eventually of much help before purchasing an inverter. Selecting the right kind of an inverter/battery system is one such important factor. And not just selecting the right inverter-battery system, it is important to select the right solution that solves your need.
In this slightly longish post, we will discuss on various power backup solutions, their pros and cons in different situations and will also help you choose the right one suitable for your need.
What are the various alternate power supply solutions?
- Inverter: It is a simple device that stores electricity from the grid in a battery and uses the DC (direct current) power from battery and converts it into AC (alternating current) that is used by appliances. Although interchangeably used, but Inverter and UPS are slightly different and we will explain that later in this post. Inverters are generally good if you want backup for small household appliances like lights, fans, TV, computers, etc. However these days’ bigger inverters can also support Refrigerators, Washing Machines and Mixer Grinders. But then adding these big appliances significantly increases the size of Inverter-Battery system and it is best if one can avoid using them on Inverter. Using inverters for ACs is strictly not suggested.
- DG Sets: DG sets use the power of Diesel to generate electricity. They generate instantaneous electricity rather than storing it. They are good for larger loads. So if you want to run ACs, Refrigerators, Motors, etc on power backup, then using DG set can be a good solution.
- Solar Power: Solar PV is slowly and steadily gaining popularity and is a great way to generate power backup. Energy from Sunlight is converted into electricity and stored in batteries. However the limitations of Solar Power are similar to that of regular inverters, so they are still not the best solution for bigger loads.
Now we will cover each of them in detail in different sections of this website, but in this article, we will mainly focus on Inverter-battery system.
First understand your Power Requirement
Before purchasing any of the solutions mentioned above, the most important thing is to understand your requirement. To evaluate the power requirement, it is very important for you to know two things upfront:
- The total load that you want to run on backup: This is essential to get the right size of inverter of power backup solution. And from size we mean the VA value. To get to the VA value, it is very important for you know the wattage (or watts) of each appliance that you want to run on the backup. More details on this are explained in the section below.
- Total number of hours for which backup is required: This is essential to size the batteries that you need (or in DG sets the litre capacity that you need).
To give you a analogy, imagine the car or vehicle that you have. It has a certain CC capacity (like 800 CC, 1200 CC, 1500 CC), which determines the power that the car has. More power can run faster or can take more load. This is similar to the VA value of inverter, which determines the load that it can take.
So what do you need to know?
Look at the list of appliances that you want to run on backup and their wattage. Check the list below to get some standard values used for calculations:
- 1 Fan – Fans Consume 75-90 Watts. For calculation we take 90 watts.
- 1 tube light – Assume 45-50 watts for standard tube lights. 28 watts for T5 tubelights.
- 1 CFL – 25 watts (default) or take the actual number if known.
- 1 Television – LED TVs take 30-100 watts depending on size. LCD TVs consume 50-150 watts. CRT takes about 120 watts.
- Set top boxes consume anywhere between 8 watts and 25 watts. Standard Definition set top boxes consume 8 Watts. HD boxes consume about 18 watts and HD DVR boxes consumed 25 watts.
- Desktops consume 120-250 watts depending on type. Those with LCD screens consume about 150 watts and those with CRT about 250.
- Laptops consume about 50 watts.
- Wi-Fi routers, modems, mobile phone chargers, etc consume 5 watts or less.
- Gaming stations and home theatre systems consume 100 watts or more depending on type and size. It should be available on product manual or on manufacturers’ website.
- A room air cooler would be about 250-300 watts.
- Induction cooker would be 1000-1500 watts (or 1 – 1.5 kW)
- Mixer Grinder would also be about 350 watts.
Suppose you want 1 Fans, 1 Tube lights, 1 CFL & 1 CRT television to operate at the time of power failure, i.e. running on inverter. Therefore the total power requirement (based on table above) comes out to be (1*90 + 1*50 + 1*25 + 1*120) = 285 watts. 285 watts here is the total load (as mentioned in point 1 above)
If you want a backup of 6 hours, then you need to store 285 x 6 = 1710 Watt-hour or 1.71 kilo watt hour (or units) of electricity. This is the storage that you need (as mentioned in point 2 above).
Now the calculation above assumes that every appliance will be “switched on” at any given point in time. Taking this exact value is safe. But if you still want to economize the size (to save costs) you can multiply the values by 0.8 if the appliances are for a 1 bed room home, 0.6 for a 2 bedroom home and 0.4 for a three bedroom home (assuming lesser number of appliances will be on at any given time).
Find the VA rating of the inverter you require
Now in an ideal situation the total Watt value (sum of wattage of all appliances as calculated above) should be the same as VA value. But due to a factor called “Power Factor” (explained in this article: link), it is not the same. For a residential house we assume power factor to be about 0.7 to 0.8 (to be super safe assume 0.7 and to be economical assume 0.8). So VA value is Total load in Watts divided by 0.8. So in the case above VA value would be:
285 / 0.8 = about 356 VA
Now you may not get an inverter of exact 356 VA but you will have to go with an inverter with VA value more than 356 (say 400 VA). As it is everything is an assumption, so we assume a value that is safe.
Find the battery size that you require
Now in the example above we needed a 356 VA inverter and we want a backup of 6 hours, then the total VAh that we need is:
356 x 6 = 2136 VAh
A regular inverter battery available in market is of 12 V, so the Ah value of battery required would be:
2136 / 12 = 178 Ah
If you go the market to purchase a battery then the first thing that you need to look at is the Ah value, which is representative of how much energy a battery can store. Typical Ah value of inverter batteries in market are: 100 Ah, 120 Ah, 150 Ah, 180 Ah, 200 Ah.
So in our example a 180 Ah battery would be ideal for the system.
What about bigger appliances?
Appliances like Refrigerator and Air Conditioners have motors in their compressors that need huge amount of current to start (unless you have inverter technology refrigerator or air conditioner). This significantly increases the VA and size requirement for an inverter.
A typical 250 lts refrigerator has a compressor which in steady state would consume about 300-350 watts, but just because it needs more current to start up, one would need about 2.5 kVA inverter to support such refrigerator. However because the compressor starts and stops as per the cooling needs, the total energy consumption per day for a refrigerator would only be about 1.5-2 units or kWh per day. So including a refrigerator in the appliance mix increases the size of the inverter significantly, but not of the battery.
Note: Inverter technology Refrigerators and ACs do not need high amount of current to start and thus if you have an appliance with inverter technology, then you can include AC or Refrigerator in the sample calculations (of other appliances) as above (and need not worry about high kVA value).
Issues with longer backup needs and Voltage of Inverter.
Many people have long power cuts (8-10 hours) and in such case, setting up inverters becomes difficult unless the load is less. As we have seen above, typical batteries available are of 12 V and of size 100 Ah, 120 Ah, 150 Ah, 180 Ah and 200 Ah. If for longer backup you need about 400 Ah battery storage, then you will have to go for two numbers of 200 Ah batteries. But this brings another challenge: that of Voltage of Inverter.
Typical small inverters (less than 1400 VA) are generally of 12 V. Which means you can either connect only 1 battery or if you want to add more batteries then you have to put them in parallel (as shown below).
But typical charging current of an inverter is about 10-15 Amp. Which means if you have one number of 200 Ah battery then it will charge in:
200 Ah / 10 A = 20 hrs
If you put two such batteries in parallel then the current will divide into two parts and thus charging time will become:
200 Ah x 2 / 10 A = 40 hrs
In such case if you have power cuts every day, then the battery itself will not charge in a full day. In fact if you put a 200 Ah battery and you need 10 hrs backup, then that itself will not happen because charging time is 20 hrs.
A possible solution is to go for an inverter with higher Voltage (say 24 V or 48 V or more). In canse you go for a 24 V inverter system, then you can connect batteries in series (as shown below), which does not divide the current (same amount of current charges both the batteries).
So if you need 200 Ah backup, then you can go for two numbers of 100 Ah batteries and put them in series. And the charging time will be
100 Ah / 10 A = 10 hrs
But then inverter with higher voltage are of higher VA value. Like 24 V system starts with 1400 VA onwards.
So it is important to go for right Voltage inverter.
What is “C rating” of batteries?
Although it is irrelevant to think about this while buying batteries, but we have got questions on it in past. If you are not curious, you can choose to ignore this section.
But if you are curious then the C rating (or the coulomb rating) of the battery (or cell), is the measure of the quantity of charge in a fully charged cell. The Coulomb capacity is provided in AH or Amp Hour unit. In other words, we can say that the C rating of a battery indicates the continuous current drawn (in amps), the cell will support. By multiplying the C rating with the cell capacity in milli-ampere-hours (mAH), the continuous current in milli-amperes (mA) of a cell can be determined. For example, in a 1,000 mAH battery with a C20 rating, the continuous current that may be drawn out of the battery is 1,000 mAH x 20 = 20,000 mA, or 20 amps (A) (divide by 1000). Keep in mind that 1,000 milliamps equals one amp. So from the above calculation, we ascertain that the battery will produce 20 amps of continuous discharge.
However, a C20 rating battery is considered ideal for the inverters and that is the C rating with which most inverter batteries are available in market. So if you ask for inverter battery, you will mostly get one with C20 rating.
Types of Inverter and How to Choose One
There are two types of inverters available in Market: Pure Sine Wave Inverter and Modified Sine Wave Inverter (or Square Wave Inverter). The choice will depend on the kind of appliances that you have.
If you want to run electronic appliances (like TVs, Computers, etc) on inverter, then go for Pure Sine wave Inverter, else you can go for Modified Sine Wave Inverter.
Please not Modified Sine Wave Inverter can cause humming noise when used with induction motors like ceiling fans, etc.
Difference between UPS and Inverter:
Although used interchangeably, there is a difference between UPS and Inverter. The basic function and architecture remains the same, but the difference is minor.
The difference between a UPS and Inverter lies in the time in which it can switch to battery power when a power cut happens. A UPS can switch to battery power in 3-5 milli seconds, whereas an inverter does it in 100 milli seconds. The choice between the two will depend on how critical is the switching to backup for the appliances that you want to support. For e.g. a desktop computer would need faster switching as you can loose data if it switches off even for a few milli seconds.
Even in UPS there are Online and Offline UPS. Online UPS are those in which the power is drawn from the battery irrespective of the availability of power from main line. This makes sure that there is no loss of work even for a few milli seconds when there is a power cut. Offline UPS is one where the power is drawn from battery only when there is power cut. All other times it is drawn from mains.
Types of Batteries and how to choose one
There are 3 types of batteries available in market: Flat Plate, Tubular and Maintenance Free. Without getting too much into details, all we can say is that Tubular Batteries are the best choice for inverters. They may cost slightly more than Flat Plate, but they will last longer. Maintenance Free batteries may sound good, but they have lesser life (4-5 years as compared to 7-8 years of a tubular battery). But the most important thing to run batteries for a longer times is to make sure that it is topped (filled) with distilled or RO water frequently and the fluid levels are maintained.
What would be the cost of 500 VA inverter and 100 AH battery?
The cost of an inverter with 500 VA would be around Rs. 3,000 and for 100 AH battery would be approximately Rs. 10,000. Therefore, if you are looking for an inverter with 500 VA and 100 AH battery, it would cost you around Rs. 13,000. Why not go online to search and compare the best deals and save the money that you may have to pay to the middle man, the quality of products and warranty is same and since now some of the e-commerce sites are providing the options of cash on delivery, there is no need to worry more about the delivery.
Other things to consider
Warranties: While purchasing batteries, one must look for maximum warranty. An inverter battery combo of the same brand is useful at times as manufacturers send their engineers for maintenance on a regular basis, which increases the life of the inverter/battery system.
In past Inverters and batteries were manufactured by different manufacturers. Su-kam, Luminous, Microtek were known manufacturers of inverter, whereas Amaron, Exide, etc were known manufacturers of batteries. But now all of them have inveter-battery combos where batteries and inverters are designed to work well with each other. So a combo system works well.
Inverter is a maintenance free product and shall not require any schedule maintenance except failure attention. Maintenance is only needed for batteries.
Efficiency of Inverter/battery system
Inverters are not 100% efficient and thus running the same appliances on inverter would cost you about 25% more. But DG sets will be slightly more expensive (which is about 33% more).
Inverters use some energy to drive its electronics and cooling fan. Battery uses additional energy during the process of charging and discharging. All this energy is wasted as heat energy. It is in the order of 8% during no load and goes up to 20% during full load. This inefficiency is generally taken care by the manufacturer and the rating mentioned is what is at the output terminal. Therefore, energy efficiency is generally not taken into account.
We hope that this information helps you choose the right inverter for your house. We have tried to include answer to every question that we get on this website, but if you still have some doubt, you can leave a note in the comments section below.