# Inverter Interview Questions & Answers

## Inverter Interview Questions And Answers

1. Question 1. How Much Power Will My Inverter Draw? I Left My Inverter On All Night And Now Nothing Works?

The power required to run an inverter is approximately 8-10% more than the power load of the appliances being run. This is due to the efficiency of the inverter. These days, quality inverters are between 90-92% efficient. An easy formula to use to work out how much DC Amps you will use from your battery is, simply divide the AC wattage of your appliance by 12 (or 24 if a 24v system) and times this number by 1.1 to get a very close estimate of the DC draw.

Inverters will draw power from your batteries when not in use, and the unit is turned on. This can vary from around .02 amps right up to 2 amps depending on the unit and design of their standby systems if you leave your inverter turned on with no load attached, the average draw from your batteries will be 1 amp per hour; 24 amps per day; or 168 amps over a week. The simplest solution to this is to just turn the inverter off when not required as the battery drain then becomes zero. Many of our inverters feature standard or optional remote on/off controls.

2. Question 2. What Size Batteries Do I Need?

The simple method of working this out, is to find the largest constant AC load you plan to run from your inverter (in most cases a microwave is the largest appliance), and divide this wattage x 5 (20% of your load). This will give you a rough idea of how much battery capacity you require to sustain your loads. With the example above, the microwave draws around 1000 watts AC, so a battery bank of around 200amp hours would be required.

3. Question 3. What Size Inverter Do I Need?

Sizing your inverter is a difficult task, and is not made any easier by different brands and manufacturers rating the power of their units at different temperature ratings. The power output of an inverter is dramatically decreases as its internal temperature rises, this is sometimes called its 5, 10 & 30 minute rating; but in reality if the inverter cannot remove the heat quick enough, then the output power will rapidly drop off. Many of the inverter brands on the market are rated to supply full output power up to 25°C and start to de-rate the output power from this point on. Some industrial models with supply full output upto a staggering 40°C.

Basically, add up all of the AC loads you want to run, then determine if you want to run all of these items at the same time. If not, choose the largest load in the system and size the inverter 50% more to cover peak surge loads.

4. Question 4. What Happens If Someone Plugs An Appliance In That They Shouldn’t Have?

The great advantage of the power inverter is its ability to surge to power output levels well in excess of its normal continuous ratings. These levels are generally twice the normal output power for 1-2 seconds to allow for starting of larger or inductive type loads. If the load is too large for the power output (e.g someone connects a hair dryer to a 500watt inverter), then the inverter simply shuts the power down. Depending on the inverter, an audible alarm will sound, or the inverter will simply restart to try the load again. Always keep in your mind that a power inverter has limitations, mainly its power size.

5. Question 5. What Inverter/size Do I Need To Run My C-pap (respirator) Machine?

For C-pap machines to function correctly they require a pure sine wave power output. These machines tend to draw around 200watts, which equates to 18amps per hour. For a normal night of sleep (7-8 hours), you would need a battery bank of around 200amp hours (and a suitable recharging system). The ePOWER 400watt inverter is perfect for basic C-pap machines; contact us for details if your machine has a heater inbuilt.

6. Question 6. My Inverter Is Running Very Hot, Is This Alright?

Power generation creates heat, so yes your inverter will get warm. For this reason it is important to keep your inverter in a location that is cool and dry, and installed in an orientation that will help assist with ventilation (e.g. you are not blocking the intake vents or fans with clothes or tools). If your inverter gets too warm, it will simply shut down until the unit cools down. As mentioned above, the hotter your inverter gets, the less AC power it will deliver (hence the new designs such as Proteins that allow for rapid removal of warm air from the inside workings).

7. Question 7. When Using My Television Off The Inverter, There Are A Number Of Fuzzy Lines Across The Screen?

Again a result of using a modified sine wave inverter. Many appliances react to this type of waveform (such as televisions generating lines across the screen, or radios having a hum through the speakers). There is not a great deal you can do to stop this except to upgrade pure sine wave inverter. Try twisting the battery cables together and then wrap them in alfoil, as well as moving the inverter as far away from the interfered appliance as possible (these are merely suggestions and may not work in every case).

8. Question 8. Can I Run My Computer / Laptop From An Inverter?

Modified sine wave inverters can be used on either a computer or laptop, however if the laptop is to only ever be powered from the inverter then a pure sine wave inverter (such as the POWER or ePRO) should be used, as the modified sine wave inverters will actually destroy the laptop battery pack.

9. Question 9. What Is An Inverter?

An inverter takes DC power (from a battery or solar panel, for example) and converts it into alternating current (AC) "household" power for running electronic equipment and appliances.

10. Question 10. Why Are They Called Inverters?

Originally converters were large rotating electromechanical devices used to convert AC to DC. It is what you have to do if you don't have semiconductor or vacuum tube rectifiers. Essentially they combined a synchronous ac motor with a commutator so that the commutator reversed its connections to the ac line exactly twice per cycle. The results are ac-in dc-out (with a lot of switching noise thrown in). If you invert the connections to a converter you put dc in and get ac out. Hence an inverter is an inverted converter.

11. Question 11. How Is An Inverter Different Than A Ups?

A UPS typically includes the inverter, battery and battery charger in one stand-alone unit. However, there are UPSs that use external batteries, and Power Stream has made inverters with battery chargers, so the differences blur as features proliferate.

UPSs also can have communication with the equipment that it is powering, which lets the equipment know that it is operating on standby, giving it shutdown warning, or communicating with the human in the loop. Inverters typically don't have this communication.

12. Question 12. What If I Want A Dc Output To Run Such Things As A Laptop From A Car Cigarette Lighter, Or Telephone Equipment At -48 Volts?

Then you want a DC/DC converter. Power Stream has many DC/DC converters just for those purposes.

13. Question 13. What Is The Difference Between Sine Wave And Modified Sine Wave?

Alternating current (AC) has a continuously varying voltage that swings from positive to negative. This has great advantages in power transmission over long distances. Power from your power company is carefully regulated to be a perfect sine wave, because that is what naturally comes out of a generator, and also because sine waves radiate the least amount of radio power during long distance transmission.

On the other hand, a sine wave is expensive to make in an inverter, and many sine wave techniques use heavy, inefficient transformers. The most inexpensive way to make AC is to switch the DC on and off--a square wave. A modified sine wave is scientifically designed to simulate a sine wave in the most important respects so that it will work for most appliances.

It consists of a flat plateau of positive voltage, dropping abruptly to zero for a while, then dropping again to a flat plateau of negative voltage, back to zero for a while, then returning to the positive voltage. This pause at zero volts puts more power into the 60HZ fundamental than a simple square wave does, so it is called "modified sine wave" instead of "square wave." Because the MOSFETs only have to turn completely on and completely off the dissipate he least amount of heat for the power generated, and so smaller semiconductors and heat sinks are needed than if you were trying to generate a real sine wave.

14. Question 14. Can I Use A Modified Sine Wave Inverter For My Medical Equipment?

For Medical equipment, oxygen generators, etc. talk to the manufacturer of the equipment. Power Stream inverters are never tested or rated with medical equipment, and we don't guarantee that they will work to save your life. For such applications please find inverters that are rated and tested for such applications.

15. Question 15. What About Square Wave Inverters?

These old-fashioned inverters are the cheapest to make, but the hardest to use. They just flip the voltage from plus to minus creating a square waveform. They are not very efficient because the square wave has a lot of power in higher harmonics that cannot be used by many appliances. Synchronous motors, for example, use the 60Hz component and turn the rest of the frequencies into heat. The modified sine wave is designed to minimize the power in the harmonics while still being cheap to make.

16. Question 16. How Do I Know If I Need A Sine Wave, Or If I Can Live With A Modified Sine Wave?

The following gadgets work well with a modified sine wave: electric blankets, computers, motor-driven appliances, toasters, coffee makers, and most stereos, ink jet printers, refrigerators, TVs, VCRs, many microwave ovens, etc.

Appliances that are known to have problems with the modified sine wave are some digital clocks, some battery chargers, most light dimmers, some battery operated gadgets that recharge in an AC receptacle, some chargers for hand tools (Makita is known to have this problem in the past). In the case of hand tools, the problem chargers usually have a warning label stating that dangerous voltages are present at the battery terminals when charging. We would like to add to this FAQ any appliances that you have had trouble with, or had success with, using modified sine wave inverters.

17. Question 17. Why Do I Hear Buzzing On My Stereo When Using A Modified Sine Wave Inverter?

Some inexpensive stereos use power supplies that cannot eliminate common-mode noise. These would require a sine wave inverter to operate noise-free. What you hear is some of the higher harmonics of the modified sine wave.

18. Question 18. Why Don't I Measure Rated Voltages When Using A Multi Meter On My Modified Sine Wave Inverter?

The rated voltage is an RMS (root mean square--they square the value to make sure it is always positive, then average it, then take the square root of the average to make up for having squared it in the first place) measurement. Most multimeters are designed to give correct RMS readings when applied to sine waves, but not when they are applied to other waveforms. They will read from 2% to 20% low in voltage. Look for a voltmeter that brags about "True RMS" readings, and that will read correctly no matter what the wave shape is.

19. Question 19. How Should I Select The Right Size Inverter?

First add up the power ratings of all the appliances, and then buy the next larger inverter! At least that is the simple answer. Note, however, that some appliances, such as table saws, refrigerators, and microwaves have a surge requirement. Power Stream inverters are designed to supply such surges, but since every appliance has its own requirements sometimes you will need to get a bigger inverter than you would otherwise think. Note that the inverter isn't the only consideration when you are pondering the mysteries of start-up surges. The battery must also be able to supply the surge power, and the cables must be able to supply the increased current without dropping the voltage too much.

20. Question 20. How Is A Microwave Rated For Wattage?

When you buy a microwave oven you want to know how intense the microwave field is, not how much the oven draws from the wall. So a microwave oven that boasts 600 watts on the box will have an input requirement of 1200 watts on the boilerplate in the back. Don't be fooled!

21. Question 21. Are Stereo Amplifiers Rated The Same Way?

Stereo manufacturers are bigger liars than politicians. Sometimes they use peak output power (milliseconds), sometimes they use power drawn from the wall, but often they just look at the competition's cartoon front and add 10%. However the truth is available: look at the boilerplate sticker, which has been evaluated by all. This will give the maximum possible power drawn, so it tends to be higher than you will actually draw.

22. Question 22. Why Do I Need Such Humongous Cables To The Battery When A Small Cord Takes The Ac Output Fine?

Power is volts times amps (Watts = V x A). So if you have a lot of voltage you don't need many amps to get a watt. Roughly you need 12 times as much current from the 12 volt battery as you need from the 110 volt AC outlet. Current is what causes cables to heat up, not voltage. That is why they use thousands of volts in long distance power transmission grids. The thing to do when you have lots of current is to lower the resistance of the cable. The larger the wire the lower the resistance. Think of the cable as a water pipe. A big pipe (wire) can carry more water (current or amperage) with less pressure (voltage), and will present less pressure (voltage) drop from one end of the pipe to the other.

Another consideration is how far the cable has to run from the battery to the inverter. Long cable runs are expensive, either in copper or efficiency, or both.

23. Question 23. Why Would You Use A 24 Volt Inverter Instead Of A 12 Volt Inverter?

At a given power rating a 24 volt inverter will need half the current as a 12 volt inverter. This makes the entire system more efficient, and since high current transistors are expensive, the inverter will be cheaper.

24. Question 24. Should I Use A Laser Printer With An Inverter?

Only if you must. Laser printers use up a surprising amount of power (due to the heated fusing rollers), and will discharge your battery faster than you expect, even on standby. If you do, make sure the inverter is rated for the power of the printer plus computer plus monitor. It doesn't do any good to have your computer brown out as soon as the printer starts to print. Ink jet printers, on the other hand, use a surprisingly low amount of power.

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