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Inverters & UPSs

, 27 May 2011 CPOL
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Comparison between inverters and UPS and calculating how long a battery can last
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Introduction

What are Inverters

Inverters are adapters to convert and amplify battery DC power into AC. A question may arise about the difference between inverters and UPS - uninterrupted power supply. The answer can be simply illustrated in a couple of points:

Inverter vs. UPS Comparison

Inverter UPS
Switching time is slow comparing to UPS, i.e. > 20 milliseconds, a typical inverter takes 500 milliseconds to switch making it reasonably unsafe for high tech electronics like computers. Switching time is relatively high 2 - 10 milliseconds, typically 8 milliseconds which is why we use them for computers.
External battery is required Internal battery is installed
Supply time can be controlled by the amps of the battery Supply time is restricted
Power transfer is not necessarily automatic Automatic power switch
Last but not least, it doesn't beep your head out Beeps the heck out of you

Background

UPS/Inverter power is measured by the Volts/Amps, i.e. Watts.

Conversion formulae

Watts ÷ 240 = Amps
Amps x 240 = Watts

Note: For calculations, you may want to replace the voltage values in this article to match with your country voltage. Here in Yemen, we are powered with 240 volts "hardly J".

You also have to keep in mind the startup watts which are consumed at the start of most appliances, as a general rule, most would consume twice as much at a startup, i.e., Watts x 2 = Approximate starting load. This formula yields a close approximation of the starting load of the appliance, though some may require an even greater starting load.

Note: Induction motors such as air conditioners, refrigerators, freezers and pumps may have a start up surge of 3 to 7 times the continuous rating.

Most often, the start up load of the appliance or power tool determines whether an inverter has the capability to power it. You may find the wordings "continuous and peak surge" on the back of an appliance.

Example: continuous - 2000 Watts, peak surge - 4000 Watts

If you don't find it, you can simply multiply Amps x Volts, i.e. (Amps x 240 AC voltage) = Watts. For example, you have a freezer with a continuous load of 4 amps, and a start up load of 12 amps:

4 amps x 240 volts = 480 watts continuous
12 amps x 240 volts = 1,440 watts starting load

To convert AC Watts to DC Amps:

AC Watts ÷ 12 x 1.1 = DC Amps

(This is the size vehicle alternator you would need to keep up with a specific load; for example, to keep up with a continuous draw of 1000 watts, you would need a 91 amp alternator)

Common Appliances

Appliance Est. Watts
Coffee pot (10 cup) 2400
Coffee pot (4 cup) 650
Toaster 800-1500
Cappuccino Maker 1250
Coffee Grinder 100
Blender 300
Microwave (600 to 1000 W cooking power) 1100-2000W (elec. consumption)
Waffle iron 2400
Hot plate 2400
Frying pan 2400
Toaster Oven 2400
Blow dryer 900-1500
Computer
- laptop
- pc & monitor
- printer-inkjet

50-75
200-400
60-75
Refrigerator/Freezer * 600
Freezer * 500-800
Appliance Est. Watts
VCR 40-60
CD or DVD Player 35
Stereo 30-100
Clock Radio 50
AM/FM car cassette 8+
Satellite dish 30+
Vacuum cleaner
 
300-1100
Mini Christmas lights (50) 25
Space Heater 1000-1500
Iron 1000
Washing machine 920
12" 3 speed table fan 230
TV - 25" color
- 19" color TV or monitor
- 12" B&W
13" color TV/VCR Combo
300
160
30
230
Game Console (X-Box) 100
Furnace Fan (1/3hp) * 2400

Common Tools

Tool Est. Watts
Jig Saw 300
Band Saw 2400
Table Saw 1800
6 1/2" circ. saw 1000
7 1/4" circ. saw 2400+
8 1/4" circ. saw 1800
Disc Sander 2400
Makita Chop Saw 1550
Makita Cut Off Saw 1000
Tool Est. Watts
1/4" drill 250
3/8" drill 500
1/2" drill 750
Shop Vac 5 HP 1000
Sabre Saw * 500
Portable Grinder * 1380
Electric Chain Saw 14" * 2400
Airless Sprayer 1/2 HP * 600
Air Compressor 1 HP * 2000

Pumps and Air Conditioners

Pump Running Starting
Well Pump 1/3 HP * 750 1400-3000
Well Pump 1/2 HP 1000 2100-4000
Sump Pump 1/3 HP * 800 1300-2900
Sump Pump 1/2 HP * 1050 2150-4100

Air Conditioner Running Starting  
7000 BTU to 10000 BTU * 1000-1500 2200-5000 A/Cs are a very difficult load because of the high start-up surge. Use the Locked Rotor Amps to determine the start up surge requirement.

Appliances and tools with induction motors (marked * in tables) may require from 3 to 7 times the listed wattage when starting. The start-up load of the appliance or tool determines whether an inverter has the capability to power it. Be sure to check the specific wattage requirements and operating instructions for appliances / tools to be used.

How Long Can My Battery Last?

To calculate the time a battery can last, use the following formula:

T = ((AH / 2) / (L / BF)) x C

where:

T - Time in hours
AH - Ampere Hours the battery is rated for
L - Load in Watts
BF - Battery factor (10 for 12V & 20 for 24V)
C - Discharge rate.

Going to the manufacture curves you can get accurate figures, for this article, let us assume that a battery lives about a year of continuous consumption and use a straight line depreciation factor @ 1/12, i.e. 0.083

1 - (0.083 * M)

where M = Months passed

The only accurate method is by measurement as all batteries lose capacity with age, charge/discharge cycles and temp variations.

It is also obvious that the more Ampere Hours you can get from a battery, the more time you will get, batteries come in the following sizes:

Battery Type Voltage Ampere Hours
22 NF 12 50
24 NF 12 75
27 NF 12 100
8 D 12 200

Battery.jpg

Using the Code

We first store values like the battery's AmpH and Volts, Watts to be consumed and the number of months the battery has been used and then apply them in the above mentioned formula.

For accuracy, in the code, we extend the battery discharge factor to 6 decimal places.

double Volts = double.Parse(bVolts.Text);
double Amps = double.Parse(bAmps.Text);
double Watts = double.Parse(cWatts.Text);
int BatteryFactor = Volts == 12 ? 10 : 20;

double DischargeFactor = 1 - (0.083333 * double.Parse(bMonths.Text));
double dTime = ((Amps / 2) / (Watts / BatteryFactor))*DischargeFactor;
int Hours = (int)dTime;
int Minutes = (int)((dTime - (double)Hours) * 60);

if (dTime > 0.1 && dTime < 99.59)
    lblMinutes.Text = Hours.ToString("00") + ":" + Minutes.ToString("00");
else
    lblMinutes.Text = "  N/A";

History

  • 27th May, 2011: Initial post

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Muammar©
Retired QSoft
Yemen Yemen
Biography?! I'm not dead yet!
www.QSoftOnline.com

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