Inverters & UPSs

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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

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

Common Tools

Pumps and Air Conditioners

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:

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

• 27^th May, 2011: Initial post