# How To Size Power Supplies by Pete Peters

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How To Size Power Supplies by Pete Peters p_peters69@hotmail.com

Watts Law P = Watts V = Voltage I = Amps To Find Desired Value Cover Letter and Perform Math P= I x E V= P / I I = P / E

Power Supply Useable Watts To find the Total Wattage (P) of a power supply, multiply the Volts (V) x Amps (I) Total Wattage X.8 = Useable Watts Example: A 120 Volt, 4 amp power supply has a total wattage of 480 Watts. Useable Watts = Total Wattage times.8 = 480 Watts x.8 = 384 Useable Watts Remember

A 100-Watt Power Supply needs 120 Volts to operate. How many Amps will the power supply use?.83 Amps = 100 Watts (P) / 120 Volts (V) (50) M5 LED lamps need 120 Volts to operate and use.025 Amps each. How many Watts will the power supply need to supply? 150 Watts = 120 Volts (V) x.025 Amps (I) x 50 Lamps The label on your power supply is damaged. You can only read the Wattage (W) 100 and the Amperage (I) 8. Use Watts Law to find out what voltage the power supply puts out. Voltage = P (watts) divided by I (amps) 100 Watts(P) / 8 Amps(I)= 12.5 Volts (V) Watts Law Examples

Sizing Your Power Supply 1) Determine the Load you will need. This is done by finding the voltage and amperage of the load or finding the total wattage of the load. 2) Use Watts Law to find the total wattage or use the wattage given on the load (light) and multiply the wattage by the number of loads (Lights). 3) Multiply the total wattage by 1.2 - this will allow a 20% increase in our Total Wattage. The amount calculated will be the size in watts for the Power Supply. 4) Remember– we do not want to operate our Power Supply at more than 80% of the Total Wattage. This is why we Multiply by 1.2. Example: You want to power eighty MR16 LEDs. Each LED is labeled as 12V 1.5W. This means that each MR16 requires 12 Volts to operate and uses 1.5 Watts each. Multiply the number (80) of MR16s times the 1.5 Watts. This equals 120 Watts. Now multiply by 1.2 times 120 Watts. This equals 144 Watts. You need a Power Supply that delivers 12 Volts and 144 Watts.