# Understanding electric power

While in a mood to purchase a Peltier strip, I visited the amazon page which stated the specs of a Peltier thermoelectric cooler, one stated 12 V and 92 W and other one stated 12 V and some 77 W.

As I already know power is the product of voltage and current drawn. And the amount of current drawn is dependent on the resistance the circuit has to the battery, the less the resistance the more the current, the more the resistance the less the current. So why should power be part of the electronic instrument's specification?

Will a set of 8 pencil cells (1.5 volts each) are not enough for each one of these two peltier devices? my assumption is that since one will draw 92 watts and another will draw 77 watts so the batteries operating 92 watts device will consume more quickly than the other batteries with 77 watts device. But eventually both can be operated on a 12 V battery with different performances depending on their power requirements, am I right?

Any two of voltage, current, and power can be specified and the third inferred. Which two are listed in the datasheet depends on what is customary to talk about for that class of device, what the datasheet writers think is more relevant, or what matters most to the design engineers that give the specs to the datasheet writer. Sometimes with lesser datasheets you get the parameters marketing wants to push because they think their device is better than the competition in that area. It varies.

If a devices is rated to require 92 Watts at 12 Volts, then you can infer it draws 7.7 Amps at that operating point. Again, you can compute any one of these numbers from the other two.

Be careful what the power spec really means. There are two different specs that have units of power for Peltier coolers. One is the electrical power the device will use, the other is the thermal cooling power. Both are relevant to different parts of the design. Possibly only one of these is listed and a efficiency spec is provided so that you can determine the other. With Peltier coolers, efficiency is a strong inverse function of input current. There should be either a equation or a graph in the datasheet showing the relationship.

As always, it's important to read the datasheet carefully. If you find a spec that doesn't make sense, come back and ask about it specifically. Otherwise there is too much general stuff to get into here.

As a separate issue, 92 Watts is way way way too much power to expect from a few "pencil cells". Go read a datasheet for any ordinary AA battery, paying close attention to maximum current and derating at high current levels.

You'll find these elsewhere of course, but

R = Resistance Ohms V = Voltage Volts I = Current Amperes or Amps W = Power Watts (sometimes P)

These three are just rearrangments of the one formula:

R = V/I
I = V/R
V = IR

These three are all the same formula with the values being substituted from the three equations above.

W = V x V / R
W = I x I x R
W = V x I

Look up battery capacities as Olin says, but set a multimeter to the 10A DC or 20A DC and short a few AA cells and measure short circuit current. That is the most they can make on short circuit - you'll get substantially less when loaded more normally. Shorting a new good quality D cell is not recommended.