How to Figure Your Needs

By Serger

Twinkle, twinkle, little star. Power equals I squared R. That’s the jingle we learned in the navy to remember the relationship between three of the four basic components which are used to define and measure basic electricity used in power generation. The fourth component being voltage. So what does that mean to you as a survivor trying to power up your VCR and TV to provide some entertainment for the team at a distant time? Not a whole lot without some background.

In this topic, the discussion rolls around to how to interpret label plate data on electrical devices to know what type and how much electrical power is needed to make the goodie of your choice operate. It will also give you tools for how to calculate power for several devices without (hopefully) burning them up or destroying the power source or prime mover and operate them all at once (maybe).

Before getting to the actual manipulation of data, let’s go over the standard model of what electricity is which is used as the tool for describing the “stuff”. The model I’ll use is the water analogy, which from all I’ve been exposed to IS the best and easiest way for people to get a handle on what electricity sorta is.

If you think of electricity as a fluid like water then a lot of the magic is taken out of understanding what happens when you flip the switch. With that model, voltage or volts is the pressure that the water flows at. Amperes or amps is the volume of the flow, resistance in ohms is what the device does to hold back the flow. Power is the amount of water at a specific pressure and flow rate your device needs to make it work and is measured in watts. Also with that model, a battery is a bucket of water setting on the shelf until you poke a hole in it (hook it up) and let the water/electricity flow from it. The transmission lines are the pipelines that bring the goody to you. A prime mover (generator) is the spring from where the water flows. Lastly, if you keep the water in the pipes you won’t get wet (shocked). I know the techies out there are groaning, but this model works and rapidly gives people who could care less an understanding of the process, enough information to perform the tasks required of them without damage to themselves or their equipment.

I’m not going to get into frequency (hertz or hz) if I can help it, because for the average person who wants to hook up their stuff, the country you got your power production equipment in will match the frequency needs of your devices. The rule of thumb is if you need 50hz at 200 volts, match it. If you need 60hz at 110 volts, match it. Hopefully the discussion won’t get more in depth than that.

Now that the model’s in your head, let's get the mathematical formulas out of the way. You need this stuff, trust me.

Voltage = E also E=IxR also
=v or vac (for our application)

Amps = I
also I=E/R also =a

Resistance(ohms) = R also R=E/I

Power (watts) = P also
P=ExI also = w

(He’s talked several paragraphs and I still don’t know if I can fire up the video system)

Ok, let’s look at your needs. As an example, we’ll use my VCR and TV, cause it’s here in front of me.

The VCR in question has the following label plate data: 110vac, 60hz, 18w.

The TV in question has the following label plate data: 110vac, 60hz, 1.2a

You have one of those static inverters which puts out 300 watts at 120vac 60hz. It’s hooked up to the battery you pulled out of the Viper after it was shot up. It’s in good condition and has a good charge. My question is, can you run your video system without damaging it? In other words do you have enough power? To tackle this one, you need to talk apples with apples and with the information you have, you’ve got apples and oranges. Let’s do some conversion. What you need is to know how many watts the two devices will draw from the inverter. So here goes. You know a watt is P=ExI and you’ve got two of the three variables out of the three:

Device 1 18w

Device 2
110v x 1.2a=132w

TOTAL
150w

Great! We can watch TV tonight! And have room for several 15watt florescent bulbs to read by if you want to . Now we come to another thing you’ll need to know. If you decide not to run the lights, how many times can you watch “Barberella” before you run the battery down? I’ll make this one easy. The battery is new, rated at 450ah (sez so right in the label plate) and had a full charge. So what’s an ah (other than the obvious, you dirty minded Rubie you). Ah is the abbreviation for the term Amp hour. An amphour is the amount of amperage a battery can put out at its rated voltage. SO, if you’ve got a 450 ah 12v battery which is fully charged, you can draw 1 amp out of it for 450 hours or 10 amps for 45 hours. And all of it at 12 volts. Now, I grant the techies that a car battery isn’t rated this heavy but this is just an example. If “Barberella” is 98 minutes long you need to know how many ah you are pulling out of your battery per viewing. So, lets say it is 102 minutes long cause the VCR needs to rewind. That works out to be 1. 7 hours of run time per viewing. Here’s the math

150w/hour x 1.7 hours = 255w at 120 volts

To get how much amperage you’re sucking out of your battery you need to plug the numbers back in`.

255w = I x
12v

255w/12v = I

21.25a = I

21.25a/hour x 1.7hours= 35.99ah per viewing

This means you’re taking about 36 ah out of the battery per showing of that great Hanoi Jane classic. This means you can watch the flick 12.5 times. In the above illustration I used ideal components with no losses which occur in the real world . The losses for inverter systems run typically 10%, so if this was a real situation, you’d only get to see Jane at her best 10 times before the battery went flat.

*Serger*

All materials at this site not otherwise credited are Copyright © 1996-2003 Trip Williams. All rights reserved. May be reproduced for personal use only. Use of any material contained herein is subject to stated terms or written permission.