Before you purchase batteries it is wise to determine the maximum electrical load you need from the batteries to run the electrical items in your sleeper when your engine is not running.

Let’s assume: 

  1. You do not have an APU or generator. 
  2. You have a coffee pot, microwave, refrigerator/freezer, blender, TV, and a computer, all running on 120-volt AC power.  Don’t forget any light bulbs that run on 120-volt AC power, for example if you have added a desk-style lamp. 
  3. Your truck is equipped with a 3000-watt inverter. 

Then determine which appliances you want to run at the same time:

  1. Will your computer be running while your TV is on? 
  2. Will you be brewing a cup of coffee while microwaving a breakfast burrito?  If your refrigerator/freezer is hardwired into the truck then it wouldn’t hurt to assume this unit is always running. 

On the back or bottom of all 120-volt AC electrical appliances you will find a tag that tells you the electrical consumption of that appliance.  It will state the designed voltage, designed hertz and how many watts the appliance will use when operating.  A watt (W) is defined as the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).Any truck that runs 120-volt appliances must have an inverter that changes 12-volt DC power from your batteries into 120-volt AC power for your appliances.  The inverter is rated in watts and will not deliver more wattage than it’s designed to deliver.  If you try to pull more watts than the inverter is designed to deliver, some appliances won’t work until there is enough capacity in the inverter to power those appliances, or if partial capacity is available in the inverter your appliance may run but without enough “power” to do the intended work. 

You may think your appliance is broken or defective when it may just be a case of low power, in which case your steaming cup of coffee may end up being lukewarm and look like weak tea.  Knowing the wattage that each appliance consumes will help you to not overload your inverter and you will be able to determine how many appliances the inverter will power simultaneously.

 

 
  • W=Watts
  • V=Volts
  •  A=Amps
  • Watts (W) = Volts (V) x Amps (A)
  • Amps (A) = Watts (W) / Volts(V)
Here are a couple formulas to help when determining the electrical load used in your truck. Here is a flashback of the dreaded story problems from school. Your coffee pot is rated at 1500 watts (from the tag on the bottom) and is connected to 120 volts through your inverter.  How much current will it draw?  Since current is measured in amps we need the second equation:  A = W/V.  A = 1500W / 120V which equals 12.5A.  Therefore running your coffee pot will pull 12.5 amps from your batteries. 

 
From the definition section of my first article you will find Amp-Hour (AH).  This is the unit of measure for a battery’s electrical storage capacity, which is obtained by multiplying the current in amps by the time in hours of discharge.  For example, a battery delivering 10 amps for 30 hours = 10 amps = 300 amp-hours (AH).
 
If the battery powering your inverter is a 300 amp-hour battery and we divide our coffee pot amps into the amp-hours we get the following answer:  300 amp-hour / 12.5 amps = 24 hours.  Theoretically if your battery is fully charged and you have a zero resistance electrical system (no electrical losses which is impossible) your battery will run your coffee pot for 24 hours.  In reality it will run your coffee pot for a shorter time.  If you follow this example and add together all the electrical appliances you use each night you will be able to get a fairly good approximation of the demand you are placing on your batteries which should help you in deciding which batteries to choose.  Having this information should also give you an idea how long your batteries will last during your 36-hour re-start without having to run the engine to recharge the batteries.

If you are using 24-volt DC appliances like those found at most truck stop travel stores, I’ve found they will list their power consumption in both amps and watts.  This is convenient since you simply need to add together the amps from the appliances you are using to determine their drain on the batteries.

As a side note, you have probably noticed that 24-volt DC appliances are much slower than 120-volt AC appliances.  The reason a 12-volt DC coffee maker takes longer to brew is due to the limitations on the 12-volt lighter plug.  Since it can only handle a limited amount of power, the manufacturer must reduce the power draw needed by the appliance through this plug.  They are limited in that the amps drawn at 12 volts can’t exceed the vehicles 12-volt plug capacity.  Hooking your 12-volt appliance directly to the battery will not make it work better because the appliance has been designed to draw a limited amount of power.

Simply put, if you want a faster cup of coffee you need an appliance designed to draw more power.

 

Comments (8)

Craig McCue

Business owner and part-time operator of a seasonal business.

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David,
That is a good question because solar panels are becoming more economical and if you can find a good place on your truck to mount a panel they provide a good way to charge your batteries when the truck isn't running such as your 34 hour restart. There are several advantages to solar panels such as they weigh far less than APU's, they are maintenance free, they are pollution free so they are very "green" and completely quiet. Also after the initial cost of the panel and installation, they are completely free to operate!

There are many variables that come into play when trying to decide how large a solar panel you require. Probably the largest variable is how much sun do you get each day. It is worth nothing that solar panels will also charge during overcast days but at a slower rate. Other variables include your electrical consumption per day and the efficiency of your system, for example small gauge wires (higher numbers) will have more resistance to electrical flow and are less efficient than larger gauge wires (lower numbers).

Solar panels are rated in watts and one of the equations above states Watts (W) = Volts (V) x Amps (A). Assuming you know the wattage of each appliance then you need to multiply that by the number of hours per day the appliance is running and that number may be a fraction of an hour such as .35 hours. This will give you the total watt hours per day for this appliance. For example a 150 watt appliance running .35 hours give you 52.5 watt hours. Do this for each electrical appliance in your truck that you run for the day and add the results together to get the total watt hours per day of electrical consumption.

Lets assume you obtain a total watt hour consumption of 925 for the day. Now assume your solar panel is a 200 watt panel. If we divide 925/200 we get 4.625 hours. This means that your solar panel will fully replace your electrical consumption for that day with 4.625 hours of sunlight. This number can vary depending on the angle of the sun in the sky, cloud cover and efficiency of your system.

The efficiency and cloud cover are self explanatory but the angle of the sun is constantly changing all day long due to the earths rotation and with the tilt of the earth on it's axis which gives us summer and winter. If your solar panel is mounted flat, which is likely on a truck to keep your overhead clearance low, and the sun is low in the sky such as in the morning or evening or during winter, your solar panel will receive fewer solar rays and will charge slower than it would at mid day in the summer when the sun is directly overhead.

I hope I've answered your question with going out on too much of a tangent. Also I've attached a good website to help with sizing your solar panel.
http://www.bdbatteries.com/panelcalculator.php

March 21, 2014 15:01:08 PM

What happens to the equation when you add the charging capabilities of a mid size solar panel? Just asking,great information,great article!

March 21, 2014 9:12:34 AM

very good article, I run inverter and apu this is help full.

March 21, 2014 8:03:32 AM

Oh no, not more word problems...lol. I definitely learned something new about electrical load, even though I had to take on a math problem!

February 09, 2014 9:11:34 AM

Excellent point William!! Thank you. Nothing in this world is free and no one has invented a machine that operates with zero consumption.

February 08, 2014 15:02:38 PM

There is not an inverter on the market that operates for free. Do not forget to figure it's electrical consumption into the total power usage.

February 07, 2014 22:38:30 PM

You are so right Linda. It is always good to know what demands we are placing on our electrical and mechanical components so as not to over work them. For those drivers lucky enough to have a generator or APU they typically provide enough power to run most anything you choose and charge batteries if properly connected. The more we know and learn, the better off we will be.

February 06, 2014 11:17:26 AM

Nice Article Craig - Even if running a generator it is good to know how much electricity we use.

February 06, 2014 6:12:49 AM