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How to determine your power requirements.


Here's some help for figuring out what you need in the form of generator/batteries/inverter, and to determine how long you have to run the generator to charge the batteries each day. I'll try to keep it simple.

Note: I am not an expert -- I have only learned this myself in the last few weeks. I'll be using 6 volt batteries (deep cycle golf cart) in my example.

A few definitions:

Volts: The amount of "force" behind an electrical flow.

Ampere or Amp: The amount of electrical flow.

Watt: The rate, or speed of an electrical flow.

A few laws:

Watts = volts times amps. If you have a 5 amp motor powered by 120 volts, the electricity will run through it at a rate of 600 watts.

Amps = watts divided by volts. A 60 watt, 120 volt light bulb will require 1/2 amp of flow.

Volts = watts divided by amps (this is not used much).

A few more definitions: Amp-hour = number of amps used in one hour. A 5 amp motor running 2 hours uses 10 amp-hours.

Watt-hour = number of watts "used" in one hour. A 60 watt light burning 2 hours "uses" 120 watthours.

Note that watt-hour changes rate of flow (watt) to amount of flow.

A fact: Batteries are rated in amp-hours, a quantity of "juice."

Another law: 6 (or 12) volt am-phours must be converted to 120 volt amp-hours. If you have a 200 amp-hour 12 volt battery, you will get 20 amp-hours at 120 volts (AC, DC, doesn't matter) because watts (rate of flow) and watt-hours stay the same. 200 amp-hours X 12 volts = 2400 watt-hours; 2400 watt-hours divided by 120 volts = 20 amp-hours

Another fact: A battery charger is rated in amps.

Now you're ready to figure it out.

You have a small refrigerator that says 2.5 amps on the sticker, and a light that says 100 watts. The reefer has to run 5 hours a day and the light is needed 5 hours a day. Let's say you also have a washing machine that uses 4 amps and you are going to use it once a week for 1 hour.

Convert all this to amp-hours.

Reefer = 2.5 amps X 5 hrs = 12.5 amp-hrs

Light = 100 watts/120 volts = .833 amps X 5 hrs = 4.2 amp-hrs

Washer = 4 amps x 1 hour = 4 amp hrs

You require a maximum of 20.7 amp-hrs per day. If you want to cut that back, you could run the washer during the day when you don't need the lights, so let's say your requirement is 16 amp-hrs.

Now determine your battery requirements. Convert 16 amp-hrs/120 volts AC to 160 amp-hrs/12 volts DC. Deep cycle batteries can usually be drained 80%, but they last longer if drained 50%, so let's use that for a guide.

Your battery capacity (50%) should equal your daily electrical requirement, 160 amp-hrs, so you need a battery, or set of batteries, with a capacity of 320 amp-hrs.

I'm going to use a set of batteries, or bank, to show you how to connect them.

Batteries can be connected in series or parallel. When connected in series, the battery voltage is multiplied by the number of batteries to determine the total output voltage. The batteries are connected positive to negative, like a flashlight. In this arrangement, the total capacity in amp-hours is that of the weakest battery, so if you have a 100 amp-hr 6 volt battery and an 200 amp-hr 6 volt battery connected in series, you will have a 12 volt, 100 amp-hr battery. When the weaker battery drains, the set is useless. (Try using one bad battery and one good one in a flashlight.)

When connected in parallel, they are connected positive to positive and negative to negative. In this case, the total output voltage, if both batteries are the same voltage, is the same as the voltage of one battery. The capacity is now the total of the capacity of each battery. So, the two batteries in the example above will now provide 300 amp-hrs at 6 volts.

You can connect batteries in a combination of series and parallel.

Let's use four 100 amp-hr 6 volt batteries. Connect two batteries in parallel. You now have one "battery" that is 6 volts, 200 amp-hr. Connect the other two batteries in parallel. Now connect the two "batteries" in series. You now have one "battery," or battery bank, that is 12 volts and 200 amp-hr.

We needed 320 amp-hrs for our example, so these batteries won't be enough. We will need four 6 volt, 160 amp-hr batteries, or two 12 volt, 160 amp-hr batteries. Let's go with four 6 volt batteries.

Trojan T105 batteries are rated at 218 amp-hrs. They cost about $70 each. This will give us plenty of extra power, in case someone leaves the reefer door open!

Next, we need an inverter. An inverter changes DC voltage to AC voltage. The input can be 12, 24, or 48 volts. 12 volt inverters are commonly used for smaller, single home applications. They are much cheaper too.

The two main types are sine wave (more expensive, a bit more inefficient), and modified sine wave.

A sine wave is not much more than a graphic representation of what AC looks like. Alternating current is actually changing direction, or polarity, 60 times or cycles per second. Looks like this (-). A sine wave inverter gives the cleanest AC power. Some claim to be better than what you get from the grid.

Modified sine wave inverters "chop off" the peaks, so the wave looks squared off on the tops and bottoms. Certain electrical devices have problems running on the resulting AC. Fluorescent lights will buzz, motors have a little trouble getting started, some sensitive electronic devices, like laser printers, will not operate, televisions get lines in the picture, etc. Computers will run OK.

Many inverters have a battery charger built in, and will monitor the battery level of charge. The good ones will turn off if the battery gets low and stop drawing from it.


When you shop for an inverter you will see what its charging rate is. The good ones will do a three-stage charge. The voltage or amperage changes at each stage to adjust to the needs of the batteries. The charging rate given is a peak rate, so if it claims to charge at 100 amps, you have to use a figure somewhat less than that. I use a value that is about 20% less.

So let's buy a sinewave inverter that is 90% efficient and has a 100 amp charger. We'll be using 160 amp-hrs per day. Due to the efficiency of the inverter, let's call that 178 amp-hrs per day.

If we say that the charger is an 80 amp charger, then the charging time will be 2.2 hours per day.


I assume that many of you are considering a generator, so I'll just briefly describe the hookup and figure out the fuel requirement for the generator.

The generator will have a 120 VAC outlet. The inverter will have three connections: one to the generator's 120V receptacle, one to the batteries, and one to the house. The connections to the generator should have a circuit breaker to protect the inverter from getting too much current (a malfunction of the generator), and the connection to the batteries should have a fuse to protect the batteries from getting too much current from the inverter. The connection to the house is made safe with a transfer switch (about $200). This looks like a circuit breaker box and wires in (easily) to your main breaker box (or fuse box in an older home). It has a receptacle where you plug in the generator (if you're not using batteries/inverter), and a switch for each circuit that you use to manually switch from generator power to grid power. This is what keeps the power from the generator from feeding into the grid, and keeps the lineman from getting fried.

When you start the generator running, the inverter decides if the batteries need charging, and takes care of that. Then your reefer wants to run. The inverter senses this and delivers the juice to it while maybe easing up on the charging a bit, or it may just pull more current from the generator, which makes it run harder. When the batteries are charged, the inverter stops charging them and just sends the juice from the generator straight to the house.

When you shut off the generator, the inverter takes the juice from the batteries. In the house, you won't even know when this is happening. The switching that the inverter is doing is done very smoothly.

Fuel requirements

How much gas or diesel do you need? If there is no load from the house when the batteries are charging, the generator will probably be running at half load. How do you know this?

The charging amperage is 80 amps. This is at 12 volts DC. Convert that to 120 volts AC and 8 amps is what the generator will be putting out. Look at the generator manual. It says it puts out 15 amps at 120 volts AC. So if the inverter is using 8 amps, then that is about half load. (A 4000 watt generator usually puts out about 35 amps).

If you're going to buy a generator be sure it will only need about half load to charge the batteries.

The manual says that the generator uses 1/2 gallon per hour at half load (it will use more at full load, of course), so if you have to run it 2.2 hours a day, you'll need about 1 gallon of fuel per day, and that's not bad!

Of course, you can do without the batteries and inverter, but you will be using the generator a lot less efficiently this way. You'll be running it whenever you need power in the house, sometimes at full load, sometimes not, and it becomes more unpredictable as to how much fuel you'll need.

One trick I will use is to plug my chest freezer into the generator directly. It has two 120V receptacles, so why not? This way, I can power the freezer for 2 hours a day while I'm charging the batteries. I can figure out how this affects the load on the generator, because I know how many amps the freezer pulls. By experimenting ahead of time, I'll know how long it will run each day. Then I can factor that in to my fuel requirement.

As a rule, the batteries are the key here. You want to get the best you can afford. Golf cart batteries are available at places like Battery Exchange. You should ask if they have blemished batteries. Those will be a little cheaper and are just as good.

Inverters are rated by how many watts they can deliver: 500, 1000, 2500, 3600, etc. Note that this is not peak wattage. A good 2500 watt inverter can usually deliver around 4000 watts for a short period of time, 10 minutes or so. This allows for motor starting, which uses more current -- up to twice as much, and up to seven times as much for air compressors, pumps, and air conditioners.

To determine what size inverter you need, you need to think of the most electricity you will need at one time. Lights on, reefer running, computer, washing machine.

If the inverter you need is beyond your budget, then you need to establish rules, like not running the washer when the reefer is running, etc.

Visit to get a higher education.

If you're in the area, consider attending the alternative energy workshop to be held at the Countryside office April 17-18. Learn from the man who installed Countryside's solar and wind system! See page 131.
COPYRIGHT 1999 Countryside Publications Ltd.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1999 Gale, Cengage Learning. All rights reserved.

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Title Annotation:planning to use a generator
Author:Lamb, Colley
Publication:Countryside & Small Stock Journal
Date:Mar 1, 1999
Previous Article:Using mechanical movements.
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