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Guide to Inverters. By Willie Bromehead

Category: General Advice | Author: |  Date posted:  | Impressions: 63582   6890

Inverters by Willie Bromehead

Just think of this! You’re on the road with the entire family; the drive is Inverters by Willie Bromeheadboring and the kids are starting to drive you nuts. Now just imagine for a minute: The kid’s can play their favourite video games or DVD’s while you listen to nonstop music from your radio at the same time. All of this can happen because of a small device called an Inverter!

Please do not think that Inverters are a cheap source of 220 volt power. There are literally hundreds of Inverters available on the market, ranging from the ''cheap'' imported versions, and there are many of them, to purchasing a decent, high quality product. To coin the popular phrase "you only get what you pay for" could not be truer when it comes to Inverters.

It is interesting to note that Inverters are mainly used by people who want to get away from it all.  They use Inverters to convert 12volt DC power to AC mains power for various appliances. Remember one very important rule-just choose the right one. The advantage of having such an energy supplying appliance is that it offers a mains power source in car, caravan, and boat … anywhere there’s a 12 volt battery to run your 220 volt appliance.

An Inverter changes DC power from a battery into conventional AC power that you can use to operate all kinds of devices ... electric lights, kitchen appliances, microwaves, power tools, TVs, radios, DVD’s, computers, and so on. You just connect the Inverter to a battery, and plug your AC devices into the Inverter ... and you've got portable power ... whenever and wherever you need it.

Inverter sizes are specified in WATTS, and range from 50 watts up to 11,000 or more, with the most popular in the 500 to 4000 watt range. Most of the smaller Inverters run off 12 volts DC, but all of the larger ones are designed to run off 24 or 48 volts.
Before you rush out to purchase an Inverter for your needs, we recommend you read this information, as this will help you understand why certain Inverters cost more and others less. This information generally applies to all makes and types of Inverters.

1. The size of the Inverter you require depends on the power in watts (or current in amps) of the appliance or equipment you want to run. You first need to determine the power consumption (WATTS) of the equipment you want to run by referring to the specification plate on the appliance or tool. Alternatively, you can find this information in the appliance manual. You will need to know both the continuous rating in watts or amps of the appliance/s, as well as the peak/surge rating in watts or amps combined (If you intend to run multiple appliances simultaneously). Without this information you will not know which Inverter you will require to install.

2. Inverters are rated in continuous power and peak/surge power that they can supply. Continuous power is the total WATTS the Inverter can support indefinitely while peak or surge power is the amount of power that the Inverter can provide for a brief period, usually when the equipment or the appliance/s starts up. A lot of appliances and equipment may have a start up peak or surge that is far greater than the continuous rating specified. 

3. To CONVERT AMPS TO WATTS you need to multiply the equipment or appliance AMPSx220 AC voltage to obtain the approximate WATTS that are required.

a. The conversion of Amps to Watts at fixed voltage is governed by the equation Watts=AmpsxVolts. For example 1 ampx220 volts=220 watts.

b. This means that: if you want to run an appliance with a continuous load of 5 Amps and a peak load of 15 Amps your calculation is as follows:

220V ACx5A=1100 Watts continuous power. And
220V ACx15A=3300 Watts peak/surge.

c. You would need an Inverter with a continuous rating of approximately 1100 watts and with a peak/surge rating of approximately 3300 watts. It is always advisable to build in a safety factor by overrating the continuous rating by between 20-25%.

d. See end of article for an example of how to calculate your needs.

4. Now you have to decide as to whether you want to go the modified sine wave or pure sine wave route. Inverters have either modified sine wave or pure sine wave output. The PURE SINE WAVE INVERTER is the best output waveform you can get out of an Inverter and all appliances are able to run off it without interference or overheating. Some of its advantages are as follows:

a. Pure or True Sine Wave Inverters provide electrical power similar to the output of your wall plug, (ESKOM power) which is highly reliable and does not produce the electrical noise interference, which may result from use of a modified sine wave unit.

b. Appliances such as microwave ovens and motors run correctly, quieter and cooler.

c. Pure sine wave reduces the audible electrical noise in fans, fluorescent lights, audio amplifiers, TV, DVD’s, Game consoles, Fax, and answering machines.

d. It is recommended not to connect a PC with an Inverter. Use your UPS with the Inverter! An Inverter will not regulate the AC Power.

Photo taken from Waeco website
Photo taken from Waeco website.

5. The MODIFIED SINEWAVE INVERTER has greater limitations as opposed to Pure Sine wave. Here are some of the appliances that may experience problems when running off Modified Sine wave Inverters:

a. The main drawback with Modified sine wave Inverters is that some appliances with timing devices, light dimmers, and some battery chargers, as well as variable speed devices may not work well, or may not work at all.

b. Modified Sine wave Inverters normally causes interference in television sets and digital clocks and radios.

c. Laser printers, computer hard drives and photocopiers.

d. Fluorescent lighting.

e. Modified sine wave Inverters are usually only protected by standard fuses and they are far more likely to failure and to give trouble.

If at all possible, and budget allows, it is best to use a pure sine wave Inverter because a pure sine wave Inverter can basically run any or most type of equipment as opposed to a modified sine wave Inverter.

Most of the smaller Inverters with an output of below 150 watts are available with a cigarette lighter adapter so that you can plug it into your car's lighter socket. Suitable to run DVD players, radios and if a good quality, even your laptop.

The DVD player in my tow vehicle. (Connected to Inverter.)
The DVD player in my tow vehicle. (Connected to Inverter.)

Inverters 300W and higher, are supplied with DC connection cables that must be connected direct to a battery.(Not through the cigarette lighter!)

Now comes the interesting part; what size and type of battery should one use?

Most vehicle batteries will provide an ample power supply for 30 to 60 minutes when the engine is off. Remember, if you use the Inverter while the engine is off, you will need to start and run the engine every hour or so for about 15-20 minutes so as to recharge the battery. The Inverter will operate normally continuously while the engine is running.

I have a good quality cigarette lighter Inverter of 110 watt output, and run a DVD player to keep the grandchildren occupied whilst on the road. The DVD player only draws 15 watt, and the 110 watt Inverter is more than adequate. The Inverter only cost about R400.00 and I have been using it for about a year now.

Photo of Inverter I use for a DVD player in my tow vehicle
Photo of Inverter I use for a DVD player in my tow vehicle

When it comes to larger Inverters, 300watt or more, it is recommend you use a deep cycle battery that will give you hundreds of charge and discharge cycles. You would then connect the Inverter to the battery to run the appliance/s. Please note: Standard motor vehicle batteries are not designed to be continuously charged and discharged as a deep cycle battery can.

If you are going to use an Inverter for extended periods of time in your motor vehicle, it makes good sense to have an extra deep cycle battery for the Inverter, and cabled to the starting battery. In other words, you should install a dual battery system in your vehicle.

Your Vehicles starting battery should not be discharged below 90% of its charged state, and deep cycle batteries should not be discharged below 50% of their charged state. If you exceed this recommended discharge state of the respective batteries, you will shorten the life expectancy of the battery/s.

Deep cycle battery with inverter connected to dual charging system
Deep cycle battery with inverter connected to dual charging system

What Capacity of Inverter is going to suite your needs?

Once you are sure that you are looking for an Inverter, it's important to calculate the capacity of the Inverter that will suit your requirement. The following table will give you a reasonable idea about the power consumed by various equipments.
Florescent Tube light-60W
Calculating your power need.

Now I'll explain to you how to use above table to calculate the right capacity Inverter for your requirement. Let's say you need an Inverter to run 2 Tube (florescent) lights, 3 Fans, 1 TV and 1 Laptop computer. As per the table above, you'll need a total power of 550 WATTS (2x60 + 3x70 + 1x120 + 1x100) to support all of your equipments at the same time.

Your Total Power Need will be 550 WATTS to run the appliances given in the above example.
Now that you know how much power is required, it's easy to get the VA rating of the inverter. In ideal conditions, the VA rating would have been the same as the power need i.e. 550VA. Now add about 20 – 25% safety factor.

You will therefore need an Inverter with a 650 – 700 watt output capacity.
Calculating the battery capacity.

Up until now, you have had determined the Inverter capacity required for your need. But the next question is that how much back up will the inverter provide and for how many hours you can run all of your equipments. And that's where battery capacity comes into the picture. It's the battery which determines the amount of hours you can run the equipment. The higher the capacity battery you have, the more hours running time you'll get.

Required power (in Wattage)xrequired running time hours (in Hours)=Inverter Voltage (in Voltage)xBattery Capacity (in Ampere Hours.)

Let's go for, as an example, for an Inverter which can give us back-up running time of up to 3 hours or so for our requirement:

650 Wx3 hours=12VxBattery Capacity
The battery Capacity=(650x3) divided by 12=160 Ampere Hours
So we need a battery of 160 Amp Hours capacity. Based on this calculation, that should be more or less sufficient for your requirement.

In conclusion.

What you must remember is: You need to work out first what your requirements are and what appliances you will be running. Then which Inverter is going to suite your needs, whether the Pure Sine Wave or Modified Sine Wave Inverter you that want to use, and then your budget!
I hope this article helps you in some way in making the right decisions.

Willie at Mountain Zebra: March 2012.
Willie at Mountain Zebra: March 2012

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