What is an inverter?

When camping in more remote locations, you have to rely on the RVs 12VDC power system. 12VDC is typically used to power lighting, heater fans, water pump, and alarm systems. The source of this power is the house battery bank. The house battery bank is charged with a generator, a solar system, or the vehicle charging system. Other devices we are most familar with use the common 120 Volts AC used in households. Microwaves, televisions, DVD players, laptop battery chargers, cannot run on the RV 12VDC system. An inverter solves this problem by inverting the 12 Volts DC to 120 Volts AC.

What is AC and DC?

DC is direct current. DC is a constant voltage remaining at a steady state. Batterues and solar cells put out a constant DC voltage. 12 VDC systems are very common because almost all automobiles use 12VDC for starting, lighting, stereos, etc. Batteries can only store and output DC. 12 V batteries. lights, pumps, and stereos are common and relativily cheap.

AC is alternating current. This means that the power or current flow goes from 0 to a maximum positive value then back to 0 then to a maximum negative value then back to 0 to repeat. The repition rate for household power in North America is 60 times a second, or 60 Hertz, commonly written as 60 Hz. Below is a picture if an ocilliscope trace of the power supplied by Southern California Edison. It is clean 60 Hz sine wave that has a frequency varience of 0 over time. North American utilities supply power like this just about everywhere. Good quality generators do the same.

Pure sine wave. The clipped end is a function of the photography.

A little science to confuse things and make eyelids heavy. What we call 120 Volts AC has a peak to peak voltage of 170 Volts. The 120 Volt figure we refer to is the Root Mean Square of the P-P Voltage. RMS is roughly .707 times the p-p.

What is pure sine wave and modified sine wave?

The inage above is a pure sine wave. Rotary generators produce a pure sine wave naturally but to convert DC to pure sine wave is more difficult. For many years this was accomplished with DC motor driven alternators. These devices were heavy, power hungry, and expensive. The advent of semiconductors has done away with rotary inverters. To make a pure sine wave is difficult and expensive to do with a so called chopper circuit so most low end inverters output a modified sine wave. A modified sine wave looks more like a square wave with a pause between positive and negative excursions.

Modified sine wave on the same scale as the pure sine wave above.

Problems with modified sine wave.

As you can see there is big difference between what is called MSW and PSW. In many applications this make no difference ut in some it does. For one thing micro wave ovens use the peak power so the lower peaks of the MSW power will not let a microwave heat as much. The abrubt transitions can cause noise in TVs and LASER printers. The area under the curve is the total power so some inverters will over heat battery chargers. The best advice I can deliver is to test carefully, check for overheating, expect mictowaves to not heat as well, learn to accept noisey TV pictures.

How do you size an inverter?

Most AC appliances give a power rating. It will be shown as either Watts or current. Volts times Amps will give watts and Watts divided by Voltage will give current. Current is the one you want. As a rough guide, 12 Volt current will be 10 times 120 VAC current. A typical RV microwave is around 1000 Watts or just over 8.33 Amps. This translates into over 83 Amps at 12 VDC. It gets more complex when powereing electric motors. Electric motors have high surge currents, sometimes as much as twice the rated current. SO to size an inverter, add up all the loads you plan to power with the inverter. Double that if there are surge currents. Below I will get into another part of the sizing question, wire size fropm teh battery to the inverter. Do be aware it is nearly impossible to power high current loads for long periods of time such as an RV airconditioner with an inverter. It can be done but you will need a huge battery bank.

How do you install an inverter?

Several small inverters vs. a single large inverter.

There is an ongoing argument

Inverter tests

I have three inverters for testing. As I get others I will add them to the test. The test setup was very simple, I did not have the tools to measure everything so I looked at output waveforms. output voltage, and input current. None of the inverters were loaded for these tests. Later I will test them with various loads.

Maxx 400 Watt

Maxx 225 Watt

XPower 175 Watt