Total Harmonic Distortion (THD) is the stated measurement related to the quality of electricity. The amount of THD will influence how your equipment may respond or perform. Get the technical low down at wikipedia.org: Total harmonic distortion, but it’s going to make your head hurt.

When power quality is discussed, you will hear terms like “clean electricity” and sine wave power. What these refer to is the lower harmonic distortion of the electrical AC wave and the ability to take square wave generated electrical power and smooth the edges to produce a phase that is smoother and less square.

“Clean electricity” is considered good with a THD of less than 6% and often stated or promoted at 5% or less.

For units that measure greater than 6% THD the electrical wave can cause sensitive electrical circuits such as computers or other minor electronics, to lose life. These components may malfunction or operate improperly.

It is common for standard generators to be in the 9% range and some to produce THD as much as 15%.

At over 5% THD you can run heating elements and the vast majority of non-sensitive appliances. The issue you run into is that TV’s may see lines through the picture or noise in the speaker of your radio. Other controllers may not operate properly. You may even get flickering in your lights although most lights can easily handle this amount of distortion.

Generally speaking you can run necessities such as appliances or tools without issue. But if you need to rely on more sensitive equipment you’ll be looking to more expensive standby type generators or inverter power generators.

What influences THD:

Factors that influence THD are primarily in the alternator construction and excitation as well as the controlling electronics for frequency and voltage.

The more you have in place for the details for any of these factors the better (lower) the THD numbers, but the greater the cost.

Although there are many detailed explanations for these influences like number of poles and wave generation, the normal consumer can consider a few basic considerations for their own individual needs.

Cost Factors:

It’s simple to say if you are getting a high-quality alternator that it will be paired to high quality engine and controls. It will cost more and sometimes significantly more, double or triple in cost, due to the upgrades across the entire unit.

Most non-commercial units are built to meet a target price. Because they are used infrequently, they are built with the least amount of material or minimum construction standard to keep costs down. Thus the lower quality and typically higher distortion.

If you have plans for more continued use, you should plan on going for higher quality generators.

What to Consider:

If you have lots of processors and LCD panels in you appliances you’re going to want to look for high quality power output.  Generally if you’re appliances are not all fancy you can get away with higher THD values which are typical of the vast majority of non-inverter generation.

Note that with less expensive units it is not uncommon for voltage and frequency control to suffer as you approach maximum output of the generators. The simply are not designed with the extra care that the high quality units are. So not only will you have a higher THD you may lose voltage and suffer from frequency (60 hz maybe come 53 hz.) as the generator engine can’t maintain full rpm at high load.

Following some general rules based on the above considerations can lead to a more pleasant user experience:

• Low quality power is acceptable for intermittent use for power tools and most appliances. THD less than 10%
• Higher quality power is required as your requirements for more need go up. THD 6% to 9%
• Highest quality “clean power” is needed for your more sensitive electronics such as LCD screens, TV’s, and other microprocessor appliances. (THD <5%, but shooting for a 3%)

Summary THD Issues:

Since most manufactures don’t publish this data unless it becomes a selling point, you can expect the 9% to 10% range to be standard. Just note the lower the cost the more likely the THD is to increase. Standard units are going to be fine for your basic backup needs for appliances to keep you in light, the food cold, and basic small appliances. Clocks and other components may not function as well unless your using higher quality power from inverter systems or very well-built standby generators.