You’ve purchased a power inverter for emergency power outages to keep your refrigerator running and your food from spoiling, then the inevitable happens. The power goes out. So, you eagerly hook up the inverter, plug in the refrigerator and…nothing. The inverter powers on, but all you get is a fault light. How could this be? You made sure to look up the power rating of the refrigerator and purchase an inverter that should have been large enough to run it. Assuming you are using the proper gauge of battery cables and have good incoming DC voltage, there is probably nothing wrong with the inverter and no, you’re not crazy. What happened is the inverter is going into an overload fault caused by a high starting surge not listed in the refrigerator manufacturer specifications. Let’s go over a few examples of devices that have unlisted starting surges, why I feel they are not listed, and general sizing recommendations.

Examples of devices that require high starting current

• Motor driven appliances, such as refrigerators, air compressors, bench grinders, electric saws, air conditioners and water pumps. Note that additional consideration is required on these motor driven appliances as the start up duration can be as long as 10 seconds.
• Lighting such as halogen and photographic strobes.
• Laser printers are known for having a high start up surge as the fuser heats up.

Why is surge rating not listed on my appliance?

To answer this question we have to look at how household utility power works and what factors play into the purchase of a new appliance. While the utility grid and our electrical panels have limits, they have the ability to absorb a quick starting surge, within reason. This ability makes knowing the surge rating of appliances unnecessary in a household situation. Inverters, on the other hand, are sensitive to appliance surges, which makes knowing these ratings more of a necessity.

Really though, what is your concern when purchasing a new appliance? How much will it cost and how does it affect your utility bill? This is why you’ll be able to find either the continuous draw or energy rating on most appliances, but not the surge. With your tools, don’t be surprised if you don’t find either, but rather a 15 or 20 amp power requirement instead. The running cost on a tool with a short runtime is not a concern; having a tool that will do the job is the main concern. The 15 or 20 amp listing is actually the required circuit rating, which is essentially useless for determining inverter size.

What is the surge requirement of my appliance?

There are two ways to find out the actual surge rating of an appliance. The first is to physically measure it with an inrush current meter. However, most people do not have one of these devices on hand. The other me method would be to find out from the manufacturer of the appliance. Be advised though, that it is not uncommon for a manufacturer not to have start up rating, or to not fully understand the question, but an inquiry is still worth the effort. Keep in mind the key phrase with motors is locked rotor amperage or LRA, not to be confused with full load amps.

If you are unable to get the information from the manufacturer, or measure it yourself, here are some sizing guidelines. I would like to offer a disclaimer with these guidelines. Although we have a good track record with these sizing estimates, I cannot emphasize enough that it is exactly that, an estimate.

• Refrigerator: 5-7 times continuous rating
• Air compressor: 7-10 times continuous rating
• Laser printer: 6-7 times continuous rating
• Electric tools: 3-7 times continuous rating
• Photographic Strobe: 4-5 times watt sec rating
• Halogen heater: 10 times continuous rating
• Air conditioner: 6-10 times continuous rating

Depending on the surge duration of the motor, it may be required to size the inverter based on its continuous rating as inverter surge durations are typically 0.5 seconds or less.

Final Thoughts

Oversize, oversize, and oversize. I’m not saying go out and get a 10,000 watt inverter for your dorm size refrigerator, but when all else fails, err on the side of caution. Not only are you more likely to have an inverter that will operate your loads, there are other benefits to over sizing an inverter as well. While some inverters can handle loads that exceed their specifications, this is not without consequence. Under sizing an inverter is one of the largest causes of premature inverter failure. You wouldn’t drive your vehicles motor at full RPM on a regular basis would you? Over sizing improves inverter longevity, generates less component heat and maintains peak efficiency.

Doing a little homework and making an informed decision can save you a lot of frustration with your inverter purchase. Feel free to contact us at DonRowe.com if you have any questions or want to talk your application over.