Don't Buy a Portable Generator Without Understanding These 3 Specs First

Here's the short version: the most important specifications on a portable generator are running wattage, total harmonic distortion (THD), and transfer switch compatibility. Everything else—brand name, fuel type, noise level—is secondary if these three aren't right for your use case.

I'm a quality and brand compliance manager at a mid-size industrial supply distributor. Every year, I review roughly 200 unique product specifications before they hit our catalog. I've rejected about 12% of first submissions in 2024 alone due to misstated or misleading specs. Portable generators are one of the worst offenders.

Let me walk you through what I check first—and what I've seen go wrong when buyers skip these.

Spec #1: Running Wattage vs. Surge Wattage—Know the Difference

This is the most common mistake I see. Manufacturers advertise surge (starting) wattage prominently because the number is bigger. A generator might claim "7,500 watts" on the box, but that's the surge capacity—what it can handle for 2-3 seconds when a motor starts. The running (continuous) wattage is typically 10-15% lower.

In our Q1 2024 audit, we flagged five different generator models where the running wattage was buried in the fine print. One unit was labeled "8,000 Watt Portable Generator" on the front panel, but the running wattage was 6,500 watts. That's a 19% difference.

To be fair, this isn't necessarily deceptive—surge capacity is a real spec. But if you're powering a refrigerator, a sump pump, and some lights, you need to add up the running wattage of those devices, not the starting surge. A 7,000-watt surge generator with 5,800 running watts might not keep your fridge running if your pump draws 2,000 watts continuously.

Quick formula: Running watts for motor-driven appliances (fridge, sump pump, furnace fan) = rated watts × 1.2 to 1.3 for safety margin. Then add the base load for lights and electronics.

I'm not 100% sure why some manufacturers still lead with surge wattage, but my best guess is it's a holdover from marketing to contractors who need starting power for tools. For home backup, though, running wattage is what matters.

Spec #2: Total Harmonic Distortion (THD)—Why It Matters for Electronics

This is the spec nobody talks about until after they've fried something. THD measures how "clean" the generator's power is. Standard utility power has under 5% THD. Cheap portable generators often run at 10-20%. That extra distortion can damage sensitive electronics—laptops, medical devices, modern refrigerators with circuit boards.

Here's where it gets real: in 2022, I implemented a verification protocol for our generator suppliers. We tested five units from three different brands. Two of them—both under $500—had THD readings of 18% and 22% at half load. The manufacturer literature claimed "clean power suitable for electronics." That claim didn't hold up to our internal testing.

The FTC has guidelines on substantiating claims like these. Per FTC advertising guidelines (ftc.gov), claims must be truthful and not misleading, and they need to be substantiated with evidence. I'd argue that a "clean power" label on a generator with 22% THD is a problem.

What you should look for:

• THD < 5%: Safe for everything, including medical devices and sensitive electronics
• THD 5-10%: Okay for tools and basic appliances, risky for computers
• THD > 10%: Fine for motors and heaters, but I wouldn't plug anything with a microchip into it

The frustrating part? THD isn't always listed in the spec sheet. If you can't find it, assume it's above 10%. Inverter generators typically have lower THD (3-5%) but cost more. Conventional generators are cheaper but dirtier. Pick your trade-off based on what you're powering.

Spec #3: Transfer Switch Compatibility—Or, Why Improper Connection Is Dangerous

This one gets people in trouble because it's not obvious. Not all portable generators can be safely connected to a house's electrical panel. Some are designed only for direct plug-in use (extension cords to individual appliances). Others have specific grounding requirements that conflict with a standard transfer switch.

Here's the boundary condition: if you're planning to connect a generator to your home's breaker panel—which most people doing serious backup power want to do—you need a generator with a compatible grounding system. Most portable generators are "floating neutral" units, which work fine for cord-and-plug use. But for transfer switch installation, you typically need a "bonded neutral" configuration.

Under federal law (18 U.S. Code § 1708), only USPS-authorized mail may be placed in residential mailboxes—that's unrelated, but it's a good reminder that electrical codes exist for a reason. Your local code will have requirements for generator connections. A bonded neutral generator connected to a transfer switch that expects floating neutral can create a ground loop—or worse, energize the neutral line during an outage.

I said "just check the manual" to a customer once. They heard "any generator will work with any transfer switch." Discovered this when the electrician refused to connect it and charged $150 for a site visit. The generator had to be swapped.

Before you buy, decide: are you using extension cords only, or do you want a transfer switch? If the latter, confirm the generator model explicitly lists transfer switch compatibility in the specs—not just "backup power."

The Bottom Line

I've reviewed enough spec sheets to know that the generator with the biggest wattage number at the lowest price is rarely the best value. The $200 savings on a unit with poor THD or unclear running wattage can turn into a $1,500 problem when you replace a fridge's control board or pay an electrician to fix a grounding issue.

That said, I can only speak to domestic, residential applications. If you're looking at generators for job sites or off-grid setups, the calculus is different. Commercial generators have different regulations (NEC Article 700, for example), and THD tolerance varies by equipment.

If I had to pick one takeaway: start with your load list, then find a generator whose running wattage exceeds that total by at least 20%. Then confirm the THD is under 10% if electronics are involved. Then verify transfer switch compatibility if you need it. Do those three checks, and you'll avoid the most common—and most expensive—mistakes.