Surge Protectors Explained: What UL 1449, Joules and Voltage Ratings Really Mean
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How we wrote this: A guide based on published material from NEMA's Surge Protection Institute, a U.S. Department of Defense surge protector evaluation document, and the Electrical Safety Foundation (linked below). We have not tested any surge protector. For whole-home protection, talk to a licensed electrician.
A surge protector is cheap insurance for a computer, TV or router, but the packaging is full of numbers that don't mean what they seem to. Here's how to read it.
A power strip isn't automatically a surge protector
Many multi-outlet strips are just extension cords with extra sockets. A surge protector contains components that divert voltage spikes away from your devices. Check the label for a surge rating and a safety listing, not just the number of outlets.
UL 1449: the label that matters most
UL 1449 is the safety standard for surge protective devices. A U.S. Department of Defense evaluation document (TSEWG TP-3) notes that a UL 1449 listing establishes a standardized suppression voltage rating that can be used to compare products. Look for:
- A UL 1449 listing (or equivalent from a nationally recognized testing laboratory)
- The Voltage Protection Rating (VPR): the let-through voltage measured in that standardized test. Lower means less voltage reaches your gear.
Why joules can mislead
Joule ratings are the biggest number on most boxes. NEMA's Surge Protection Institute FAQ warns that comparing joule ratings "can be misleading." It says there's no clear standard for measuring them, and that many industry standards (UL) and guidelines (IEEE) don't recommend comparing joules. They focus on tested performance such as the VPR and nominal discharge current instead.
The DoD document says something similar. It calls the joule rating "not an important performance attribute" for a UL 1449-listed device that has passed operational testing. It also advises treating advertised response time with caution: what matters is let-through voltage.
Where surges come from
ESFI's surge protection overview notes that recent editions of the National Electrical Code have extended surge protection requirements. ESFI notes that the 2020 code added surge protection requirements for residences, to protect large appliances, equipment such as HVAC systems, and smoke alarms. That kind of protection is installed by an electrician. Plug-in protectors still help at the device level, because ESFI's related material says 60 to 80% of surges originate inside buildings, from equipment switching on and off.
Buying checklist
- UL 1449 listed (look for the mark and the listing on the product)
- VPR printed on the label; lower is better
- An indicator light showing protection is still active
- Coax or Ethernet protection if you're protecting a modem or TV, since surges can travel on those lines too
- Enough spaced outlets for bulky adapters, so you don't need to chain strips together
What to look for: (Look for: UL 1449 listed surge protector with protection indicator light)
Surge protector or UPS?
A surge protector handles spikes. A UPS also keeps devices running through an outage, and most include surge protection. For a desktop computer or a VoIP phone setup, a UPS may be the better buy. See how to size a UPS.