What Is Startup Surge? Why Your Power Station Needs Extra Watts

You plug your fridge into a portable power station. It flickers on for half a second, then the power station shuts down. The capacity is plenty. The wattage rating looks fine. So what happened?

Startup surge happened.

What is startup surge?

Startup surge (also called inrush current) is the brief spike of power a device needs when it first turns on. It lasts less than a second, but during that moment, the device pulls far more watts than its normal running power.

Think of it like pushing a heavy cart. Getting it moving takes a big shove. Once it’s rolling, you only need a light push to keep it going. The big shove is the surge. The light push is the running wattage.

Which devices have startup surge?

Anything with a motor or compressor has significant startup surge. The spike is typically 2 to 5 times the running wattage.

Common examples:

Refrigerator: runs at 100-250W, surges to 800-1,200W
Freezer: runs at 80-200W, surges to 600-1,000W
Air conditioner (window unit): runs at 500-1,500W, surges to 1,500-4,500W
Sump pump: runs at 300-800W, surges to 1,000-2,400W
Power tools (drill, saw): varies widely, surge is typically 2-3x running watts

Devices without motors generally have little to no startup surge:

Laptops, phones, and chargers — no surge
LED lights — no surge
Routers and modems — no surge
CPAP machines — no meaningful surge
TVs and monitors — negligible surge

Why does this matter for backup power?

Every UPS and portable power station has two power limits:

Continuous watts — how much it can deliver steadily
Surge watts (or peak watts) — how much it can handle for a brief spike

If your device’s startup surge exceeds the power station’s surge rating, the unit will shut down to protect itself. It doesn’t matter if the battery has plenty of capacity. The instantaneous demand is too high.

This is the most common reason a “big enough” power station fails to run a fridge. The battery has the energy. The continuous watts are sufficient. But the surge rating is too low.

How to check surge ratings

Power station manufacturers usually list two numbers:

Rated output: 1,800W (this is the continuous limit)
Surge/peak output: 3,600W (this is the brief spike limit)

For a UPS, look for the VA rating and check the spec sheet for surge capacity. Not all UPS units handle high surge well — most are designed for electronics, not motors.

How to handle surge when sizing

There are a few approaches:

Option 1: Choose a unit with a high enough surge rating. This is the simplest. If your fridge surges to 1,200W, make sure your power station can handle at least 1,200W peak. Most quality units handle 2x their continuous rating.

Option 2: Avoid plugging in surge-heavy devices at the same time. If you have a fridge and a freezer, plug in one first, let it start, then plug in the other. Staggering startup reduces the combined surge.

Option 3: Use a soft-start device. Some aftermarket devices gradually ramp up power to a compressor, reducing the surge spike. These are common for RV air conditioners but less practical for most home use.

A common mistake

People add up all their running wattages, find a power station that exceeds the total, and assume they’re covered. They forget about surge entirely.

Here’s what that looks like:

Fridge (150W) + router (15W) + lamp (10W) = 175W running
A 300W power station looks like plenty
But the fridge surges to 1,000W at startup
The 300W station can’t handle it and shuts down

The fix: always check the surge rating, not just the continuous rating.

How our calculator handles surge

When you add a device with startup surge (like a fridge) in our calculator, it automatically enables the surge buffer. This adds a 30% margin to your capacity target to account for the extra energy demand during startup spikes.

For devices without surge (laptops, routers, phones), no buffer is added — you don’t pay for headroom you don’t need.

Use our calculator to get a safe capacity target that accounts for surge automatically. Just pick your devices and we’ll tell you what you need.