Watts vs Wh vs Surge Watts

Quick answer

Watts (W) measure power — how fast a device consumes energy, or how much a power station can deliver, at any given instant. Watt-hours (Wh) measure energy — the total amount of electricity a battery has stored, which is what determines how long it can supply that power for.

Continuous (or rated) watts is the power a power station can deliver sustained, hour after hour, without overloading its inverter. Surge (or peak) watts is extra power available for a fraction of a second — enough to let certain appliances start up, but not something the power station can sustain.

A simple worked example ties watts and watt-hours together: a 100 W device running for 5 hours consumes approximately 500 Wh (100 W × 5 h) before conversion losses. The rest of this page explains where that number comes from, why "before conversion losses" matters, and how continuous and surge watts fit into sizing a power station correctly.

Watts vs. watt-hours: they are not the same unit

The easiest way to keep these straight is a simple analogy: watts are like the flow rate out of a tap (gallons per minute), while watt-hours are like the total amount of water sitting in a tank (gallons). A high flow rate doesn't tell you how much water is in the tank, and a full tank doesn't tell you how fast it can be poured out — you need both numbers to know what you can actually do with it.

W ≠ Wh. Watts describe power — an instantaneous rate. Watt-hours describe energy — an accumulated total over time (power × time). A power station's Wh rating is its "tank size"; the watts it can output, and the watts your devices draw, describe flow rates in and out of that tank. Confusing the two is exactly how people end up comparing a power station's battery size against a device's power draw as if they were on the same scale, when they measure fundamentally different things.

How to calculate runtime

Once watts and watt-hours are separated correctly, converting between them for a runtime estimate takes two short steps — the same two steps every calculator on this site runs automatically:

Usable Energy (Wh) = Battery Capacity (Wh) × Efficiency

Runtime (hours) = Usable Energy (Wh) ÷ Total Power (W)

The efficiency step matters because no inverter or battery system converts stored DC energy into usable AC power at 100% — some is lost as heat in the conversion. This site uses 85% as a typical planning figure, though real-world efficiency varies by power station model, age, and load. Worked example: 1000 Wh × 0.85 = 850 usable Wh. Divide that by a 100 W load (this site's own TV default, among others): 850 Wh ÷ 100 W = 8.5 hours. This is exactly what the Runtime Calculator computes the moment you enter your own numbers.

What are continuous / rated watts?

A power station's continuous (sometimes called "rated") watt rating is the maximum power its inverter can deliver sustained, without overheating or shutting down. This is a completely separate spec from its Wh battery capacity, and it's the one that determines whether a power station can run your devices at all — not just for how long.

This is why a large battery doesn't automatically mean a capable power station: if your devices require 700 W continuously, a power station rated for 500 W is not suitable even if it has a large Wh battery — its inverter simply can't deliver the power your devices are asking for, regardless of how much energy is sitting in the battery behind it. Always check continuous output watts as its own spec, separate from the Wh number on the box.

What are surge / peak watts?

Some appliances need a brief burst of extra power — well above their normal running wattage — just to start up. This is usually called surge, peak, or inrush power, and it typically lasts a fraction of a second as a motor or compressor spins up from a standstill.

This matters most for devices with a compressor or motor: refrigerators, air conditioners, water pumps, and some power tools are the classic examples. A power station needs a surge rating high enough to get past that initial spike, or the device may fail to start even though the power station's continuous rating would comfortably run it once it's already running.

Resistive loads — things like simple light bulbs, phone chargers, or a laptop charger — generally have little or no significant startup surge, since there's no motor or compressor spinning up. There is no single universal surge multiplier that applies to every appliance: the actual surge depends on the specific appliance, its motor/compressor design, and its condition. This site uses roughly 3× running watts as a rough planning figure for compressor-driven devices like refrigerators and air conditioners (see the refrigerator and air conditioner guides) — treat it as a starting point for planning, not a guarantee, and always check your power station's rated surge/peak watts against your specific appliance.

Watts and Wh: a quick reference table

These are theoretical energy calculations (power × time) before conversion losses — useful for building intuition about how watts and watt-hours relate, separate from the efficiency step covered above:

• 100 W for 1 hour = 100 Wh
• 100 W for 5 hours = 500 Wh
• 500 W for 2 hours = 1000 Wh
• 1000 W for 30 minutes = 500 Wh

How many watts does a power station need?

Start by adding up the running wattage of everything you'd realistically run at the same time — not everything you own, just what's actually plugged in simultaneously. Using the example preconfigured in the calculator above (a TV, a router, a laptop and some lights), the combined total is 195 W:

• TV: 100 W
• Router: 10 W
• Laptop: 65 W
• Lights: 20 W
• Total: 195 W

How much headroom to leave on continuous watts

The power station's continuous output rating needs to comfortably exceed that combined figure — not just match it exactly. A common approach elsewhere on this site is to add roughly 15–25% of headroom on top of the calculated total, which for the 195 W example above works out to a continuous rating of at least around 225–245 W. Most power stations sold today comfortably clear this for a light electronics load like this one; headroom becomes a genuinely binding constraint mainly once heavier appliances (an air conditioner, a microwave, power tools) enter the mix.

How many Wh do I need?

Required watt-hours depend on four things: total power (W), how many hours you need it covered, efficiency, and — for appliances that cycle on and off, like a refrigerator — duty cycle (see below). The formula is the size formula used across this site:

Required Capacity (Wh) = (Total Watts × Hours) ÷ Efficiency

Applying the site's 85% efficiency default to the 195 W example above, here's how long a few common power station capacities would run it — the same runtime formula from earlier, just applied across multiple sizes. This is exactly what the Size Calculator does in reverse — give it your own devices and target hours, and it recommends a capacity directly instead of you working the formula by hand:

• 500 Wh: 500 × 0.85 = 425 usable Wh → 425 ÷ 195 ≈ 2h 11m
• 1000 Wh: 1000 × 0.85 = 850 usable Wh → 850 ÷ 195 ≈ 4h 22m
• 2000 Wh: 2000 × 0.85 = 1700 usable Wh → 1700 ÷ 195 ≈ 8h 43m
• 3000 Wh: 3000 × 0.85 = 2550 usable Wh → 2550 ÷ 195 ≈ 13h 5m

Cycling appliances: why a refrigerator isn't a flat number

Not every device draws a constant number of watts the whole time it's plugged in. A refrigerator's compressor cycles on and off to maintain temperature, often running only 30–50% of the time (its duty cycle) — treating its nameplate/rated wattage as if it ran continuously would badly overstate real energy use.

This site's 150 W refrigerator figure is already an average running watts number that accounts for that cycling, not the compressor's instantaneous draw while active, and not the nameplate figure printed on some spec sheets. On top of that averaged draw, the compressor also produces the startup surge discussed above each time it kicks on. See the dedicated power station for refrigerator guide for the full breakdown, including worked runtime examples at several capacities.

Common mistakes

A handful of mix-ups account for most sizing regrets:

• Confusing W with Wh — comparing a device's power draw directly against a battery's energy capacity as if they were the same kind of number.
• Choosing a large Wh battery with insufficient output watts — a power station can have a huge battery and still be unable to run a device whose wattage exceeds its continuous output rating.
• Ignoring startup surge — assuming that if a power station's continuous rating covers a device's running watts, it can also start it, without checking the surge/peak rating separately.
• Assuming advertised battery capacity equals usable AC energy — the rated Wh figure is reduced by inverter/conversion losses (this site's 85% default) before it reaches your devices.
• Assuming a refrigerator runs continuously at its nameplate/rated wattage — compressor cycling means real average consumption is meaningfully lower than the nameplate figure, though it still needs surge headroom to start.
• Forgetting that multiple devices add together — a router, some lights and a laptop each look negligible alone, but their combined wattage and combined energy draw both matter for sizing.

Practical examples

Applying the runtime formula (capacity × 85% efficiency ÷ watts) to this site's own device figures and calculator presets, here's roughly what a 1000 Wh power station gets you across a few common scenarios — each links to a dedicated guide with a full breakdown and its own capacity table:

• TV (100 W): 850 ÷ 100 ≈ 8h 30m
• Refrigerator (150 W average running watts, cyclic load with ~3× startup surge): 850 ÷ 150 ≈ 5h 40m
• Gaming PC (350 W under active load): 850 ÷ 350 ≈ 2h 26m
• PS5 / Xbox (200 W active gameplay): 850 ÷ 200 ≈ 4h 15m
• Camping setup (LED lights, phone charging, a fan — 90 W combined): 850 ÷ 90 ≈ 9h 27m
• RV (fridge, fan and router — 210 W combined): 850 ÷ 210 ≈ 4h 3m
• Blackout essentials (fridge, router, phone chargers and lighting — also 210 W combined, coincidentally the same total as the RV example): 850 ÷ 210 ≈ 4h 3m