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Watt-Hours vs Watts: 5 Power Station Formulas

Updated 2026-10-21 · Published by Charland & Co. · Based on published specs; we have not tested the products discussed.

Quick answer: Watts are how fast a device uses power; watt-hours are how much energy the battery stores. Runtime hours = battery Wh x 0.85 / load W, so a 1,024 Wh station on a 50 W load lasts about 17.4 hours (1,024 x 0.85 = 870; 870 / 50 = 17.4).

Watts measure how fast a device uses power. Watt-hours measure how much energy is stored in the battery. For a portable power station, the useful math comes down to five formulas: runtime, battery size, solar yield, surge, and cold-weather usable capacity. A 1,024 Wh station on a 50 W load lasts about 17.4 hours after efficiency losses.

Watts vs Watt-Hours in Plain Words

Watts tell you the speed of the power. A 50 W light uses energy at 50 watts while it is on. Watt-hours tell you the amount of energy available. A 1,000 Wh battery is like a tank that holds 1,000 watt-hours.

Think of a car. Watts are like miles per hour. Watt-hours are like the gallons of fuel in the tank. You need both numbers to know how far you can go and whether the engine can handle the load when it starts.

On a power station label, you will see both a watt rating (continuous and surge) and a watt-hour rating. The watt-hours set how long things can run. The watts set whether the station can start and keep running the device.

Formula 1: Runtime Hours

Formula:

Runtime hours = (Battery Wh × 0.85) ÷ (Device watts)

Example:

(1,024 × 0.85) ÷ (50) = (870.4) ÷ (50) = 17.4 hours

Common mistake: Forgetting the 0.85 factor. This accounts for inverter and system losses. If you skip it, you might think the station will last about 20.5 hours instead of 17.4 — roughly 18% too long (1 / 0.85).

Formula 2: Battery Size Needed

Formula:

Battery Wh needed = Daily Wh × Days × 1.2

Example:
If your daily load is 500 Wh and you need power for 2 days, the calculation is:

500 × 2 = 1,000 Wh \quad then \quad 1,000 × 1.2 = 1,200 Wh

Common mistake: Skipping the 20% margin. This can lead to the station running out of power early when real use is higher than expected.

Formula 3: Solar Yield Per Day

Formula:

Solar Wh per day = Panel W × Peak sun hours × 0.75

Example:
A 200 W panel in an area with 5 peak sun hours:

200 × 5 = 1,000 \quad then \quad 1,000 × 0.75 = 750 Wh/day

Peak sun hours: These vary by location and season. Use the NREL PVWatts tool to find your local peak sun hours. SOURCE: NREL / PVWatts

Common mistake: Using total daylight hours instead of peak sun hours. This can overestimate solar yield by a large margin.

Formula 4: Surge Power

Formula:

Surge W = (Total running W - Largest running W) + Largest starting W

Example:
If your total running load is 1,000 W, the largest running device is 150 W, and the largest starting device is 1,200 W:

(1,000 - 150) + 1,200 = 850 + 1,200 = 2,050 W

Common mistake: Ignoring the brief starting surge of the largest motor. This can cause the station to trip when the compressor kicks on.

Formula 5: Cold-Weather Usable Capacity

Formula:

Cold-weather usable Wh = Wh × 0.85

This factor stacks with the inverter efficiency:

Cold-weather usable Wh = Wh × 0.85 × 0.85 = Wh × 0.7225

Example:
A 1,000 Wh battery in cold weather:

1,000 × 0.85 = 850 \quad then \quad 850 × 0.85 = 722.5 Wh usable

LiFePO4 batteries: Most LiFePO4 batteries should not be charged below 32°F unless the station has battery heating. For example, EcoFlow lists the DELTA 3 Classic charge range as 32-113°F (EcoFlow DELTA 3 Classic spec: charge 32-113°F). Check your manual for specifics.

Common mistake: Using the room-temperature number for winter camping. This can lead to unexpected power loss in cold conditions.

Putting It Together

Here is a realistic weekend camping workflow that uses all five formulas. Daily energy is built from hours of use, not from assuming every load runs 24 hours.

Example Setup

Step 1: Daily Wh

20 × 6 + 150 × 12 = 120 + 1,800 = 1,920 Wh/day

Step 2: Solar Yield Per Day

Solar Wh per day = 200 × 5 × 0.75 = 750 Wh/day

Step 3: Net Daily Need After Solar

Net daily need = 1,920 - 750 = 1,170 Wh

Step 4: Battery Size Needed

Battery Wh needed = 1,170 × 2 × 1.2 = 2,808 Wh

Step 5: Pick a Station Class

A ~3,000 Wh class station matches this trip. It is in the right ballpark for camping, unlike a multi-10 kWh bank sized for continuous cabin loads.

Step 6: Cold-Weather Check on the Station You Pick

Cold-weather usable Wh = 3,000 × 0.85 × 0.85 ≈ 2,168 Wh AC

Cold nights plus a cloudy day that cuts solar make this tight. Plan to cut loads or add panel watts if the forecast looks poor.

Step 7: Surge Check

Surge W = (170 - 150) + 1,200 = 20 + 1,200 = 1,220 W

Make sure your station (or hybrid generator) has at least 1,220 W of surge capacity for the fridge start.

Final Notes

Subtract solar before you multiply by days and 1.2. Size for the weekend you actually run, not for every watt left on for 24 hours. Always check the manufacturer's surge and cold-weather numbers for your unit.

Worked Example (Same Numbers, Condensed)

  1. Daily Wh = 1,920.
  2. Solar = 200 × 5 × 0.75 = 750 Wh/day.
  3. Net = 1,920 − 750 = 1,170 Wh/day.
  4. Battery = 1,170 × 2 × 1.2 = 2,808 Wh → pick ~3,000 Wh.
  5. Cold check = 3,000 × 0.85 × 0.85 ≈ 2,168 Wh usable AC.
  6. Surge = 170 − 150 + 1,200 = 1,220 W.

Who This Is Not For

This guide is not for people who need continuous power for homes, businesses, or large-scale operations. It is intended for weekend and seasonal cabin, camper, and van owners who want to size portable power stations, batteries, and solar systems for short-term use. If you need power for more than a few days or for a full-time home, consult a professional and use larger, more robust systems designed for continuous use.

FAQ

Why multiply by 0.85?

Power stations lose some energy when the inverter turns battery DC into the AC your devices use. The 0.85 factor is a practical average that keeps the estimate realistic.

Do I always need the 1.2 safety margin?

It is wise for trips where you cannot easily recharge or when loads may run higher than planned. For short, known uses you can size closer to the raw daily total.

What if my solar panel gets less than the peak sun hours I used?

The daily Wh drops in proportion. Clouds, shade, or a lower sun angle all reduce output, so the NREL number for your area is the better starting point.

Can I charge the station in freezing weather?

Most LiFePO4 stations should not be charged below 32°F unless they have built-in battery heating. Check the manual for your unit; EcoFlow's DELTA 3 Classic, for example, lists charging at 32-113°F (EcoFlow DELTA 3 Classic spec: charge 32-113°F). Running from the battery is usually fine; charging is the restriction.

Do these formulas apply to both power stations and generators?

Runtime, battery size, solar, and cold-weather math are for battery stations. The surge formula is the one you use when sizing a generator (or a station's AC inverter) to start motors. Continuous watts still have to cover everything you run at once.

What does the 1.2 buffer actually cover?

It is a planning margin for inverter losses you did not model separately, partial charges, and one extra cloudy stretch. It is not a guarantee of multi-day autonomy without solar.

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