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Nameplate Wh is the energy stored in the cells. AC delivery at the wall is lower. WattList plans usable AC Wh as nameplate × 0.85, so a 1,024Wh pack is about 870Wh for 120V loads (same example as our homepage meter). That is planning math, not a lab certificate for every SKU.
If you only remember one thing from this page: multiply the brochure number by 0.85 before you decide how long a fridge, router, or overnight load will last. Then check that the station’s continuous and surge watts can start the load.
More load-by-load sizing lives in our fridge + Wi‑Fi + CPAP Wh guide. This page owns the usable-vs-nameplate rule every WattList guide points at.
Watts vs watt-hours (30 seconds)
Watts 
= how hard the load pulls right now.
Watt-hours (Wh) = how long the tank lasts.
A 1,000W kettle on a 500Wh pack is a surge/continuous-watts problem, not a “big tank” problem. A 40W overnight device on a 300Wh pack is a Wh problem. You need both: enough continuous/surge watts to start and run the load, and enough usable Wh to cover the hours.
Where the Wh goes
Brochure Wh is cell capacity. AC outlets turn that DC energy into 120V through an inverter and a battery management system (BMS). Pure sine stations typically lose roughly 10–20% as heat and overhead. WattList’s planning default sits in the middle of that band: × 0.85.
Rough loss stack (ranges, not SKU lab data):
| Loss | What it is | Typical planning range |
|---|---|---|
| DC→AC inverter | Conversion heat under load | ~8–15% (worse at tiny loads) |
| Inverter idle | AC left on with almost nothing plugged in | Often tens of watts continuous |
| BMS reserve / cutoff | You rarely empty the pack to true zero | Small slice of nameplate |
| Fans / display | Small continuous draw | Usually minor vs inverter |
| DC outputs (USB-C / 12V) | Skip most of the AC inverter tax | Better % when the device allows DC |
0.85 is a planning rule, not a lab measurement for every unit. Real packs vary with load size, temperature, age, and whether AC stays on overnight. Our homepage meter says the same: illustrative AC usable share 85%; example math only.
Nameplate → usable at ×0.85
| Nameplate Wh | Usable AC Wh (×0.85) |
|---|---|
| 512 | ~435 |
| 1,024 | ~870 |
| 2,048 | ~1,741 |
Use this table on any product page: find the rated Wh, multiply by 0.85, then divide by your running watts for hours.
WattList formulas (copy-ready)
Illustrative planning math. Not a lab measurement.
- Usable AC Wh ≈ Nameplate Wh × 0.85
- Hours ≈ (Nameplate Wh × 0.85) ÷ Running watts
- Optional buffer: add about 10–15% headroom for cold, aging cells, fan draw, and surprise loads.
If the load can run on USB-C or 12V DC, you usually skip most inverter loss. Still plan a buffer; do not invent a brand-specific DC efficiency number from this page. Prefer manufacturer DC cables when the device supports them.
Worked examples (illustrative)
These use round planning watts so you can see the shape of the math. Substitute your label watts. None of these are WattList lab runs.
CPAP-class overnight (~40W × 8h)
CPAP note: Draws vary widely. Humidifiers and heated hoses can dominate. Check your device’s labeled wattage and ask the manufacturer about approved DC cables. A power station is only a battery + inverter. This is not medical advice.
Raw energy at the device: 40W × 8h = 320Wh.
From a nameplate pack on AC: 320 ÷ 0.85 ≈ 376Wh of nameplate capacity (before buffer).
With a 15% buffer: plan roughly 430Wh of nameplate, or more if your label is higher than 40W.
On a 1,024Wh station (~870Wh usable AC): 870 ÷ 40 ≈ ~22 hours of that 40W load alone, before fridge, lights, or idle tax. If your machine supports an approved DC path, expect better efficiency than AC; still verify with the maker.
Full overnight mix (fridge + Wi‑Fi + CPAP) belongs in the sizing guide, not as a fake one-size Wh claim here.
Fridge cycling
Fridge nameplates (compressor start / peak) are not average watts. Compressors cycle. Ambient heat, dirty coils, and door openings raise the daily Wh bill.
Shopping method:
- Estimate daily Wh from average watts × hours (or a meter).
- Apply ÷ 0.85 when you map that need onto a product-page nameplate.
- Add buffer for a warm kitchen or a long outage.
Example shape only: 70W average mini-fridge × 12h = 840Wh at the load → about 990Wh of nameplate on AC before buffer. A ~1kWh station can do a careful half-day stretch; aggressive 24h fridge duty usually wants more Wh, solar recovery, or shedding. Details and class picks: Wh sizing and apartment blackout kits.
Starlink / always-on ~45W
Always-on AC loads punish idle. At 45W continuous:
| Nameplate | Usable (×0.85) | Hours at 45W |
|---|---|---|
| 512 | ~435 | ~9.7 h |
| 1,024 | ~870 | ~19 h |
| 2,048 | ~1,741 | ~39 h |
Leave the inverter on all night with only a dish and a few watts of “nothing else,” and idle draw still eats the tank. Prefer DC or USB-C when the kit allows it; turn AC off when nothing needs it. Kit-specific averages: how long a power station runs Starlink. Same formula there: (Wh × 0.85) ÷ watts.
Practical rules you can use tonight
- Multiply nameplate by 0.85 before you trust a runtime claim.
- Prefer USB-C / 12V when the device allows it.
- Turn the AC inverter off when nothing needs AC. Idle is a silent tax.
- Oversized inverter + tiny overnight load is a bad match; idle can rival the load.
- Cold weather: expect less usable output; add buffer (especially backcountry / elk camp nights).
- “Solar generator” on the box still means battery + optional panels. Nameplate Wh does not refill itself.
- After usable Wh is settled, confirm continuous and surge watts can start the load.
How this changes shopping on WattList
WattList rankings and kit math use usable Wh so brochure numbers do not over-promise hours. When two stations list similar nameplates, the one with the right continuous watts, chemistry, weight, and recharge path still wins for your trip. Both should still be sized with the same ×0.85 lens.
Soft next steps (not a full roundup):
- Budget lane: best portable power stations under $500
- Midrange tanks: best portable power stations under $1,000
- Brand feel for weekends: Jackery vs EcoFlow vs Bluetti
- Rolling home: van life power stations
- Grid-down apartment nights: apartment / condo blackout kits
- Load math: how many Wh for fridge, Wi‑Fi & CPAP
- Panels that actually match the input: foldable solar for power stations
- Station + panel + cable: portable power station starter kit
- All field guides: WattList Guides hub
FAQ
Is usable Wh the same as nameplate Wh?
No. Nameplate Wh is cell storage. Usable AC Wh is what you can plan to deliver through the inverter after conversion and BMS overhead. WattList’s default is about 85% of nameplate for AC planning.
Why does WattList multiply by 0.85?
Pure sine inverters typically lose roughly 10–20%. We pick 0.85 as a single planning default so every guide, widget, and product note speaks the same language. It is illustrative, not a measurement of every SKU.
Do I lose less energy on USB-C or 12V than on AC?
Usually yes. DC paths skip most of the AC inverter conversion. Use manufacturer-approved cables when the device supports them. You still want a buffer for cable, converter, and idle losses.
Why is my runtime shorter than the marketing claim?
Marketing often shows best-case cell Wh, a light load, or DC. Real nights add inverter idle, fans, cold, aging, and higher average watts than the brochure assumed. Recalculate with (nameplate × 0.85) ÷ your running watts.
Should I turn the inverter off overnight?
Yes, if nothing needs AC. Leaving AC on for a tiny load burns idle watts all night. Charge phones on USB-C, run 12V fridges on DC when possible, and switch AC on only when you need it.
Does cold weather change usable Wh?
Yes. Cold packs deliver less and protect themselves earlier. Plan extra headroom, often in that same 10–15% buffer band, more if you are winter camping.
Size with usable Wh, then watts
Size the trip with usable Wh (nameplate × 0.85), then pick a station whose continuous and surge watts can start the load. Specs on WattList are manufacturer-page checked; rankings are not for sale.
Corrections or a missing loss you want documented: Contact WattList or email [email protected].
