Battery Runtime Basics: Watts → Watt-hours + Real-World Efficiency (With Calculator)

January 21, 2026 ZacharyWilliam

Power Outage Prep · Runtime planning that actually matches real life

The clean math is simple. The real world isn’t. This guide shows how to estimate battery runtime in a power outage using watts and watt-hours, then adds the parts most “quick formulas” skip: inverter losses, reserve, and appliances that cycle on/off. You’ll also get a practical load planner and a quick way to estimate fridge energy from the yellow EnergyGuide label.

Back to the main guide: Power Outage Checklist (24/48/72 Hours) · Related: Power Priorities: What to Run First

Use UDPOWER’s Runtime Calculator Battery Unit Conversion Tools

Battery runtime planning for a power outage showing watts to watt-hours math with a portable power station powering a router and lamp

The only two numbers you need: W and Wh

Watts (W) = how hard a device pulls

Think of watts as “speed.” A 10W router sips power. A 1,000W microwave gulps it. When the outage starts, watts help you decide what belongs on your priority list.

Need help choosing what to run first? Use the priority ladder: Power Priorities: What to Run First.

Watt-hours (Wh) = how much energy you have

Watt-hours are “fuel in the tank.” If your power station is 1,200Wh and your load is 100W, the clean math says ~12 hours. In the real world, you plan a little lower (we’ll show you how).

If you’re converting between mAh, Ah, Wh, kWh, etc., use: Battery Unit Conversion Tools.

The clean formula

Runtime (hours) ≈ Battery Wh ÷ Load W (then we adjust for real-world losses below).

If you don’t know a device’s watts: check the label, check the power brick, or measure with a plug-in watt meter. Guessing is fine for a first pass, but a $20 meter can save you a lot of “why did it die so fast?” later.

Real-world efficiency: “usable Wh” (the number you should plan with)

Batteries store DC energy. Many household devices use AC. Converting DC → AC costs energy. On top of that, power stations have their own system overhead, and most people keep a little reserve so they don’t hit 0% at the worst moment.

Planning formula (what we use in the calculator below)

Usable Wh ≈ Battery Wh × Efficiency × (1 − Reserve)

Output path	What’s happening	Practical planning range	Best move during an outage
AC outlets	Battery DC → inverter → AC → your device’s power supply	0.80–0.90	Use AC for true AC loads; keep total watts reasonable to avoid extra loss
USB / DC ports	Less conversion (often more efficient for small electronics)	0.85–0.95	Whenever possible, charge phones/laptops via USB-C PD instead of AC bricks
Reserve	Intentional buffer so you don’t hit 0% unexpectedly	5%–15%	For multi-day outages, a small reserve keeps options open (especially overnight)

Mobile tip: swipe the table sideways.

A quick “good enough” default

If you don’t want to overthink it: start with 0.85 efficiency and 10% reserve. Then test once with your real devices and adjust.

Fridge + cycling loads: the duty-cycle shortcut (and a smarter label trick)

A refrigerator is the classic trap. The label might show a high “running” number, but the compressor cycles on and off. That means the useful number is the average over time.

Shortcut #1: Duty-cycle average

If you know the running watts and you have a rough idea how often it runs:

Average W ≈ Running W × Duty cycle (example: 150W × 0.33 ≈ 50W average)

This is great for “good enough” planning, especially when you don’t have a meter yet.

Shortcut #2: Use the yellow EnergyGuide label

Many major appliances show estimated annual energy use in kWh/year. That number is already “real-world average-ish” because it’s based on standardized testing.

Average W ≈ (kWh/year × 1000) ÷ 8760

Want to learn what EnergyGuide is and why it’s on appliances? FTC: Using the EnergyGuide label

EnergyGuide (kWh/year)	Average watts (approx.)	What that means for runtime planning
300	~34W	Efficient fridge / smaller unit (average draw)
350	~40W	Common modern efficiency range
400	~46W	Still manageable with a solid outage plan
500	~57W	Multi-day outages likely need cycling strategy + recharging
650	~74W	Older/larger units: plan for faster battery drain
900	~103W	Very power-hungry: battery-only is usually short-lived
1400	~160W	Old inefficient units: generator/solar hybrid becomes more realistic

Mobile tip: swipe the table sideways.

Outage reality check

If your plan depends on keeping food safe, pair your power plan with the actual food safety timeline. The FDA/USDA guidance is a helpful baseline: Food safety during power outages. For a full fridge/freezer decision guide, use: Food Safety During a Power Outage.

Runtime calculator (efficiency + reserve)

Tip: Start with 0.85 & 10% reserve

Prefer a full-page tool? Use: Portable Power Station Runtime Calculator.

1) Single-load runtime

2) Quick guidance (when you’re in “outage mode”)

Group loads: add up watts for devices you’ll run together (router + lamp + chargers).
Run big loads in windows: fridge, cooking, and pumps often belong in short bursts.
Use the most efficient ports: USB-C PD beats AC bricks for many electronics.
Have a refill plan: if the outage might last days, plan for solar/car charging.

If you’re using a fuel generator as part of your plan, keep it outdoors and away from openings. CDC guidance: Carbon monoxide safety for generators. If you’re deciding between battery vs generator, use: Portable Power Station vs Generator.

Outage load planner (multi-device, per day)

This is the part that actually reduces stress: a simple daily budget. Put your “must-run” items in the list. You’ll instantly see what fits for 24, 48, and 72 hours.

How to use this planner

Enter watts and hours used per day for each device.
For charging-only items (phone/laptop), estimate how many hours of charging per day you’ll do.
Then compare your daily Wh to your usable battery Wh (from the calculator above).

Device	Watts (W)	Hours/day	Wh/day (auto)
Wi-Fi router + modem			—
LED lamp(s)			—
Phone charging			—
Laptop top-up			—
Small fan			—
Other (your pick)			—

Mobile tip: swipe the table sideways.

Planner totals

Total Wh/day: —

Total Wh (72 hours): —

Use this to sanity-check your 3-day plan. For water planning to match the same window: Emergency Water for a 3-Day Outage.

If you’re counting on solar

Solar recharging changes everything in a multi-day outage—but only if you stay inside input limits. Before you connect panels (especially in series), read: Solar Charging During an Outage: Input Voltage Safety.

Examples: what “1,000Wh-class” vs “2,000Wh-class” feels like

Instead of promising exact hours (your devices decide that), it helps to think in “energy budgets.” Below are two UDPOWER reference points using a conservative planning setup: 0.85 efficiency and 10% reserve.

Model	Battery (Wh)	Usable Wh (0.85 × 90%)	100W load (approx.)	Notes for outage planning
UDPOWER S1200	1191Wh	~911Wh	~9.1 hours	Great for essentials (Wi-Fi, lights, charging) and short “windows” for bigger loads. AC output is 1200W (1800W surge), solar input 12–75V up to 400W, UPS ≤10ms. Product page
UDPOWER S2400	2083Wh	~1593Wh	~15.9 hours	Bigger buffer for multi-device days and longer outages. 2400W AC output (UDTURBO surge up to 3000W), solar input 12–50V up to 400W, UPS ≤10ms. Product page

Mobile tip: swipe the table sideways.

A practical takeaway

If your “must-run” set is mostly communication + lights + charging, you can usually stretch runtime a long way. If your must-run set includes big continuous heating/cooling, battery-only plans get hard fast. That’s why prioritization matters: What to Run First.

Don’t forget: surge / startup watts

Motors and compressors can spike at start-up. Always verify both running watts and surge needs. If you’re backing up medical devices, plan extra conservatively and test your setup in advance. CPAP planning: CPAP Battery Backup During a Power Outage.

Common mistakes that wreck runtime (and how to avoid them)

Mistake: using “max watts” as “average watts”

Appliances cycle. Your fridge’s label might show a high running number, but it doesn’t run at that rate 24/7. Use a meter, or use the EnergyGuide kWh/year method above.

Mistake: charging everything through AC

Many small electronics are happier (and often more efficient) on USB-C or DC. In an outage, that efficiency difference adds up over 48–72 hours.

Mistake: no “schedule” for big loads

If you run the fridge, lights, chargers, and cooking device all at once, your battery drops fast. Pick windows: fridge window, cooking window, charging window.

Mistake: forgetting the outage isn’t just power

Pair your energy plan with food + water basics, especially for multi-day outages. Start here: 24/48/72 Checklist.

Where UDPOWER fits (quiet essentials)

For most households, the biggest “win” is staying functional: phones charged, a few lights, internet (if available), and strategic use of larger loads. That’s exactly where portable power stations are most convenient—especially because they can run indoors without fumes.

Two common starting points

S1200 (1191Wh, 1200W AC, 1800W surge; solar 12–75V up to 400W; UPS ≤10ms): a strong baseline for essentials-first backup. View S1200
S2400 (2083Wh, 2400W AC, surge up to 3000W; solar 12–50V up to 400W; UPS ≤10ms): more breathing room for multi-device plans. View S2400

If your plan includes solar recharging, don’t skip: Input Voltage Safety.

The fastest way to get accurate for your home

Pick your top 6 loads (Tier 1–2 items).
Measure or label-check watts for each.
Run a 1-hour test during normal life.
Then lock your plan in writing (so outage you doesn’t have to think).

Author note: This article is for planning and education. Always follow your device manufacturer guidance, and test critical setups (especially medical devices) before relying on them.

Sources & further reading

External links open in a new tab and are marked nofollow.

FTC: What the EnergyGuide label is and how to use it — consumer.ftc.gov
U.S. Department of Energy: Estimating appliance energy use (W, hours, kWh math) — energy.gov
FDA: Food safety during power outages (fridge/freezer time windows) — fda.gov
CDC: Carbon monoxide safety guidance for generators — cdc.gov

Back to blog

Battery Runtime Basics: Watts → Watt-hours + Real-World Efficiency (With Calculator)