How to Calculate Watt-Hours: Formula, Examples, and the Mistakes That Burn Buyers

The formula in one line

(Wattage × Hours Used Per Day) ÷ 1,000 = Daily kWh.

That’s the formula the U.S. Department of Energy publishes on its appliance energy-use page, ¹ and it’s the only one you need 90% of the time. Watts is the rate (instantaneous power). Hours is duration. The product is energy.

A 1,500 W microwave running 10 minutes per day is 1,500 × (10 / 60) = 250 Wh. Round trip in a single line.

A worked example: how long can I run a fridge?

This is the question that drives most power-station purchases. Take a modern ENERGY STAR full-size refrigerator drawing about 150 W when the compressor is on. You have a Jackery Explorer 1000 v2 (1,070 Wh nominal, LFP, 4,000 cycles per Jackery’s spec sheet) and you want to know: how long can it carry the fridge?

Step 1 — Adjust the battery for the inverter. A pure-sine inverter at AC output runs roughly 90% efficient on a continuous load. Usable AC energy ≈ 1,070 × 0.90 = 963 Wh.

Step 2 — Adjust the load for duty cycle. A fridge doesn’t run 24 hours straight. The DOE recommends dividing total plugged-in time by three for compressor runtime estimation. ¹ So you’re really powering a 150 W load for one-third of wall-clock time.

Step 3 — Solve.

• Compressor-only runtime: 963 ÷ 150 = ~6.4 hours of compressor time.
• Wall-clock runtime: 6.4 × 3 = ~19 hours before the battery hits empty.

Match against the DOE-method estimate for daily fridge energy (~1,200–1,800 Wh for an ENERGY STAR full-size unit) and the math agrees: a 1 kWh battery covers about three-quarters of a day.

Watt-hours vs amp-hours: when each matters

Watt-hours (Wh) measure energy directly. Amp-hours (Ah) measure charge — and only become useful once you multiply by voltage:

Ah × V = Wh.

A “100 Ah” battery says nothing without the voltage. A 100 Ah battery at 12 V is 1,200 Wh. The same nameplate at 24 V is 2,400 Wh. At 48 V, 4,800 Wh. Battery shoppers who only look at Ah end up comparing apples to oranges across systems.

Power station marketing has mostly moved to Wh because it’s directly comparable. Most legacy lead-acid and 12 V LiFePO4 RV/marine batteries still spec in Ah — that’s the only reason the conversion matters in 2026.

Five real-world examples worked end-to-end

1. Charging a laptop

A 65 W USB-C laptop charger feeding a laptop using ~45 W average. Five hours of work = 45 × 5 = 225 Wh. A 512 Wh Jackery Explorer 500 v2 covers a long workday with margin.

2. Running a CPAP overnight

ResMed AirSense 11, no humidifier, ~30 W average over an 8-hour night = 30 × 8 = 240 Wh. Add the heated humidifier and tubing and you’re at ~75 W average = 600 Wh/night. A 1 kWh battery covers a single CPAP night easily; two nights is a 2 kWh job once you account for inverter losses.

3. A weekend campsite

12 V cooler at 60 W with the compressor running about 12 hours of 24 (a 50% duty cycle in summer) = 60 × 12 = 720 Wh/day. LED string lights at 15 W × 6 h = 90 Wh. Phone charging × 4 = 80 Wh. Coffee press, no electric kettle.

Daily total ≈ 890 Wh. Two nights = ~1,800 Wh — fits in a 2 kWh-class power station with one solar recharge in between.

4. The 24-hour blackout, with electronics

Refrigerator (~1,500 Wh adjusted for cycling), Wi-Fi router (~250 Wh), three phone charges (60 Wh), one LED ceiling light × 4 hours (40 Wh), a 55″ TV × 3 h (300 Wh).

Total ≈ 2,150 Wh. A 2 kWh class battery rides this with discipline (no microwaving meals); 3 kWh gives margin.

5. Off-grid weekend cabin

Cabin scenario from the wattage chart: ~5,500 Wh for two days. A 3 kWh battery (e.g. Bluetti Elite 300, 3,014 Wh nominal, 6,000-cycle LFP) handles this in one charge, or with a 200 W solar panel topping up between draws.

What the spec sheet doesn’t always tell you

A “2,048 Wh” power station is not the same as a “2,048 Wh that you can use.” Three things eat at the headline number:

1. Inverter efficiency — the conversion from DC battery to AC outlet is 85–92% in modern units. Cheap inverters drop into the 80% range under low load. Always read the inverter spec, not just the battery spec.
2. Depth of discharge (DoD) protection — LFP cells last longest if not pushed below ~10%. Some units enforce this with a battery management system; others let you drain to ~0% but burn cycles faster. The published “cycles to 80% capacity” figure assumes a specific DoD — usually 80% or 100%. Check the footnote.
3. Calendar aging — even unused, lithium cells lose 1–3% capacity per year. Five years in, your “2 kWh” battery is more like 1.7–1.85 kWh.

Together: the realistic usable capacity of a brand-new 2,048 Wh LFP power station to AC loads is closer to ~1,750–1,900 Wh. Plan around that number, not the headline.

How to size your battery — backwards

Most people start with the battery and work forward. The right move is to start with the loads and work backward:

1. List the appliances you actually want to keep running. (See the wattage chart if you don’t know the numbers.)
2. Multiply running W × realistic hours = daily Wh per appliance.
3. Sum them. Add 25% margin for inverter losses and surge headroom.
4. That’s your minimum battery size.

For a typical “essentials only during a 24-hour blackout” use case — fridge, lights, router, phones, a TV — the answer almost always lands between 1,500 and 2,500 Wh. That’s why the 2 kWh LFP class is so popular.

Use the calculator instead

If the math feels tedious, the WattBunker sizing calculator does this exact computation against the catalog:

Sources

Inverter efficiency figures (85–92% pure-sine) and LFP depth-of-discharge protection are industry conventions cross-referenced against published spec sheets for the Jackery Explorer series, EcoFlow DELTA 2 family, and Anker SOLIX C2000 Gen 2. Cycle counts cited (4,000 LFP / 6,000 LFP) are from the manufacturers’ own product pages; refresh against the current Amazon listing before any consequential purchase.