Watt Hours Explained: Wh vs W vs Ah
Why a 1,000 Wh box is not 1,000 usable watt-hours, how amp-hours convert at 12 V, and the three losses that decide real runtime.
energy · battery runtime
local · auto-updatingPortable Power Station Runtime Calculator
How long will a power station last at your load? Enter the battery's watt-hours and the devices you want to run. Get the runtime with depth of discharge, inverter losses and standby draw counted — or the capacity you need for 12, 24, 48 or 72 hours.
the formula
Runtime (hours) = (watt-hours × depth of discharge × inverter efficiency) ÷ (average load + standby draw). At the defaults — 1,000 Wh, 90% depth of discharge, 88% inverter efficiency, 5 W standby — that is 792 Wh of usable energy, so a 220 W average load runs about 3 hours 31 minutes.
battery + loads
The number on the box. 1,000 Wh = 1 kWh.
Sets the depth of discharge.
Most stations: 85–90%.
The inverter's own idle use.
The station's AC rating.
Peak rating for motor starts.
The target drives the capacity.
Loadouts
how the math works
How the runtime is calculated
Four inputs decide the answer: usable watt-hours, the average load, the standby draw, and the station's output rating. Here is what each one does.
Usable watt-hours, not nameplate watt-hours
A portable power station is sold by its nameplate capacity, but three things sit between that number and the energy that reaches your devices. The depth of discharge is the share of the pack the battery management system will let you use — about 90% for LiFePO4, 85% for NMC lithium and 50% for lead-acid. The inverter efficiency is what the DC-to-AC conversion keeps, typically 85–90% on the stations manufacturers publish. The rest is the station's own standby draw, a few watts the inverter burns just being switched on.
Runtime (hours)
= (Wh × depth of discharge × inverter efficiency) ÷ (average load + standby draw)
→ 1,000 Wh × 0.90 × 0.88 = 792 Wh usable; 792 ÷ (220 + 5) = 3.53 h
= (Wh × depth of discharge × inverter efficiency) ÷ (average load + standby draw)
→ 1,000 Wh × 0.90 × 0.88 = 792 Wh usable; 792 ÷ (220 + 5) = 3.53 h
The same 1,000 Wh pack on a lead-acid battery is a different machine: 1,000 × 0.50 × 0.88 = 440 Wh usable, a little over half the runtime. That is why a "1,000 Wh" lead-acid box and a "1,000 Wh" LiFePO4 station are not interchangeable, and why the chemistry selector exists.
Average watts: the number a watt-hour label hides
Very few loads draw their rated watts all day. A refrigerator is the obvious case: the compressor pulls its full 700 W for part of the hour and nothing in between, so its true demand is an average. The calculator applies a duty cycle to every device, which turns nameplate watts into average watts:
| Device | Rated watts | Duty cycle | Average watts |
|---|---|---|---|
| Full-size refrigerator | 700 W | 10% | 70 W |
| Chest freezer | 120 W | 33% | 40 W |
| LED TV (55 in) | 100 W | 100% | 100 W |
| Microwave | 1,000 W | 5% | 50 W |
| Space heater | 1,500 W | 50% | 750 W |
This is the correction most runtime claims skip. A 700 W refrigerator sounds impossible for a 1,000 Wh station until the duty cycle is applied — then it is a 70 W average, and the station lasts about ten and a half hours. A 1,500 W heater is the reverse: its thermostat cuts it to 750 W on average, which is still an entire 1,000 Wh station in little over an hour.
Typical runtime by capacity
At the calculator's defaults — 90% depth of discharge, 88% inverter, 5 W standby — these are the hours you get from a single device running alone. The same capacity figures drive the full runtime chart.
| Device (average) | 300 Wh | 500 Wh | 1,000 Wh | 2,000 Wh | 3,600 Wh |
|---|---|---|---|---|---|
| Phone charger (10 W) | 15 h 50 m | 26 h 24 m | 52 h 48 m | 105 h 36 m | 190 h 05 m |
| Wi-Fi router (30 W) | 6 h 47 m | 11 h 19 m | 22 h 38 m | 45 h 15 m | 81 h 28 m |
| Laptop (50 W) | 4 h 19 m | 7 h 12 m | 14 h 24 m | 28 h 48 m | 51 h 50 m |
| LED TV, 55 in (100 W) | 2 h 16 m | 3 h 46 m | 7 h 33 m | 15 h 05 m | 27 h 09 m |
| Full-size refrigerator (70 W) | 3 h 10 m | 5 h 17 m | 10 h 34 m | 21 h 07 m | 38 h 01 m |
| Space heater (750 W avg) | 0 h 19 m | 0 h 31 m | 1 h 03 m | 2 h 06 m | 3 h 47 m |
Hours = usable watt-hours ÷ (device average + 5 W standby), rounded to the nearest minute. A 300 Wh station yields 237.6 Wh usable; a 3,600 Wh unit yields 2,851 Wh.
Sizing a station instead of timing it
The second mode runs the formula backwards. Decide how many hours you need, add up the average load, and divide by the same efficiency chain:
Required capacity (Wh)
= (average load × hours) ÷ (depth of discharge × inverter efficiency)
→ a 125 W essentials load for 24 h = 3,000 Wh ÷ 0.792 = 3,788 Wh
= (average load × hours) ÷ (depth of discharge × inverter efficiency)
→ a 125 W essentials load for 24 h = 3,000 Wh ÷ 0.792 = 3,788 Wh
Blackout essentials — a refrigerator averaging 70 W, a router at 30 W, an LED bulb at 10 W and a phone charger at 10 W, plus 5 W standby — draw 125 W. That load needs 1,894 Wh of nameplate capacity for 12 hours, 3,788 Wh for 24, 7,576 Wh for 48 and 11,364 Wh for 72. In the sizes stations actually ship in: 2,048 Wh, 4,096 Wh, 7,680 Wh and 12,000 Wh. Past about 10,000 Wh you are outside the portable class and into a home battery or a generator. The sizing guide works the full example.
Output watts are a separate limit
Watt-hours tell you how long; the continuous output rating tells you whether the device will run at all. A 1,000 Wh station with a 1,000 W inverter cannot hold a 1,500 W heater — the inverter shuts down in seconds, with the battery still 90% full. Motors add a second limit: a full-size refrigerator may run at 700 W but start at about 2,200 W, so a station needs a surge rating above that or the compressor will stall on start-up.
The meter beside the calculator compares the selected devices running at once against the continuous rating. If it crosses 100%, the answer is not a bigger battery — it is a smaller load or a station with more AC output.
What these figures assume
- 01A new battery at room temperature. Capacity fades with cycles and with cold: a pack stored below freezing can deliver well under its rated watt-hours, and LiFePO4 units are commonly rated for 3,000–5,000 full cycles to 80% capacity.
- 02Typical duty cycles. Appliance averages are representative, not measured on your unit. Use the nameplate plus your own observation when the stakes are high — a fridge in a hot garage cycles far more than one in a cool kitchen.
- 03Published inverter efficiency. The 88% default is mid-range for the 85–90% most manufacturers quote, and it assumes AC output. USB and 12 V DC outlets bypass the inverter and usually waste less.
- 04No charging during discharge. Solar input extends runtime, and the calculator does not model it. A 200 W panel in good sun roughly halves the net drain on a clear day, and does nothing at night.
- 05Continuous, not surge, load. The runtime number assumes the load holds steady. A heater's thermostat, a fridge's compressor and a microwave's short bursts are already expressed as averages — do not apply the average twice.
Common power station questions
How long will a 1,000 Wh power station last?
It depends on the load. At the calculator's defaults — a 90% depth of discharge, an 88% inverter and a 5 W standby draw — 1,000 Wh becomes 792 Wh of usable energy. A 50 W laptop runs about 14 hours 24 minutes, a 100 W TV about 7 hours 33 minutes, and a full-size refrigerator averaging 70 W about 10 hours 34 minutes.
Why isn't runtime just capacity divided by watts?
Because a battery is never emptied completely and the inverter does not pass energy without loss. Usable watt-hours = nameplate capacity × depth of discharge × inverter efficiency, so a 1,000 Wh pack at 90% depth of discharge and 88% efficiency delivers 792 Wh. Divide that by the load plus the station's own standby draw.
What size power station do I need for 24 hours?
For blackout essentials drawing about 125 W — a refrigerator averaging 70 W, a router at 30 W, an LED bulb at 10 W, a phone charger at 10 W and 5 W standby — 24 hours needs 3,000 Wh of delivered energy, and 3,000 ÷ 0.792 = 3,788 Wh of nameplate capacity. The next common size is a 4,096 Wh station.
Can a power station run a space heater?
Only briefly. A 1,500 W heater on a thermostat duty cycle averages 750 W, so a 1,000 Wh station at the defaults lasts 792 ÷ 755 = 1.05 hours, about an hour. Many 1,000 Wh units are also rated below 1,500 W continuous output, so the heater can trip the inverter before the battery is empty.
Does the output rating matter as much as the watt-hours?
Yes, and it fails first. Watt-hours decide how long a load runs; the continuous output rating decides whether it runs at all. A 1,000 Wh station with a 1,000 W inverter cannot hold a 1,500 W heater even on a full battery, and a refrigerator that starts at 2,200 W needs a surge rating above that figure.
power station guides
Three guides behind the calculator
What a watt-hour actually is, a runtime chart for every common capacity, and the sizing method for a 12, 24, 48 or 72-hour outage.
Portable Power Station Runtime Chart
Hours for 300, 500, 1,000, 2,000 and 3,600 Wh against eight common loads — fridge, TV, laptop, router, heater and more.
What Size Power Station Do I Need?
The four-step sizing method with a 24-hour worked example, a capacity-by-outage-length table, and the output-rating trap that no watt-hour label warns you about.