Article — Watt-Hours Calculator
Watt-hours calculator for battery capacity
A watt-hour (Wh) is the energy delivered by one watt of power for one hour, or 3,600 joules. Battery capacity in watt-hours equals voltage multiplied by amp-hours: Wh = V x Ah. A 12 V, 100 Ah lead-acid battery stores 1,200 Wh; a Tesla Model 3 Long Range pack holds 75 kWh.
Watt-hours are the single best unit for comparing energy storage across different chemistries and voltages. Amp-hours alone are misleading because the same charge at a higher voltage carries more energy. Manufacturers publish both, but watt-hours are what matter when you want to know how long a device will run.
What are watt-hours
The watt-hour is an energy unit derived from the watt, the SI unit of power. One watt sustained for one hour delivers exactly one watt-hour, which equals 3,600 joules. The kilowatt-hour (kWh) is a thousand watt-hours and is the unit electricity utilities use for billing.
The distinction between watts and watt-hours matters because consumers see both on appliance labels. Watts is the instant draw; watt-hours is the total over time. A 60 W bulb running for 5 hours uses 300 Wh of energy. A 1,500 W heater running for 8 hours uses 12,000 Wh, or 12 kWh.
The 2019 SI redefinition fixed the joule through the kilogram, the metre, and the second. One watt-hour is now exactly 3,600 J by definition, with no measurement uncertainty.
The watt-hours formula
For batteries, the working equation is straightforward.
Wh = V × Ah Ah = Wh / VWh = P × t (hours) 1 kWh = 1,000 Wh1 Wh = 3,600 J 1 Wh = 3.412 BTUExample calculations:
- Phone battery — 3,500 mAh at 3.85 V. Convert mAh to Ah (divide by 1,000): 3.5 Ah. Then 3.5 × 3.85 = 13.5 Wh.
- Laptop battery — spec sheet lists 57 Wh. At 11.4 V nominal, that is 57 / 11.4 = 5.0 Ah.
- Power tool pack — 18 V, 5.0 Ah = 90 Wh. A 6.0 Ah upgrade pack at the same voltage stores 108 Wh, 20 percent more energy.
- Solar storage — 48 V system bank rated at 200 Ah holds 9,600 Wh, or 9.6 kWh.
Watt-hours vs amp-hours
Amp-hours measure charge. Watt-hours measure energy. They diverge as soon as voltage changes. A 100 Ah, 12 V battery and a 100 Ah, 24 V battery share the same charge capacity but the higher-voltage pack stores twice the energy (2,400 Wh vs 1,200 Wh).
Manufacturers exploit this confusion. A 20,000 mAh power bank advertised on Amazon is rated at the internal cell voltage (3.7 V), which is 74 Wh. When it charges your phone at 5 V via USB, conversion losses and the voltage difference mean you get closer to 50 to 60 Wh of useful energy. Two of three full phone charges, not the seven the box implies.
Watt-hours in modern batteries
Energy density (Wh/kg or Wh/L) tells you how much battery you need for a given range or runtime. The standard chemistries:
- Lead-acid = 30 to 40 Wh/kg, cheap, heavy, common in cars and UPS units
- NiMH = 60 to 100 Wh/kg, used in older hybrids and Eneloop cells
- Li-ion (NMC) = 150 to 220 Wh/kg, EVs and laptops
- LFP (LiFePO4) = 90 to 140 Wh/kg, long cycle life, safer, growing in EVs and home storage
- Solid-state (lab) = 300 to 500 Wh/kg targets, not yet at scale
A 75 kWh Tesla Model 3 Long Range pack weighs about 480 kg. That gives 75,000 / 480 = 156 Wh/kg at the pack level, which is lower than the cell-level 220 Wh/kg because of the structural housing, cooling, and BMS hardware.
The Hoover Dam generates about 4 billion kWh per year, or 4 TWh. A single year of its output could fully charge 53 million Tesla Model 3 LR battery packs.
Watt-hours and lithium air-travel limits
Airline safety rules use watt-hours rather than amp-hours so the limits make physical sense across voltages. The FAA and IATA cap spare lithium-ion batteries at 100 Wh in carry-on, with up to two batteries between 100 and 160 Wh allowed with airline approval. Anything above 160 Wh has to ship as cargo.
Most laptops sit at 40 to 99 Wh. Large camera batteries, drones, and e-bike batteries often exceed 100 Wh. The rating is printed on the battery itself. Spare lithium batteries are never allowed in checked baggage, regardless of capacity.
Watt-hours on your electricity bill
Utilities bill in kilowatt-hours (kWh). The average U.S. household uses about 10,500 kWh per year per the EIA, or 30 kWh per day. Climate, electric heating, and house size shift this substantially: a small efficient apartment may use 6,000 kWh per year, a large electrically heated home in a cold climate over 30,000 kWh.
To work out a single appliance's contribution, multiply watts by hours per day, then divide by 1,000 to get kWh. A 1,500 W space heater running 8 hours a day uses 12 kWh. At a 17 cent per kWh rate, that is $2.04 per day, or $61 per month. The same heater at half the daily runtime would cost $30.50.
Phantom load (devices in standby) adds up. A typical U.S. home loses 50 to 100 W around the clock to chargers, set-top boxes, and idle electronics. That is 600 to 1,200 kWh per year, $100 to $200 at average rates.
Common watt-hours mistakes
The math is simple; the unit confusion is not.
- Treating Ah as energy — 100 Ah at 12 V is 1,200 Wh; 100 Ah at 48 V is 4,800 Wh. Always compare in Wh, never Ah alone.
- Forgetting depth of discharge — lead-acid batteries should only be discharged 50 percent for long life. A 100 Ah lead-acid bank gives 50 usable Ah, not 100. LFP can use 90+ percent of nameplate.
- Mixing mAh and Ah — 3,000 mAh = 3.0 Ah, not 3,000 Ah. Off by 1,000.
- Ignoring inverter losses — running a 1,000 W AC load from a 1,000 Wh DC battery does not give one hour. Inverter efficiency (typically 85-95 percent) cuts the runtime by 5 to 15 percent.
- Confusing watts with watt-hours — watts is rate; watt-hours is total. A 1,500 W heater for one hour uses 1,500 Wh. Two hours uses 3,000 Wh.