Watts ↔ Watt-hours
Energy = power × time. Enter any two values; the third is solved for you.
Battery Sizing Lab
How big a battery bank do you really need? We divide by depth-of-discharge and inverter efficiency — the two losses most online calculators quietly skip.
🔋 Reverse: what can my battery actually run?
Solar Panel Match
How many panels to cover a daily energy need? Uses peak sun hours and a 75% real-world derate (heat, dust, wiring, controller losses).
Device Runtime Table
Pick a battery size and see honest runtimes for common devices. Wattages are typical rated draws — your device label wins.
| Device | Typical watts | Runtime |
|---|
Runtime = battery Wh ÷ device W. Real devices cycle on/off (fridges) or draw less than rated (laptops idle) — treat as a planning estimate.
Worked examples
Three calculations, checked by hand
Every number below was verified manually before this page was built.
1 · Watts → watt-hours
A 100 W work light runs for 5 hours. How much energy is that?
E = P × t = 100 W × 5 h = 500 Wh
That's 0.5 kWh — the unit your electricity bill uses. Try it in the converter tab above.
2 · Battery sizing with honest losses
A 300 W fridge-room fan setup runs 8 h/day. Daily load = 300 × 8 = 2,400 Wh. With a Li-ion bank (90% DoD) and a 90% efficient inverter:
bank = 2,400 ÷ (0.9 × 0.9) = 2,962.96 Wh
At 48 V that's 61.73 Ah — or 246.91 Ah at 12 V. A "2,400 Wh battery" would leave you ~19% short; this is the mistake most thin calculators make.
3 · Solar panel match
You need 3,000 Wh/day. A 400 W panel in 5 peak sun hours with a 75% derate yields:
400 × 5 × 0.75 = 1,500 Wh/day per panel
3,000 ÷ 1,500 = 2 panels — rounded up, because half a panel doesn't exist.
Methodology
How the numbers are computed
The core formula
Watts measure power (rate of energy flow). Watt-hours measure energy (power sustained over time). The relationship is simple multiplication:
E (Wh) = P (W) × t (h)
Run a 60 W bulb for 2.5 hours and you've used 150 Wh. A watt-hour is 3,600 joules — exactly one watt sustained for one hour (3,600 seconds).
Battery math, honestly
A battery's nominal energy is Ah × V: a 12 V, 100 Ah battery stores 1,200 Wh. But you can't use all of it:
| Chemistry | Safe depth of discharge | 12 V 100 Ah usable |
|---|---|---|
| LiFePO4 (lithium iron) | 100% | 1,200 Wh |
| Li-ion (NMC etc.) | 80–90% | 960–1,080 Wh |
| Lead-acid / AGM | 50% | 600 Wh |
Draining lead-acid to zero a few times can kill it permanently. Then, if your load is AC, the inverter eats another 5–15%. This lab divides your load by both factors — many calculators skip this and undersize your bank by 30% or more.
Solar derate
Panels are rated in lab conditions (25 °C, perfect angle). In the real world, heat, dust, wiring, and controller losses shave ~25% off. A 75% derate is the honest default; the Solar Match tab lets you adjust it.
Honest limitations
⚠️ Ideal math vs. the real world
- Wires get warm: long cable runs lose 2–5% as heat — size cables for your amps.
- Lead-acid + heavy loads (Peukert): pulling 1,500 W from a small lead-acid bank yields less than Ah × V suggests. Lithium barely cares.
- Inverter idle draw: inverters sip 5–30 W just being on — significant for tiny loads.
- Cold: battery capacity drops in freezing weather (especially lead-acid).
- Devices cycle: a fridge rated 150 W might average 60 W — runtimes in the table are planning estimates, not promises.
Scope boundary
🧭 What this tool does — and doesn't — cover
- This tool (#389): watts ↔ watt-hours with time, battery sizing with DoD + inverter honesty, solar panel matching, device runtimes. Head unit: watts.
- #384 kw-to-wh (planned sibling): the same ideas scaled to kilowatts — household appliances, EV charging, whole-home loads.
- #381 kw-to-joules (planned sibling): power → energy in joules for physics/engineering contexts.
- #379 energy-cost-calculator: turns kWh into money with tariffs — use it when you need the bill, not the physics.
- #282 electrical-unit-converter: the hub for V/A/Ω unit conversion, Ohm's law, and resistor codes — the circuit-theory companion to this page.
FAQ
Frequently asked questions
How do you convert watts to watt-hours?
Multiply watts by hours: Wh = W × h. A 100 W light running for 5 hours uses 100 × 5 = 500 Wh. If the time includes minutes, divide them by 60 first — a 60 W bulb for 2 h 30 min uses 60 × 2.5 = 150 Wh.
What is the difference between watts and watt-hours?
Watts measure power — the rate energy flows right now. Watt-hours measure energy — power sustained over time. Think of it like speed versus distance: 100 W is how fast energy flows, 500 Wh is how far it went after 5 hours.
How do I convert watt-hours back to watts?
Divide watt-hours by hours: W = Wh ÷ h. A battery that delivered 500 Wh over 5 hours supplied an average of 100 W. You need the time period — watt-hours alone can't tell you the wattage.
How many watt-hours are in a 12V 100Ah battery?
Nominally 1,200 Wh (12 × 100). Usable energy depends on depth of discharge: about 960 Wh for Li-ion at 80%, 1,200 Wh for LiFePO4 at 100%, but only 600 Wh for lead-acid at a safe 50%. The Battery Lab tab computes this for your setup.
How do I calculate what size battery I need?
First find your daily load: device watts × hours per day. Then divide by depth of discharge and inverter efficiency: bank Wh = load ÷ (DoD × efficiency). Example: 2,400 Wh ÷ (0.9 × 0.9) = 2,963 Wh of battery — then divide by system voltage for amp-hours.
What is depth of discharge (DoD) and what value should I use?
DoD is the fraction of a battery you can safely use. Use 100% for LiFePO4, 80–90% for Li-ion, and 50% for lead-acid/AGM. Draining lead-acid to zero repeatedly can destroy it, which is why a '1,200 Wh' lead-acid battery only gives you ~600 usable Wh.
How many solar panels do I need for my daily watt-hour usage?
One panel's daily yield = panel watts × peak sun hours × 0.75 (real-world derate). Divide your daily Wh need by that yield and round up. Example: 3,000 Wh/day ÷ (400 W × 5 h × 0.75 = 1,500 Wh) = 2 panels. The Solar Match tab does this with adjustable sun hours.
What are peak sun hours?
Peak sun hours (PSH) compress a whole day's sunlight into equivalent hours of full 1,000 W/m² sunshine. Typical values: ~3 in cloudy/temperate regions, ~5 in sunny areas, ~6.5 in deserts. A 400 W panel in 5 PSH theoretically makes 2,000 Wh — then the 75% derate brings it to ~1,500 Wh real.
How long will a battery run my device?
Divide usable watt-hours by device watts: hours = Wh ÷ W. A 960 Wh usable battery runs a 40 W CPAP for 24 hours, or a 60 W TV for 16 hours. Real devices cycle on and off (fridges) so treat results as planning estimates.
Why is my actual runtime shorter than the calculation?
The simple formula is ideal. Real losses include: inverter waste (5–15% for AC loads), warm cables (2–5% on long runs), inverter idle draw (5–30 W just being on), cold weather (less capacity), and lead-acid's Peukert effect under heavy loads. This page's battery lab accounts for DoD and inverter losses explicitly.
Are watts the same as amps? What about watt-hours vs amp-hours?
No — watts = volts × amps. A 12 V device drawing 5 A uses 60 W. Similarly, watt-hours = amp-hours × volts: a 100 Ah battery at 12 V stores 1,200 Wh, but the same 100 Ah at 48 V stores 4,800 Wh. Voltage is the missing piece that makes Wh the universal energy unit.
How many watt-hours are in a kilowatt-hour (kWh)?
Exactly 1,000. Your electricity bill uses kWh: 500 Wh = 0.5 kWh. The converter tab shows kWh, joules, megajoules, and BTU equivalents for every result, so you can match any unit your bill or spec sheet uses.
Do I need an inverter, and how much energy does it waste?
Only if your devices run on AC mains power. Inverters typically waste 5–15% (90–95% efficient for quality pure-sine units, ~85% for budget ones). DC-only setups — USB devices, 12 V fridges, LED strips — skip the inverter entirely and keep that energy. The Battery Lab lets you set efficiency to 100% for DC loads.
Can this tool convert kilowatts to watt-hours?
Yes for the math — 1 kW = 1,000 W, so convert kW to W first (or enter decimals). But if you work mostly in kilowatts (home appliances, EV charging, whole-house loads), our dedicated kw-to-wh converter (#384) is built for that scale with appliance presets.
How is this different from an energy cost calculator?
This tool answers the physics question: how much energy (Wh) a device uses or a battery holds. An energy cost calculator answers the money question: what that energy costs at your tariff. Use this page first to find the Wh, then our energy-cost-calculator (#379) to turn it into dollars, euros, or rupiah.
Is my data private when I use this converter?
Yes. This page runs 100% in your browser — every calculation happens locally in JavaScript. No inputs are sent to any server, no cookies track your numbers, and it keeps working offline once loaded.