Quick answer
A 100Ah 12V battery holds about 1,200Wh (100Ah × 12V). Runtime is simply usable Wh ÷ device watts, where usable Wh = 1,200 × your depth-of-discharge limit — roughly 960Wh at 80% for lithium, 600Wh at 50% for lead-acid. So a 60W device runs about 16 hours on lithium, 10 hours on lead-acid. A fridge, which cycles on and off, lasts far longer than the raw math suggests — see the duty-cycle section below. The formula is the same for every battery and every device; once you learn it, you never need a lookup table again.
The formula (once, clearly)
There is exactly one runtime formula, and it has three steps:
Step 1 — Amp-hours to watt-hours:
Ah × V = Wh
A 100Ah battery at 12V: 100 × 12 = 1,200Wh. This is the total energy stored. Amp-hours alone tell you nothing until you multiply by voltage — that’s the first mistake everyone makes.
Step 2 — Apply your depth of discharge (DoD):
Wh × DoD = usable Wh
You cannot drain a battery to zero without damaging it. Lithium is safe to about 80–90%, lead-acid to about 50%. So usable energy is 1,200 × 0.80 = 960Wh (lithium) or 1,200 × 0.50 = 600Wh (lead-acid).
Step 3 — Divide by your device’s watts:
usable Wh ÷ W = hours
A 60W device on lithium: 960 ÷ 60 = 16 hours. On lead-acid: 600 ÷ 60 = 10 hours.
That’s the whole method. The three mistakes that wreck the answer:
- Forgetting voltage. Ah ≠ Wh. A 100Ah 12V battery (1,200Wh) is not the same as a 100Ah 24V battery (2,400Wh) — see the scaling section.
- Ignoring DoD chemistry. Draining lead-acid to 80% kills it fast. The chemistry table below is not optional.
- Ignoring inverter losses. If your device runs on AC (wall plug), the inverter burns 10–15% of the energy as heat — and often draws power even when idle. That’s the next section.
Why chemistry changes everything
“100Ah” means different real energy depending on what the battery is made of. The chemistry sets your safe DoD, which sets your usable Wh, which sets your runtime.
| Chemistry | Safe DoD | Usable Wh (100Ah @ 12V) | Typical cycle life |
|---|---|---|---|
| LiFePO4 (lithium) | 80–90% | 960–1,080Wh | 2,000–5,000 cycles |
| AGM / sealed lead-acid | 50% | ~600Wh | 500–1,000 cycles |
| Flooded lead-acid | 50% | ~600Wh | 300–700 cycles |
Two batteries both labeled “100Ah” can deliver very different usable energy — a lithium one gives you up to 1,080Wh usable, a lead-acid one only 600Wh. That’s a 44% difference in runtime for the same label. It’s also why lithium costs more: you pay for usable energy and cycle life, not just amp-hours. For the full trade-off, see our lithium-ion vs lead-acid comparison.
AC devices: the inverter tax
If your device plugs into a wall outlet, it runs on AC. Your battery outputs DC. The inverter that converts between them is not free — it costs you 10–15% of the energy as heat, and it draws a small idle current even with nothing plugged in.
Worked example, before and after the inverter:
A 100W AC device, no inverter math: 960 ÷ 100 = 9.6 hours.
With a realistic 85% efficient inverter: the battery must supply 100 ÷ 0.85 ≈ 118W. Runtime: 960 ÷ 118 ≈ 8.1 hours.
That’s 1.5 hours lost to the inverter — about 15%. For a small device like a 10W router, the inverter’s idle draw can matter more than the device itself, so a DC-powered router is often the smarter choice. When sizing an inverter, also check our battery cable size for inverter guide — undersized cable is a common hidden loss.
The lookup table
Here’s the runtime for a 100Ah 12V battery across common draws, using 80% DoD for lithium (960Wh usable) and 50% for lead-acid (600Wh usable). Every number is the formula — usable Wh ÷ watts — nothing else.
| Device (typical draw) | Watts | Lithium runtime | Lead-acid runtime |
|---|---|---|---|
| Wi-Fi router | 10W | 96h | 60h |
| CPAP (no humidifier) | 30W | 32h | 20h |
| LED TV | 60W | 16h | 10h |
| Laptop + monitor | 100W | 9.6h | 6h |
| Fridge (average running watts)* | 150W | 6.4h | 4h |
| Power tool / space heater | 300W | 3.2h | 2h |
| Microwave / kettle | 600W | 1.6h | 1h |
*The fridge row is the trap. A 150W fridge does not run at 150W continuously — it cycles. The 6.4h figure is what you’d get if it ran flat-out, which it never does. The real answer is in the next section.
The fridge asterisk: duty cycle decoded
A fridge’s compressor runs in cycles, not continuously. Typical duty cycle is 30–50% — the compressor is on only a third to half of the time. That changes everything.
Effective watts = running watts × duty cycle
A 150W fridge at 40% duty cycle draws an average of 150 × 0.40 = 60W. On lithium: 960 ÷ 60 = 16 hours — not 6.4. On lead-acid: 600 ÷ 60 = 10 hours.
So a 100Ah lithium battery can realistically run a modern fridge for about 16 hours, and a lead-acid one for about 10 hours — roughly 2.5× the naive table number. Duty cycle varies with ambient temperature, door openings, and fridge age, so treat these as estimates. For the full method of measuring your fridge’s real running watts, surge, and daily Wh, see our what size solar generator to run a refrigerator guide — we won’t duplicate its measuring steps here. And if the load is a medical device like a CPAP, our CPAP battery backup guide applies the same formula to that specific case.
Devices with surges and cycles
Three device types bend the formula, and each bends it differently:
Anything with a motor (fridge, freezer, pump, fan) has a surge — a brief start spike of 2–4× running watts. Surge matters for inverter sizing more than runtime: a 150W fridge might need 600W of surge capacity for a fraction of a second. The battery’s Wh math is unchanged — the surge is too brief to drain meaningful energy — but the inverter must be able to deliver it or the device never starts.
Anything with a thermostat (fridge, freezer, water heater) has a duty cycle — it cycles on and off to hold a setpoint. Use effective watts (running watts × duty cycle), not nameplate watts, for runtime.
Anything that charges (phone, laptop, power tool battery) draws a taper current — high at first, then dropping as the battery fills. A laptop might pull 60W for the first hour and 20W after. The formula still works, but your runtime is longer than the peak-watt math suggests because the average draw is lower.
From runtime to system: how fast will solar refill it?
Runtime tells you how long the battery lasts; solar tells you how fast it comes back. The full sizing method lives in our solar system sizing guide — here’s the one-paragraph version. If you drained 960Wh usable from a lithium battery, a 200W panel in 4 good sun hours at ~80% efficiency delivers about 200 × 4 × 0.8 = 640Wh — roughly two-thirds of a full recharge in one good day. A 100W panel delivers about half that, so it extends runtime but won’t keep up with heavy use.
One charging note: don’t slam a battery from zero at maximum current. Lithium batteries typically charge at 0.2–0.5C — for a 100Ah battery, that’s 20–50A — and lead-acid prefers gentler rates. Your charge controller or solar generator handles this automatically; just know that “from zero to full in one sunny afternoon” is usually optimistic.
Bigger/smaller batteries: scale the same math
The formula doesn’t change — only the Wh does. Here’s the quick scaling table at 12V, plus one 24V row to show why voltage matters.
| Battery | Total Wh | Usable Wh (lithium 80%) | Usable Wh (lead-acid 50%) |
|---|---|---|---|
| 50Ah @ 12V | 600 | 480 | 300 |
| 100Ah @ 12V | 1,200 | 960 | 600 |
| 200Ah @ 12V | 2,400 | 1,920 | 1,200 |
| 100Ah @ 24V | 2,400 | 1,920 | 1,200 |
The 10-second scaling rule: double the amp-hours, double the runtime. Double the voltage, double the runtime. A 100Ah 24V battery holds the same energy as a 200Ah 12V battery — 2,400Wh — because 100 × 24 = 200 × 12. That’s why higher-voltage systems use thinner cable and smaller batteries for the same energy. For the full trade-off, see our 12V vs 24V vs 48V solar guide, and for how battery capacity fits into a whole system, our battery capacity explainer.
FAQ
Can a 100Ah battery run a TV overnight?
What do two 100Ah batteries in parallel give me?
How low can I drain a 100Ah battery?
Does cold reduce battery capacity?
Image Prompts
Placement: H2-1, after the formula intro. Concept: a clean three-step formula pyramid — Ah × V → Wh → DoD → hours — with each step in a distinct colored block and a small battery icon at the base. Composition: pyramid centered, steps stacked vertically with arrows, minimal text labels, soft studio lighting, photorealistic battery at bottom, crisp vector overlay, 16:9.
Placement: H2-2, in the chemistry section. Concept: side-by-side comparison bars showing usable Wh per 100Ah — lithium at 960Wh versus lead-acid at 600Wh — with a subtle “wasted” shaded portion on the lead-acid bar. Composition: two vertical bars on a clean white background, lithium in green, lead-acid in gray, clear Wh labels, flat infographic style, readable at thumbnail size, 16:9.
Placement: H2-4, beside the lookup table. Concept: a horizontal runtime tape-measure graphic stretching across device icons — router, CPAP, TV, laptop, fridge, microwave — with each icon placed at its lithium runtime position on the tape. Composition: tape measure as the horizontal axis, device icons spaced along it, warm neutral palette, flat illustration style, clean and scannable, 16:9.