Amp-Hours vs Watt-Hours: Use Voltage to Estimate Outage Runtime

For outage planning, compare watt-hours (Wh), not amp-hours (Ah) alone. Watt-hours include battery voltage, so they can be compared with the wattage and operating time of lights, routers, fans and other essential loads.

Use this sequence:

1. Nominal watt-hours = volts × amp-hours
2. Usable watt-hours = nominal Wh × usable-capacity fraction
3. Available AC watt-hours = usable Wh × inverter efficiency
4. Estimated runtime = available Wh ÷ load watts

The result is a planning estimate, not guaranteed runtime.

Amp-hours and watt-hours measure different things

An amp-hour measures electric charge. For example, a 10-amp load operating for two hours removes 20Ah. A watt-hour measures energy and includes voltage, which is why it is more useful when comparing battery systems or planning loads. Victron’s battery-monitor documentation explains the current-times-time calculation for Ah and notes that effective capacity changes with discharge rate and temperature (Victron SmartShunt manual).

Rating	What it tells you	Best use
Ah	Charge delivered over time	Comparing batteries at the same voltage
Wh	Energy capacity, including voltage	Comparing different-voltage batteries and estimating runtime
W	Power used or supplied at a moment	Checking whether an inverter can operate a load

Two 100Ah batteries do not necessarily store the same nominal energy:

• 12V × 100Ah = 1,200Wh
• 24V × 100Ah = 2,400Wh
• 48V × 100Ah = 4,800Wh

This is why “100Ah” by itself is not enough. Use the battery’s labeled nominal voltage, not a momentary meter reading. A battery labeled 12.8V and 100Ah, for example, has a nominal capacity of 1,280Wh rather than 1,200Wh.

The conversion Wh = V × Ah and the load calculation W × hours = Wh also appear in Renogy’s battery-sizing guidance.

Account for usable capacity and conversion losses

The nameplate calculation gives nominal energy. It does not mean every watt-hour will reach an AC appliance.

Suppose you have a 12V, 100Ah battery:

• Nominal energy: 12V × 100Ah = 1,200Wh
• Planned usable capacity: 1,200Wh × 80% = 960Wh
• After an assumed 90% inverter efficiency: 960Wh × 90% = 864Wh available to AC loads

At a steady 60W load:

864Wh ÷ 60W = 14.4 hours

Treat the 80% usable capacity and 90% inverter efficiency as example inputs—not universal specifications. Use the discharge limit, efficiency information and cutoff settings in the manuals for your exact battery and inverter. For a DC load that does not pass through an inverter, omit inverter efficiency but include losses from any DC converter.

A battery monitor can improve the estimate, but it cannot make runtime certain. The Victron monitor cited above calculates state of charge from current flow while accounting for factors including Peukert efficiency and charge efficiency. Its manual says the time-remaining display should be treated only as a guideline when the load fluctuates heavily.

Quick planning table for a 12V, 100Ah battery

This table starts with 1,200Wh of nominal capacity and assumes 90% inverter efficiency. Choose only the usable fraction permitted by the battery manufacturer.

Planned usable capacity	AC energy after 90% efficiency	20W load	60W load	120W load
50%	540Wh	27 hr	9 hr	4.5 hr
80%	864Wh	43.2 hr	14.4 hr	7.2 hr
100%	1,080Wh	54 hr	18 hr	9 hr

Do not automatically select 50%, 80% or 100% based only on broad labels such as “lead-acid” or “lithium.” Construction, age, operating conditions and the manufacturer’s limits matter. Trojan Battery’s maintenance guidance notes that battery type, equipment loads, cable size and climate vary by system and recommends using the equipment manufacturer’s battery specification.

Add the actual outage loads

For each device, multiply watts by expected operating hours, then add the results:

Essential load	Example draw	Planned use	Energy needed
Router	12W	12 hr	144Wh
LED lamp	8W	5 hr	40Wh
Fan	35W	8 hr	280Wh
Total			464Wh

These are illustrative inputs. Measure each device or consult its label and manual. Cycling equipment such as a refrigerator does not draw its running wattage continuously, while motors may briefly require substantially more power at startup.

To reverse the calculation and size a battery for this 464Wh plan, assuming 80% usable capacity and 90% inverter efficiency:

464Wh ÷ 0.80 ÷ 0.90 = 644Wh minimum nominal capacity

That is the mathematical minimum under those assumptions. Add a reserve for battery aging, cold conditions, variable loads and an outage lasting longer than planned.

Capacity and output are separate checks:

• Wh determines approximate duration.
• Continuous W determines whether the inverter can carry the running load.
• Surge W determines whether it can start a motor or compressor.

A large Wh rating does not guarantee enough starting power. Conversely, a high-watt inverter attached to a small battery may operate a heavy load only briefly.

UPS runtime should be checked against the manufacturer’s load-specific runtime information rather than calculated from battery labels alone. Schneider Electric says runtime depends on both the supported load and the technical characteristics of the particular UPS (APC UPS Selector guidance).

The practical comparison rule

When comparing batteries for an outage:

• Convert Ah to Wh using each battery’s nominal voltage.
• Apply the manufacturer-permitted usable capacity.
• Account for inverter or converter losses.
• Divide available Wh by the measured or documented load in watts.
• Check continuous and surge output separately.
• Keep a reserve rather than planning to the calculated limit.

If a portable power station already lists capacity in Wh, start with that figure instead of comparing its internal Ah rating. For more on the relationship between charge, energy and runtime, see Battery Charge: Usable Energy for a Device. If the outage may last longer than a battery can cover, compare recharging and fuel options in Portable Power Station vs Generator for PSPS Events.