Battery labels can make capacity comparisons confusing. This guide explains voltage, amp-hours, watt-hours, usable energy, and runtime with repeatable formulas, reference conversions, a load-estimation worksheet, and worked examples for household, automotive, RV, solar, and portable-power applications.
Overview
Battery capacity is the amount of energy a battery can store, but manufacturers may describe it in different ways. Small rechargeable batteries are commonly labeled in milliamp-hours (mAh), deep-cycle batteries in amp-hours (Ah), and power stations in watt-hours (Wh). Voltage (V) describes electrical pressure; amps (A) describe current; amp-hours describe how much current a battery can provide over time; and watt-hours describe energy in a form that is easier to compare with a device's power demand.
The basic conversion is:
Watt-hours (Wh) = Volts (V) × Amp-hours (Ah)
For a device with a known power draw, a simple runtime estimate is:
Runtime in hours = Usable watt-hours ÷ Device watts
These are starting estimates rather than guarantees. Actual results depend on inverter losses, battery age, temperature, discharge rate, battery chemistry, the device's duty cycle, and the amount of capacity that the battery management system or manufacturer allows you to use.
Battery capacity reference table
| Nominal voltage | Rated capacity | Approximate nominal energy | Common context |
|---|---|---|---|
| 1.2 V | 2,000 mAh (2 Ah) | 2.4 Wh | Rechargeable AA NiMH cell |
| 3.7 V | 5,000 mAh (5 Ah) | 18.5 Wh | Single lithium-ion cell or small battery pack reference |
| 12 V | 50 Ah | 600 Wh | Small lead-acid or lithium auxiliary battery |
| 12.8 V | 100 Ah | 1,280 Wh | Typical nominal LiFePO4 battery specification |
| 24 V | 100 Ah | 2,400 Wh | Series-connected battery system or 24 V storage bank |
| 48 V | 100 Ah | 4,800 Wh | Higher-voltage solar or backup battery system |
The figures in this table use nominal voltage. A battery's measured voltage changes during charging and discharging, so nominal voltage is a practical comparison value, not a constant operating voltage.
How to estimate battery runtime
Start by identifying the device's actual power requirement. Look for a watt rating on the product label, power adapter, user manual, or manufacturer specification. If only volts and amps are listed, use:
Watts = Volts × Amps
For USB equipment, a 5 V device drawing 2 A uses approximately 10 W at that input. A laptop adapter marked 20 V and 3.25 A has a maximum rated input of approximately 65 W, although the laptop may draw less during light work and more while charging or performing demanding tasks.
Next, convert the battery rating to watt-hours. If the battery is labeled in mAh, divide by 1,000 to get Ah before multiplying by voltage:
Wh = V × (mAh ÷ 1,000)
Then apply a usable-energy factor. For a direct-current load, this may account for the battery's recommended depth of discharge and conversion losses. For an alternating-current load powered through an inverter, also allow for inverter efficiency. A useful planning formula is:
Estimated runtime = Rated Wh × Usable-capacity factor × Conversion-efficiency factor ÷ Load watts
For an initial estimate, you might model a lead-acid system at a lower usable fraction than a lithium system because regularly using all of its rated capacity may shorten service life. The correct factor depends on the product instructions, operating conditions, and your priorities. Use the battery maker's stated usable capacity when available rather than assuming every labeled watt-hour is available to the load.
Inputs and assumptions
Before comparing batteries, record the following inputs. This prevents a high amp-hour number at one voltage from appearing more capable than a lower amp-hour number at a different voltage.
- Battery voltage: Use the nominal voltage listed by the manufacturer. Do not combine the Ah rating of one voltage system with the voltage of another.
- Rated capacity: Convert mAh to Ah, then calculate nominal Wh. Check whether the rating is measured under a specified test condition.
- Load power: Use watts when possible. For motors, compressors, refrigerators, pumps, and other inductive loads, account for startup demand as well as running demand.
- Duty cycle: A refrigerator, furnace fan, or pump may not run continuously. Estimate average watts over the period you care about, while still checking peak demand.
- Usable capacity: Leave room for the battery's recommended depth of discharge, low-temperature limits, and automatic shutoff settings.
- Conversion losses: Include an inverter, DC converter, USB regulator, or other electronics between the battery and load.
- Conditions: Temperature, battery age, charging history, cable size, and high discharge rates can all change the result.
Simple load-estimation worksheet
Copy this worksheet for each use case:
- Device or appliance: __________
- Running watts: __________ W
- Peak or startup watts, if relevant: __________ W
- Hours of operation: __________ h
- Daily energy required: running watts × hours = __________ Wh
- Battery voltage and rated capacity: __________ V, __________ Ah
- Nominal battery energy: volts × amp-hours = __________ Wh
- Estimated usable and efficiency factor: __________
- Estimated delivered energy: nominal Wh × factor = __________ Wh
- Estimated runtime or days of backup: delivered Wh ÷ required Wh = __________
For multiple devices, calculate each device's watt-hours and add them. If they run at the same time, add their running watts to check the inverter or power station's continuous output rating. Check peak output separately.
Worked examples
Example 1: A 12 V lead-acid battery and a 30 W accessory
A 12 V, 50 Ah battery has a nominal capacity of 12 × 50 = 600 Wh. Suppose your planning assumption makes 50% of that capacity usable, and a converter is 90% efficient. Delivered energy is approximately 600 × 0.50 × 0.90 = 270 Wh. A 30 W accessory would therefore run for about 270 ÷ 30 = 9 hours under these assumptions.
This is a planning estimate. A higher current draw, cold weather, an aging battery, or a different depth-of-discharge target can reduce the result. For a starter battery, do not treat the calculated capacity as permission to deeply discharge it; starting batteries and deep-cycle batteries are designed for different jobs.
Example 2: A 12.8 V, 100 Ah LiFePO4 battery for an RV load
The nominal energy is 12.8 × 100 = 1,280 Wh. If your design uses an assumed 90% usable fraction and a 90% inverter efficiency, estimated delivered energy is 1,280 × 0.90 × 0.90 = 1,036.8 Wh. A combined AC load averaging 120 W could run for approximately 1,036.8 ÷ 120 = 8.6 hours.
Confirm that the battery's battery management system supports the required continuous and surge current. Read the product documentation for low-temperature charging restrictions and charging equipment compatibility. For a broader chemistry comparison, see our AGM versus LiFePO4 battery guide.
Example 3: A portable power station for a laptop and phone
A power station rated at 500 Wh does not normally deliver 500 Wh to AC outlets because its inverter and internal electronics consume some energy. If you use a planning factor of 0.85, the estimated AC energy is 500 × 0.85 = 425 Wh. A laptop averaging 65 W and a phone charger averaging 10 W together require 75 W while both are operating. Estimated runtime is 425 ÷ 75 = 5.7 hours.
Direct USB-C or DC outputs may produce a different result from the AC outlet because they avoid, or change, some conversion stages. Compare like with like when reading portable power station reviews.
When to recalculate
Revisit your calculation whenever an input changes. Recalculate after replacing a battery with a different voltage, chemistry, Ah rating, or usable-capacity specification. Do the same when adding a refrigerator, inverter, solar controller, CPAP machine, heater, pump, or other significant load to an RV or backup system.
Update the estimate as a battery ages or as real runtime differs from the model. If the battery reaches low-voltage shutdown sooner than expected, check the load's actual wattage, cable connections, inverter efficiency, temperature, and discharge settings before simply buying more capacity. Also recalculate when changing from an AC power supply to a direct DC or USB connection.
For a buying decision, use this final checklist:
- List every device and its running and startup watts.
- Convert each battery option to watt-hours using its nominal voltage.
- Apply realistic usable-capacity and efficiency assumptions.
- Check continuous and surge output ratings, not capacity alone.
- Verify chemistry-specific charging, temperature, installation, and safety requirements.
- Leave practical reserve capacity instead of sizing to the exact calculated minimum.
Capacity calculations help compare products, but they do not replace the manufacturer's installation and charging instructions. Keep the battery type, charger, wiring, fuses, and load within their specified limits. When your devices or operating conditions change, repeat the worksheet so your battery choice remains appropriate.
