Battery Sizing Sheet

The arithmetic

WATT-HOURS

amp-hours x volts (mAh ÷ 1000 x V)

RUNTIME

usable Wh ÷ average watts

Usable Wh

Wh x depth % x efficiency %

Single conversion

about 90% efficient

Battery to battery

75-80% — losses at BOTH ends

Lithium depth

nearly all of it

Lead-acid depth

50% — below that shortens its life

⚠ 10,000 mAh at 3.7 V

is about 6,660 mAh at USB 5 V

⚠ Cold

cuts capacity; CHARGING below 0°C damages it

Batteries and loads

This setup

Device

Date

Capacity mAh

Voltage

ENERGY (Wh)

Discharge depth %

Efficiency %

USABLE Wh

Average draw W

RUNTIME

At USB 5 V (mAh)

Measured runtime

Sizing a battery honestly

• Compared in WATT-HOURS, not mAh
• Average draw used, not the idle or the peak figure
• Depth of discharge set for the chemistry — 50% for lead-acid
• Losses counted at BOTH ends if charging another battery
• Inverter losses included if running mains equipment
• Age allowed for — an old pack holds well under its label
• Cold weather considered, and charging below freezing avoided
• Result checked against a real measured runtime where possible

The power bank surprise

A 10,000 mAh pack holds about 37 Wh at its 3.7 V cell voltage. Delivered through a 5 V USB port at 90% efficiency that is roughly 33 Wh, or about 6,660 mAh at 5 V — two thirds of the number on the box, before the phone loses a little more converting it back down. The label is not dishonest; it is measured at the cell voltage, and the port runs at a different one.

Half of a lead-acid battery is not yours

Routinely discharging lead-acid below about 50% shortens its life dramatically, so a 100 Ah battery should be planned as 50 Ah. This is the most common reason an off-grid or backup system ends up sized at half what it needed — and lithium behaves completely differently, which is why the setting is a question rather than an assumption.