Energy Meaning
Watt hours describe stored energy. Milliamp hours describe charge at a chosen voltage. The two values are linked, but they are not equal alone. Voltage must be known before a fair conversion is possible. This calculator helps compare power banks, battery packs, small electronics, lights, tools, and backup systems.
Why Voltage Matters
A ten watt hour pack can show different milliamp hour ratings at different voltages. At five volts, it equals more charge than it does at twelve volts. This is why battery labels often confuse buyers. A power bank may advertise capacity at cell voltage, while output happens at another voltage. Always match the voltage used in your estimate.
Efficiency And Real Use
Real devices lose energy during conversion. Boost circuits, cables, heat, and charging electronics reduce usable output. The efficiency option lets you include that loss. Use one hundred percent for an ideal conversion. Use eighty five to ninety five percent for many good converters. Use lower values for poor adapters or hot conditions.
Planning Better Batteries
The result can support trip planning, repair work, solar projects, and portable charging estimates. It can also help compare two packs with different labels. Use nominal voltage for the battery side. Use output voltage for delivered energy estimates. For runtime planning, divide the final milliamp hours by the device current in milliamps.
Good Input Habits
Enter watt hours from a label, test report, or specification sheet. Enter voltage carefully. Small voltage errors can make large capacity errors. Choose sensible decimal places for clean results. Save the CSV when you need spreadsheet records. Save the PDF when you need a simple report for notes, clients, or team review.
Common Battery Context
Phone cells often use a nominal voltage near 3.7 volts. USB output commonly uses 5 volts. Laptop packs may use higher series voltages. Tool batteries may be 12, 18, 20, or 40 volts. These labels can be marketing names, so check the real nominal voltage when precision matters. Better voltage data creates better milliamp hour estimates.
Safety Note
Do not exceed rated voltage or current. Use protected cells and suitable chargers. Confirm pack limits before connecting loads. Capacity math helps planning, but safe wiring and manufacturer guidance still matter.