Measure a phone top-up at the USB port

Choose a top-up that resembles your trip

If you typically charge from 35% to 75%, measure that interval rather than running the phone flat just for a test. Note the start and end battery percentages and the usual phone settings. A measurement with the screen off will be different from one while you are using a map and recording a track.

Use a meter, cable and charger that are designed to work together at the equipment’s supported voltage, current and charging mode. Follow their instructions. A meter may change how a charger and phone negotiate a charge, so keep the measurement arrangement consistent when comparing two tests. Avoid a damaged cable or port.

Write down the energy rather than the peak reading

A meter showing a momentary power of 9 W has not yet told you how much energy the charge uses. Watt-hours account for time as well as power. For example, a steady 6 W over 45 minutes would deliver 4.5 Wh. An actual phone charge may vary in power as it proceeds. Use the accumulated energy readout if available.

If a meter reports only charge, follow the mAh to Wh note. The relevant voltage is at the measurement point. A voltage that changes during a test is a reason to prefer the integrated watt-hour figure over multiplying the final charge by one guessed voltage.

Use the result on one side of the budget

Suppose the chosen top-up uses 7 Wh at the port. Three similar top-ups are a 21 Wh demand on that side of the budget. A bank labelled 37 Wh at the cells may deliver less than that label at the port. Estimating the number of charges explains how that difference enters the arithmetic.

Run the assumption in Power budget or compare it with a measured output from the bank. Do not subtract a conversion loss a second time from an output value that already includes it. Leave separate energy for the use you would prioritise in a delay, as in the reserve note.

Repeat the condition that matters

Do another pass with the screen and map behaving as they would on the trip. If reception is limited, run the offline map test and write down which radios were active. Compare the amount used and keep the test conditions with the result. A small spreadsheet can be useful even if it has only two rows.

Remember what the meter did not see. It did not measure the phone’s full day in the pack or the power needed for other devices. Keep those as distinct items. If a future cable or phone changes the result, retest the one top-up before reusing the old budget.

Reset the accumulated reading before the next measurement. If a display also records a maximum power, give it a separate column. It helps explain a charger difference but cannot replace the energy total used in the budget.