High-capacity power banks for field teams: runtime maths

Key Takeaways

• Voltage conversion and heat losses typically consume 15-25% of a power bank's rated mAh capacity, so the number printed on the device is never fully available to your equipment.
• Watt-hours (Wh), not mAh, is the reliable unit for comparing power banks across different devices, since mAh alone doesn't account for voltage, and converting requires multiplying mAh by voltage and dividing by 1000.
• Real-world charging efficiency for most electronics runs 80-90%, which needs to be factored in alongside the power bank's own internal losses to get an honest runtime estimate.
• The basic runtime formula, capacity (mAh) ÷ device draw (mA), only works when both figures are at the same voltage; for cross-voltage comparisons (a 5V phone vs a 12V field device), the Wh-based formula is the one that actually holds up.

A power bank rated at 20,000 mAh sounds like it should charge a 4,000 mAh device five times over. In practice it charges it closer to three and a half times, not because the manufacturer overstated the rating, but because two separate loss mechanisms, internal conversion losses and device charging inefficiency, both eat into that number before it reaches the device.

Why mAh alone doesn't tell the real story

The fundamental runtime formula is straightforward: runtime (hours) equals battery capacity (mAh) divided by device consumption (mA), so a 3,000 mAh battery powering a device drawing 150 mA lasts approximately 20 hours (Miniwebtool, battery life calculator, retrieved 2026-09-10). That formula only holds cleanly when both the battery and device operate at the same voltage. Power banks and field devices frequently don't, which is why mAh alone is an incomplete unit for comparing or sizing across different equipment: it doesn't carry voltage information, and two batteries with identical mAh ratings at different voltages store meaningfully different amounts of actual energy.

Converting to watt-hours: the unit that actually compares fairly

Watt-hours is a more reliable basis for comparison because it already incorporates voltage: to convert, multiply the capacity in milliampere-hours by the voltage, then divide by 1000 (ProCalcLab, power bank watt-hour calculator, retrieved 2026-09-10). A 20,000 mAh power bank rated at 3.7V internal cell voltage stores roughly 74 Wh, and that Wh figure is what should be compared against a device's actual power draw in watts, rather than comparing mAh ratings across devices that may run at different voltages internally.

The two loss mechanisms that reduce real-world runtime

Voltage conversion and heat losses eat 15-25% of the rated capacity in typical power banks, arising from the boost/buck conversion circuitry needed to step the internal cell voltage up or down to match the output port's voltage standard (VoltCalcs, power bank runtime calculator, retrieved 2026-09-10). Separately, on the receiving end, real-world device charging efficiency runs 80-90% for most electronics, accounting for further losses in the device's own charging circuitry (Symbo Battery, how to calculate battery runtime, retrieved 2026-09-10). These two loss mechanisms compound rather than substitute for each other: a power bank losing 20% internally, feeding a device that's 85% efficient at receiving that charge, delivers roughly 0.8 × 0.85 ≈ 68% of the rated capacity as genuinely usable charge, not 80% or 85% alone.

The full runtime formula for field equipment

For field devices specifically, the reliable formula is: runtime equals battery watt-hours multiplied by overall efficiency, divided by device wattage (Symbo Battery, retrieved 2026-09-10). This requires knowing the device's actual power draw in watts (not just its charging current), and applying a realistic combined efficiency figure (accounting for both the power bank's internal losses and the device's charging efficiency) rather than assuming the full rated capacity is available. Run your specific device's wattage and the power bank's rated Wh through the power bank runtime calculator using this formula, rather than the simpler mAh ÷ mA shortcut, whenever voltage differs between power bank and device.

What this means for equipping a field team

For a field team relying on a shared power bank to keep multiple devices running across a working day, sizing based on the printed mAh rating without applying the loss corrections above routinely overestimates how many device-charges the bank actually delivers, sometimes by 30% or more once both loss mechanisms are combined. Build the day's power budget from each device's actual watt-hour need, apply a realistic 65-75% combined efficiency factor to the power bank's rated capacity, and confirm the connector, voltage protocol, and output wattage the power bank supports actually match what each field device requires, since a capacity mismatch on paper is meaningless if the connector or protocol doesn't support the device's actual charging profile. Once the day's watt-hour budget is worked out, comparing options against the high-capacity power bank range narrows the field faster than shopping by mAh rating alone, since connector and output wattage compatibility rule out most listings before capacity even becomes the deciding factor.

Frequently asked questions

Why does my 20,000 mAh power bank only charge my phone about 3.5 times instead of 5?

Because voltage conversion losses in the power bank (typically 15-25%) and charging inefficiency in the phone itself (80-90% efficient) both reduce the usable energy delivered, compounding to leave roughly 65-75% of the rated capacity genuinely available as device charge.

Is mAh or Wh the better spec to compare when choosing a power bank?

Watt-hours, since it already accounts for voltage and gives an apples-to-apples comparison across devices and power banks operating at different voltages. mAh alone can be misleading when comparing products with different internal voltage designs.

How do I size a power bank for a field device with a different voltage than my phone?

Convert both the power bank's capacity and the device's power draw to watt-hours and watts respectively, then use the Wh-based runtime formula (Wh × efficiency ÷ device watts) rather than the simpler mAh ÷ mA formula, which only works reliably when both are at the same voltage.

The bottom line

The number printed on a power bank is a starting point, not the usable capacity. Voltage conversion losses and device charging inefficiency together typically claim 25-35% of that rated figure, and sizing a field team's power budget without accounting for both is the most common reason a "big enough" power bank runs out before the shift does.

Figures were verified on 10 September 2026 against published power bank and battery runtime calculation sources. Actual efficiency losses vary by specific power bank and device model; confirm manufacturer efficiency specifications where available before finalising a field power budget.