A typical power bank draws wall energy close to its Wh rating plus 10–30% losses, so a 10,000 mAh unit uses about 40–50 Wh per full charge.
Wondering what a portable battery adds to your energy bill? You can pin it down with a few simple numbers. Below you’ll see the math, clear examples, and a quick table.
Power Bank Electricity Use – Quick Method
Every pack lists milliamp-hours (mAh). Inside, the cells sit at about 3.6–3.7 volts. Multiply mAh by 3.7 and divide by 1,000 to get watt-hours (Wh). That’s the stored energy. Charging isn’t perfect, so add charging and converter losses. The math stays simple overall.
The Three-Step Rule
- Find Wh: capacity (mAh) × 3.7 ÷ 1,000.
- Add losses: multiply by 1.1 to 1.3 for charge and conversion overhead.
- Convert to kWh: divide Wh by 1,000 to estimate cost with your local rate.
The ranges cover charger efficiency, pack electronics, and heat. Better gear lands lower; budget gear higher.
Typical Energy And Cost By Size
Use this as a fast reference for one full charge from empty. Local rates vary, so the cost is a small fraction of a cent in many places.
| Rated Capacity | Stored Energy (Wh) | Wall Energy (Wh)* |
|---|---|---|
| 5,000 mAh | 18.5 Wh | 20–24 Wh |
| 10,000 mAh | 37 Wh | 41–48 Wh |
| 20,000 mAh | 74 Wh | 81–96 Wh |
| 27,000 mAh | 99.9 Wh | 110–130 Wh |
| 40,000 mAh | 148 Wh | 163–192 Wh |
*Includes a 10–30% allowance for losses during charging and DC-DC conversion.
Why Wh, Not mAh, Tells The Real Story
mAh alone skips voltage, so it can mislead. Wh bakes voltage into the number and matches what your meter and bill use. Most cells in these packs are rated near 3.7 V. That’s why a 10,000 mAh pack stores around 37 Wh, not 50 Wh. If a box prints Wh, prefer that figure every time.
Backed By Battery Standards
The industry treats a single lithium-ion cell as 3.6–3.7 V nominal. Multiply mAh by that voltage to get Wh, the correct energy unit. You can see this explained in Battery University’s article on nominal voltage and Wh math. For typical energy prices, the U.S. Energy Information Administration lists monthly averages in its average retail rates.
Real Examples You Can Copy
Example 1: Everyday 10,000 mAh Pack
Wh = 10,000 × 3.7 ÷ 1,000 = 37 Wh. Add 20% losses → 44.4 Wh from the wall. In kWh, that’s 0.0444. At $0.15 per kWh, cost per full charge sits near $0.0067—two-thirds of a cent.
Example 2: Big 20,000 mAh Pack
Wh = 20,000 × 3.7 ÷ 1,000 = 74 Wh. Add 15% losses → 85.1 Wh = 0.0851 kWh. At $0.30 per kWh (some cities), one full charge is about $0.0255—two and a half cents.
Example 3: Travel-Size 5,000 mAh
Wh = 5,000 × 3.7 ÷ 1,000 = 18.5 Wh. Add 25% losses → 23.1 Wh = 0.0231 kWh. At $0.10 per kWh, that’s a quarter of a cent.
What Affects How Much Energy The Wall Sees
Charger And Cable Quality
Good USB-C PD chargers waste less energy than bargain bricks. Long, thin cables drop voltage and create heat, which means the pack may draw longer to reach full. A shorter, well-made cable can shave a bit off the total.
Pack Efficiency And Electronics
Inside the case, the battery sits at cell voltage and a step-up converter delivers 5–20 V to your phone or laptop. During charging the pack, another converter takes wall DC to the cell. Each step loses a slice of energy. Many well-built packs land near 80–90% round-trip efficiency under light to moderate loads, and lab tests on specific models often chart numbers in that range.
Charge Rate And Heat
Fast input (say 45–65 W) shortens time, but a hard push can raise losses a bit. Slower input takes longer; the overhead from lights and quiescent circuits adds up. The difference is small for pocket packs yet can show on jumbo models.
State Of Charge
Packs pull current aggressively at first, then taper near the top. Leaving the pack at 100% on the charger for hours wastes trickle overhead. Unplug soon after the indicator says full.
How To Estimate Charge Time
Divide the pack’s Wh by the charger’s real input watts, then adjust for losses. If your pack accepts 30 W and your charger can deliver it, time ≈ Wh ÷ (0.9 × input W). For smaller wall warts, bump the denominator down to 0.8 to account for conversion and taper.
Charger Power Vs. Filling Time
| Charger Power | 10,000 mAh Time* | 20,000 mAh Time* |
|---|---|---|
| 18 W | ~2.5–3.0 h | ~5–6 h |
| 30 W | ~1.5–2.0 h | ~3–4 h |
| 45 W | ~1.2–1.6 h | ~2.5–3.5 h |
| 65 W | ~1.0–1.4 h | ~2–3 h |
*Assumes healthy cable, a pack that accepts the listed input, and ~85–90% end-to-end charging efficiency.
How Much Does Using The Pack On Devices Waste?
When you charge a phone from the pack, energy again passes through converters and heat losses. If a pack stores 74 Wh, you won’t deliver 74 Wh to the phone—more like 60–66 Wh with a good unit. Review labs that log input and output energy often publish efficiencies in the 75–85% zone, and some premium models do a bit better at moderate power.
What About Leaving It Plugged In?
Many packs sip power while sitting full—LEDs and standby chips draw tiny current. It’s small, yet over days it adds up. Unplug the wall charger after topping up to avoid background drain.
Self-Discharge: Energy Lost While Sitting
Lithium-ion chemistry loses a small slice while stored. The common pattern: a few percent right after charging, then about 1–2% per month, with protection circuits adding around 3% per month. That’s why a drawer-stored pack seems a bit low weeks later.
Practical Storage Tips
- Store near mid-charge for long breaks.
- Top up every couple of months.
- Avoid hot dashboards and heaters.
Step-By-Step: Figure Your Own Cost
Grab Two Numbers
- Your pack’s capacity in mAh (or Wh if printed).
- Your energy price in kWh from the bill or utility site.
Do The Math
- Convert to Wh if needed: mAh × 3.7 ÷ 1,000.
- Add 10–30% to cover losses.
- Divide by 1,000 for kWh and multiply by your rate.
That number is your cost per full charge. Even for big packs, the figure is usually pennies at most.
Ways To Trim Waste Without Sacrifice
Pick A Solid Charger
Use a quality USB-C PD charger near the pack’s rated input. Oversized bricks can idle less efficiently at light load, and undersized ones stretch charge time.
Match Cable To Power
Thick, short USB-C cables reduce voltage drop at higher watts, which keeps conversions in their sweet spot.
Charge In Cooler Rooms
Heat is the enemy of both lifespan and efficiency. A cool desk beats a sunny windowsill.
Don’t Trickle Forever
Unplug after the pack is full. Leaving it on the wall encourages tiny, endless top-off cycles.
Method Notes And Sources
Voltage and Wh math: industry references cite 3.6–3.7 V per Li-ion cell, and Wh = V × Ah. For price context, check the monthly average rates from your utility or national statistics office. Self-discharge figures reflect typical lithium-ion behavior: a few percent drop soon after charging, then about one to two percent per month, with protection circuits adding three percent monthly.