Charging a typical 10,000 mAh power bank costs only a few cents, based on your local price per kWh.
Here’s the short version: a battery pack’s energy is measured in watt-hours (Wh). Multiply its milliamp-hours by 3.7 volts, divide by 1,000, then multiply by your electricity rate. Add a small margin for losses and you’ve got the bill to refill it from empty.
Quick Answer, Then The Method
The math is simple at home. Use this formula:
Formula You Can Trust
Cost = (mAh × 3.7 ÷ 1000 ÷ η) × Price_per_kWh
Where η (eta) is the overall charging efficiency. Real setups land in 0.85–0.95, combining cell efficiency and electronics losses. Lithium-ion cells sit around 3.6–3.7 V nominal, and modern boost/buck converters in packs and chargers can hit peak efficiencies around the 90% mark. That’s why a small efficiency range gives realistic, wallet-level numbers.
Early Estimates For Popular Sizes
To ground things, we’ll use the latest U.S. residential average price published by the Energy Information Administration and show round-number costs for common capacities. If your rate is different, the next sections show how to slot yours into the same equation.
| Capacity (mAh) | Energy Stored (Wh) | Estimated Cost Per Full Charge (US avg) |
|---|---|---|
| 5,000 | 18.5 | $0.04–$0.05 |
| 10,000 | 37 | $0.07–$0.08 |
| 20,000 | 74 | $0.13–$0.16 |
| 27,000 | 99.9 | $0.18–$0.21 |
These ranges assume the July 2025 U.S. residential average of about 17.5¢/kWh and an efficiency between 85% and 95%. If your utility is higher or lower, your numbers scale linearly.
Why The Numbers Look Small
Portable batteries store modest energy compared with home appliances. A 10,000 mAh pack holds ≈ 37 Wh. That’s a tiny fraction of a kilowatt-hour, so even pricey regions still land in pocket-change territory. The only time the bill nudges up is when electricity unit rates spike.
Where The Watts Go
Cell Voltage And Energy
Most packs use a single lithium-ion cell (or cells in parallel) with a nominal voltage around 3.6–3.7 V. Energy in watt-hours equals voltage times amp-hours. That’s why you convert mAh at 3.7 V to Wh before you multiply by your tariff.
DC-DC Conversion And Heat
Power banks step voltage up or down for USB-A, USB-C, or USB-C PD. Those converters aren’t perfect; real devices often show mid- to high-90s peak efficiency on datasheets, and lower figures under heavy load or at the extremes. Add small charger losses on the wall side and the combined η lands in the 0.85–0.95 band used above.
Work Through One Complete Example
Say you’ve got a 20,000 mAh model. First, convert to watt-hours: 20,000 × 3.7 ÷ 1000 = 74 Wh stored. With a typical 90% overall efficiency, energy drawn from the wall is 74 ÷ 0.90 ≈ 82.2 Wh, or 0.0822 kWh. At 17.5¢/kWh, the cost is 0.0822 × $0.175 ≈ $0.0144—about one and a half cents. With a less efficient setup at 85%, it’s still roughly two cents.
Use Your Own Tariff In Seconds
Step-By-Step
- Find capacity on the label (mAh).
- Compute Wh: mAh × 3.7 ÷ 1000.
- Pick η = 0.90 (safe middle), or 0.85 if your pack runs warm while charging.
- Wall energy = Wh ÷ η, then divide by 1000 for kWh.
- Multiply by your local price per kWh.
Regional Snapshot: UK Example
Unit rates in Great Britain are governed by the price cap for standard variable tariffs. For October–December 2025, the typical direct-debit electricity unit rate is around 26.35 p/kWh, varying by region. Plug that into the same math and you’ll see costs ≈1.5× the U.S. average shown earlier.
What Changes The Bill
Fast Charging Profiles
USB-C PD raises voltage and power briefly during the constant-current phase. The electronics juggle conversion on both the wall charger and the pack, which adds loss. With quality parts, the impact is small—usually cents over months, not dollars.
Ambient Temperature
Batteries are happier at moderate temperatures. Hot garages or sun-baked dashboards waste some energy as heat during charge and reduce long-term health.
Old Cells And Aging
A pack that’s seen hundreds of cycles may show more internal resistance. That raises losses slightly and shortens runtime per charge, which can nudge cost per usable Wh up a hair.
Close Variant Keyword: Charging A Power Bank Cost — Realistic Ranges
People ask what they’ll actually pay at the outlet for common sizes. Here’s a focused look using today’s representative unit prices, so you can eyeball your bill in under a minute.
| Unit Rate | 10,000 mAh Cost | 20,000 mAh Cost |
|---|---|---|
| US avg ~$0.175/kWh | $0.07–$0.08 | $0.13–$0.16 |
| GB typical ~£0.2635/kWh | £0.09–£0.11 | £0.18–£0.21 |
How To Lower The Tiny Cost Even Further
Charge In Cooler Rooms
Cooler cells waste less energy. Indoors beats a hot car.
Use A Quality Wall Charger
Well-designed adapters convert AC to DC more efficiently and hold efficiency across a wide range of loads. That trims losses before the pack even sees the power.
Stop At 80–90% When You Can
If you don’t need a full battery for the day, partial charges keep heat lower and extend cell life. That reduces how often you cycle the pack, which means fewer total kilowatt-hours over the year.
Capacity Labels, Wh, And Why USB Numbers Look Different
Most labels print milliamp-hours at the battery’s native voltage near 3.7 V. USB outputs at 5 V or higher for fast protocols. When you raise voltage, you lower available amp-hours; the constant is energy in watt-hours. That’s why an honest calculation starts at Wh, not mAh.
Troubleshooting: Your Pack Seems To Cost More
Warm To The Touch While Charging
Heat points to conversion losses. If it’s hot, η is closer to 0.85 than 0.95. Re-run the math with 0.85.
Charging Through A Weak Cable
Thin, long leads drop voltage at higher currents. The pack compensates by drawing more current, wasting a bit more as heat in the cable. Try a short, certified USB-C lead.
High Local Rates
Some regions bill well above the averages shown here. Use the formula and your actual unit price; the result scales one-for-one.
Method, Sources, And Assumptions
Energy price data comes from official sources. The U.S. residential average around 17.5¢/kWh is from EIA’s Electric Power Monthly (July 2025). Unit rates for Great Britain come from the Ofgem cap for October–December 2025. Battery voltage and efficiency ranges follow standard lithium-ion references and vendor data sheets for modern boost converters. Real households see minor variation based on room temperature, chargers, and cables, so we present ranges, not single-point claims.
Want to double-check the inputs? See the EIA residential price table and Ofgem’s energy price cap unit rates. Both update routinely; swap in your local price per kWh and you’ll get current numbers for your home.
From One Charge To Your Monthly Bill
One refill costs cents. What about a routine? If you top up a 10,000 mAh pack twice a week, that’s 104 cycles per year. Using the mid-point cost from the first table, that’s about $7–$8 for the entire year at the U.S. average.
Quick Scenarios
- Light use: 10,000 mAh once a week → ~$3–$4 per year.
- Commuter use: 20,000 mAh twice a week → ~$14–$17 per year.
- Heavy travel: 27,000 mAh three times a week → ~$28–$34 per year.
Does Fast USB-C PD Raise Your Costs?
Fast profiles trade time for power, then slow near the top. The peak window adds a tiny bump in conversion loss, but the difference per charge is pennies at most.
What About Solar Chargers And Off-Peak Rates?
Many utilities offer time-of-use pricing. If your plan has cheaper overnight rates, plug the pack in before bed. With portable solar, the grid cost is zero, though charge time stretches.
Common Device Pairings And Size Picks
Picking capacity by need keeps wasted energy low. Here’s a quick guide for everyday gear you actually use.
Phones
Modern phones sit in the 10–20 Wh battery range. A 10,000 mAh pack gives one to two full refills. If you shoot lots of video or run GPS often, move to 20,000 mAh.
Tablets And Handhelds
Large tablets run 25–40 Wh batteries. Two top-offs from a 20,000 mAh pack is realistic, or one from a compact 10,000 mAh in a pinch.
Myths That Waste Your Time
“Leaving It Plugged In All Night Burns Money”
Once topped up, quality chargers idle at a few hundred milliwatts of draw. That’s pennies per month. Unplug for safety and tidy cables, not for fear of a big bill.
“Higher mAh Always Means Better Value”
Capacity you never use is energy you carry for no benefit. If your phone needs a single boost on busy days, a compact pack keeps both weight and yearly energy spend low.
Safety And Lifespan Tips That Also Save Energy
Keep Packs Off The Dashboard
Direct sun bakes cells and casings. Cooler storage preserves capacity and trims resistive losses during the next charge.
Avoid Deep Depletion
Shallow cycles are easier on lithium cells. Topping up sooner reduces heat and softens the energy you need from the wall for each session.
Retire Swollen Or Damaged Units
Bulging casings or odd smells point to failing cells. Recycle safely and replace; inefficient, failing packs can run hot and waste power.
Turn The Formula Into A Habit
Once you’ve used the equation with your own rate, you can price any pack in seconds. Spot a deal on a 30,000 mAh model? Multiply by 3.7, divide by 1000, adjust by your go-to efficiency, and multiply by the tariff. That skill keeps energy costs transparent.