Divide mAh by 1,000 to get amp hours, then multiply by the voltage to get watt hours. That single voltage step is the part most people skip, and it is the reason two batteries that convert mAh to watt hours correctly can look identical on a spec sheet while storing completely different amounts of energy. A 20,000mAh number means nothing until the voltage behind it is known.
Battery listings lean on mAh because the number looks big. Watt hours is the honest unit, because it accounts for both how much charge a cell holds and how hard that charge pushes. Without voltage, mAh is half a measurement.
The math takes about ten seconds once the two inputs are in hand. The harder part is knowing which voltage belongs in the equation, since packs quote one figure and outputs deliver another. Anyone sizing a system should start by working out the daily energy budget first, then convert every candidate battery into the same unit.
Quick Answer
To convert mAh to watt hours, divide the mAh rating by 1,000 to get amp hours, then multiply that result by the battery voltage. A 20,000mAh cell pack at 3.7V holds 74Wh. The same 20,000mAh figure at 12V holds 240Wh, so voltage decides the answer.
Key Takeaways
- mAh measures charge, watt hours measures energy, and voltage is the bridge between them.
- The formula is mAh divided by 1,000, multiplied by volts, which gives watt hours.
- Identical mAh ratings at different voltages store wildly different amounts of energy.
- Power banks quote cell voltage at 3.7V, not the 5V that leaves the USB port.
- Comparing two batteries by mAh is only valid when both run at the same voltage.
Step by Step
1. Find the capacity in mAh
The mAh rating sits on the label, the listing, or the printed side of the cell. It states how much charge the battery holds, not how much energy. Write the raw number down before touching any other spec.
2. Divide by 1,000 to get amp hours
Milliamp hours are simply amp hours scaled up by a thousand. So 20,000mAh becomes 20Ah, and 5,000mAh becomes 5Ah. This step just moves the decimal point three places to the left.
3. Find the voltage that capacity was measured at
This is the step that gets skipped, and it is the one that matters most. Lithium ion cells are rated at a nominal 3.7V, LiFePO4 cells at 3.2V, and assembled packs at 12V, 24V, or 48V. The voltage must match the same point in the system as the capacity figure.
4. Multiply amp hours by volts
Amp hours times volts gives watt hours directly. So 20Ah at 3.7V works out to 74Wh, while 20Ah at 12V works out to 240Wh. Divide watt hours by 1,000 when a kilowatt hour figure is needed.
Why mAh Alone Tells You Almost Nothing
A capacity of 20,000mAh describes charge, and charge on its own carries no information about usable energy. Run that figure at 3.7V and the pack holds 74Wh. Run the same figure at 12V and it holds 240Wh, which is more than three times the energy.
Nothing about the mAh number changed between those two cases. Only the voltage moved, and the stored energy moved with it. That is why a marketing headline built on mAh alone cannot be compared across product categories.
The gap widens further at higher pack voltages. A 100Ah battery at 12V stores 1,200Wh, or 1.2kWh. The same 100Ah at 48V stores 4,800Wh, four times as much, from a cell count that is four times larger.
This is exactly why serious solar listings quote watt hours and rarely bother with mAh. Watt hours already contains the voltage, so no hidden variable is left. Anyone comparing storage options should convert every option to watt hours before shortlisting.
Which Voltage to Use
The correct voltage is the one the capacity figure was measured at, not the one the device outputs. Lithium ion cells carry a nominal rating of 3.7V, which is an average across the discharge curve. That 3.7V figure is what almost every power bank capacity claim is built on.
Pack voltage is a different number entirely. Wire cells in series and the voltages add, so four LiFePO4 cells at 3.2V make a 12.8V pack. Wire them in parallel instead and capacity adds while voltage stays put.
This distinction explains a common frustration with power banks. A 20,000mAh bank holds roughly 74Wh at the cell level, but the USB port delivers at 5V. Divide 74Wh by 5V and the theoretical output is about 14,800mAh, before any losses.
Conversion losses then cut that further. Boosting 3.7V up to 5V costs energy as heat, and typical real world efficiency lands somewhere between 80 and 90 percent. So a 20,000mAh bank realistically delivers somewhere around 12,000 to 13,000mAh at the port, which is why the phone charge count never matches the box.
None of that is deception in the strict sense, since the cell rating is accurate. It just answers a different question than the buyer is asking. Reading the watt hour figure instead sidesteps the whole problem, and the better solar charging banks worth considering print it clearly.
Power and Energy Are Different Things
Watts and watt hours look similar and mean very different things. Watts is a rate, describing how fast energy moves at any instant. Watt hours is a quantity, describing how much energy exists in total.
A kettle rated at 1,000W draws energy at that rate whenever it runs. Run it for six minutes, which is a tenth of an hour, and it consumes 100Wh. Run the same kettle for a full hour and it consumes 1,000Wh, or 1kWh.
The formula is watts multiplied by hours, which yields watt hours. Go the other direction by dividing watt hours by watts to find runtime. A 1,200Wh battery running a 100W load lasts roughly 12 hours before inverter losses.
This is where battery sizing becomes practical rather than theoretical. List every device, note its wattage, estimate its daily runtime, and add the watt hours up. That total is the number to match against battery capacity when picking a power station that fits the load.
What Trips People Up
Comparing mAh across different voltages
Two batteries can only be ranked by mAh when both operate at the same voltage. A 10,000mAh pack at 12V beats a 20,000mAh pack at 3.7V on stored energy, at 120Wh against 74Wh. Convert both to watt hours and the ranking becomes obvious immediately.
Expecting full mAh out of the USB port
Cell capacity and output capacity are separate numbers. Voltage conversion from 3.7V up to 5V reduces the mAh figure proportionally, then efficiency losses take another slice. Expect roughly 60 to 70 percent of the printed mAh to actually reach a phone.
Mixing nominal and maximum voltage
A lithium ion cell reads about 4.2V when full and around 3.0V when empty. The 3.7V nominal figure represents the working average across that range. Using 4.2V in the calculation inflates the watt hour result by roughly 13 percent.
Forgetting depth of discharge
Stored watt hours and usable watt hours differ, particularly with older chemistries. Lead acid banks are typically limited to about half their rated capacity to avoid damage, while LiFePO4 tolerates far deeper cycling. Two packs with identical watt hour ratings can therefore deliver very different amounts of usable energy.
Recommended Reading
- how charge moves through a solar storage system
- how the two main storage chemistries compare
- capable power stations at the budget end
- choosing between a pocket bank and a full station
Frequently Asked Questions
How do you convert mAh to watt hours by hand?
Divide the mAh figure by 1,000, which converts it into amp hours. Then multiply those amp hours by the battery voltage to get watt hours. For example, 20,000mAh becomes 20Ah, and 20Ah at 3.7V gives 74Wh, so the whole calculation needs only two numbers and one multiplication.
What voltage should be used for a power bank?
Use 3.7V, the nominal voltage of the lithium ion cells inside. Manufacturers rate capacity at the cell level, so 3.7V matches the printed mAh figure. Using 5V instead, the USB output voltage, produces a watt hour number that overstates the pack by a wide margin.
Why does a 20,000mAh power bank not deliver 20,000mAh?
The 20,000mAh rating applies at 3.7V inside the pack, giving about 74Wh. Delivering that energy at 5V through USB yields roughly 14,800mAh in theory. Conversion losses then remove another 10 to 20 percent, so around 12,000 to 13,000mAh actually reaches the device.
How many watt hours is a 100Ah battery?
Multiply the amp hours by the system voltage. A 100Ah battery at 12V holds 1,200Wh, which is 1.2kWh, while that same 100Ah rating on a 48V system holds 4,800Wh. Always check the pack voltage before quoting a watt hour figure, since the gap between voltage classes is enormous.
What is the difference between watts and watt hours?
Watts measures a rate of energy flow at a given moment, while watt hours measures a total quantity of stored energy. Multiply watts by hours of runtime to get watt hours. A 1,000W kettle running for six minutes uses 100Wh, while the same kettle running a full hour uses 1,000Wh.
Can two batteries be compared using mAh alone?
Only when both batteries operate at exactly the same voltage, because matched voltage means a higher mAh rating really does hold more energy. Across different voltages the comparison breaks down completely, since voltage scales the stored energy directly. Converting both figures into watt hours removes the ambiguity in a single step.
How do watt hours translate into runtime?
Divide the battery watt hours by the load in watts, so a 1,200Wh battery powering a 60W load lasts about 20 hours in theory. Real runtime falls short, because inverters, heat, and depth of discharge limits all consume part of that budget. Planning for roughly 80 percent of the calculated figure is sensible.
