mAh to Watt Hours Explained: Voltage, Capacity and Real Runtime Checks

Two storage packs sit side by side on a supplier data sheet. One is listed as 20,000 mAh, the other as 74 Wh. Most buyers assume the first one is the stronger product. In fact both hold exactly the same amount of energy, and the purchase decision made on the bigger number is usually the wrong one.

Watt hours measure energy — the work a battery can actually deliver. Milliamp hours measure charge — current flowing over a period of time. You can compare watt hours directly. You can only compare mAh when you know the voltage it was measured at.

What follows is the conversion, the four places where numbers get quietly distorted, and the checks worth running before a purchase order is signed.

The short answer: watt hours come from mAh, voltage and nothing else

Convert charge into energy with one line: Wh = (mAh × V) ÷ 1,000, where V is the nominal voltage of the cell or pack the mAh rating was measured on. Run it backwards and mAh = (Wh × 1,000) ÷ V.

Because the divisor is fixed and the voltage is not, identical mAh figures can describe very different products. 10,000 mAh at 3.7 V is 37 Wh. The same 10,000 mAh at 12 V is 120 Wh — more than triple the energy hidden behind an identical-looking headline.

That is why airlines cap carry-on batteries at 100 Wh instead of quoting a mAh limit, and why freight paperwork for storage cabinets is written in kWh. Watt hours are an absolute energy quantity; they do not shift when the internal architecture changes.

Converting a real pack, four steps

Find the correct voltage basis. Use the nominal pack voltage, not the charger input. A 51.2 V LFP pack is sixteen 3.2 V cells in series; an NMC pack reaches a similar figure with thirteen or fourteen cells.
Convert mAh to Ah by dividing by 1,000. A 280,000 mAh cell is 280 Ah.
Multiply. 280 Ah × 51.2 V = 14,336 Wh, or about 14.3 kWh.
Sanity-check the claim. If a 100 Ah, 51.2 V pack is sold as "5 kWh", the arithmetic does not work. Ask which node the seller measured — cell, pack or output.

Keep one anchor number in mind: at 51.2 V, 1 kWh is roughly 19.5 Ah. It converts most LFP data sheets in seconds and exposes inflated capacity claims long before a sample ships.

Reference table: mAh to Wh at common voltages

Use the table as a cross-check rather than a replacement for a data sheet. Values assume nominal voltage and are rounded, with no depth-of-discharge or conversion losses applied.

Stored energy in watt hours for a given charge rating at four nominal voltages; the voltage term is the reason a single mAh figure cannot travel between systems.
Charge (mAh)	3.2 V (LFP cell)	3.7 V (NMC cell)	12 V	51.2 V
5,000	16 Wh	18.5 Wh	60 Wh	256 Wh
10,000	32 Wh	37 Wh	120 Wh	512 Wh
20,000	64 Wh	74 Wh	240 Wh	1,024 Wh
50,000	160 Wh	185 Wh	600 Wh	2,560 Wh
100,000	320 Wh	370 Wh	1,200 Wh	5,120 Wh

Read the diagonal, not the row. The same 20,000 mAh is a 74 Wh pocket device at cell voltage and a 1 kWh battery at 51.2 V.

Why mAh stops being useful above cell level

Cell data sheets stay in mAh because that is how cells are graded and matched on a production line. System data sheets move to kWh because that is the unit a site consumes. Almost all confusion starts at the join between those two layers.

A 280 Ah LFP cell appears as 280,000 mAh on some supplier pages — a big, memorable number. At 3.2 V nominal it stores about 896 Wh. Wire sixteen of them in series and you have 51.2 V and 14.3 kWh, with the mAh figure untouched. Add a second string in parallel and capacity doubles again while voltage stays exactly where it was.

The layering matters when you buy. Battery cells are graded and matched before they become battery modules, and modules before packs. A mAh number quoted at one layer rarely survives intact at the next.

LFP Battery Cells for Energy Storage SystemsLFP cell series spanning 50–314 Ah with model-dependent cycle life, positioned for residential, commercial, grid and industrial storage builds.View Product →

Four ways the numbers get distorted

Voltage basis swaps. A 20,000 mAh power bank holds 74 Wh at 3.7 V cell voltage, but the same stored energy quoted at a 5 V USB output reads as 100 Wh. Neither statement is false; they describe different nodes. Ask which one the seller means.
Nameplate versus usable. Depth of discharge sets the real figure. LFP systems typically run 90–95% DoD, flooded lead-acid stops near 50%. A 10 kWh nameplate at 90% DoD delivers 9 kWh to the load.
Round-trip efficiency. Charge and discharge losses run 85–95% on most lithium systems. A 100 kWh cabinet may hand back 88–92 kWh, and the gap widens at high C-rates.
Rating conditions. Capacity is quoted at 0.2C and 25 °C. At 1C, or at 0 °C in an outdoor cabinet, expect visibly less — a difference that appears in January, not in the brochure.

A pre-purchase checklist, written in watt hours

Nameplate energy in Wh or kWh, never Ah alone.
Voltage basis plus series and parallel configuration, confirmed in writing.
Usable energy after the stated depth of discharge.
Round-trip efficiency at your expected C-rate, not at a laboratory 0.2C.
Continuous and peak power, so the pack is never asked for more than its rating allows — a 5 kWh pack at 0.5C gives 2.5 kW.
Certification scope: UL 1973 and IEC 62619 cover cells and modules; system-level approval often needs more.
Price per usable kWh, the only comparison that survives a change in voltage.

Where watt hours turn into a sizing decision

A household drawing 14 kWh a day and wanting 8 kWh of overnight cover should be shopping in kWh from the first email. The formula can produce an equivalent mAh figure at a given voltage, but nobody sizes a home system that way, and no installer should.

Residential Energy Storage Pack with Modular CapacityStackable home battery expandable from 5 to 40 kWh, with IP65 enclosure, app monitoring, backup switching and compatibility with major solar inverters.View Product →

Commercial sites repeat the same arithmetic at a larger scale. A 215–233 kWh liquid-cooled cabinet covers a mid-size industrial load with headroom for peak shaving; a 400 kWh unit suits longer autonomy or heavier duty cycles. Both are already labelled in kWh, which happens to be the unit the electricity bill arrives in.

Liquid-Cooled Commercial and Industrial Storage CabinetOutdoor LFP cabinet at 215/233 kWh with liquid cooling, bidirectional PCS and scalable EMS for single-unit or parallel C&I deployment.View Product →

When cells, modules, packs and systems come from one manufacturer, the mAh-to-Wh conversion stays consistent from the module data sheet through to the container nameplate. That consistency is worth more than it sounds: mismatched voltage bases are one of the most common causes of a system arriving undersized.

The takeaway

If a specification gives you only mAh, ask for the voltage before you compare anything. Then convert once, in writing, and keep the number in the quotation. Watt hours are the only figure that survives a change of voltage, chemistry or configuration — and the only one worth putting into a purchase order.