How to Size a Battery Pack: Runtime to Amp-Hours

Most battery enquiries start with a capacity someone else chose: “we need 40 Ah”. That number is usually inherited from a lead-acid pack or copied from a competitor, and it is the wrong place to start. Capacity is an output of the calculation, not an input. Start from the job the battery has to do and work backwards.

Key takeaways

Capacity is an output of the calculation, not an input. Start from four things: load power while working, hours per charge, the recharge window, and the lowest operating temperature.
Work in watt-hours and convert at the end: 1,200 Wh is 25 Ah on a 48V pack and about 17 Ah on 72V. Always quote the voltage beside the amp-hours.
Size for current before capacity. A 5 kW golf cart motor on 48V draws over 100 A continuous, so a 40 Ah pack built from 1C cells is being asked for 2.5C and will not survive it.
Divide by drivetrain efficiency – around 0.85 battery to wheels – before you convert to watt-hours.
Add the margins: 15-25% for duty below about 10 °C, design to 80% depth of discharge rather than 100% (it roughly doubles cycle life), and check the charger can refill the pack inside the window available.

Start from the job, not the battery

Write down four things before anything else: how much power the load draws while working, how many hours it works per day or per charge, how much time it has to recharge, and the lowest temperature it will be used at. Everything else follows from those. If you cannot state the second one — hours per charge — then range or distance is the substitute, and you convert it with an energy consumption figure.

The four-step calculation

Step 1: turn hours and watts into watt-hours

Multiply average power in watts by hours of operation. “Average” is the word that matters: a 2,000 W motor on a cargo trike does not draw 2,000 W all day. On a mixed route with stops and level ground it averages perhaps a third of that. Divide by system efficiency — around 0.85 for a battery-to-wheels drivetrain — to get the energy the battery must deliver.

Step 2: choose the voltage class

Voltage is set by the machine, not by the battery: 36V and 48V for e-bikes and light scooters, 60V and 72V for mopeds and motorcycles, 48V for golf carts and most low-speed vehicles. Higher voltage means lower current for the same power, which means less heat and thinner cable — but it also means more cells in series and a tighter matching requirement. The trade-offs are set out in our 48V vs 72V comparison.

Step 3: convert watt-hours to amp-hours

Divide the required watt-hours by the nominal pack voltage. A 1,200 Wh requirement on a 48V pack is 25 Ah; on 72V it is about 17 Ah. This is the number to quote to a supplier — always together with the voltage, because amp-hours alone tells you nothing about energy.

Step 4: check the current, not just the capacity

Divide the maximum continuous power by the pack voltage to get the current the pack must deliver, then check both the cells and the BMS against it. A 48V pack feeding a 5 kW golf cart motor has to supply over 100 A continuous — a 40 Ah pack built from cells rated for 1C is being asked for 2.5C and will not survive it, whatever the capacity says. This is where most undersized packs fail, and it is a BMS sizing question as much as a cell question.

The margins people forget

Cold: lithium loses usable capacity below about 10 °C and more below freezing — budget 15 to 25 percent for winter duty
Ageing: cells are usually warranted to 80 percent of original capacity, so a pack sized to the exact requirement fails the duty in year three
Depth of discharge: design to 80 percent DoD rather than 100 percent and the cycle count roughly doubles
Charging window: if the machine must recharge in four hours, the charger current has to fit the pack — a 100 Ah pack at 0.2C takes five hours
Peak events: hill starts, acceleration and hydraulic loads are short but large, and the BMS trip setting must sit above them
Accessories: lights, hydraulics, refrigeration and inverters are often left out of the calculation

Three worked examples

Commuter e-bike, 40 km per charge

A pedal-assist e-bike on mixed urban roads uses roughly 10 to 15 Wh per kilometre; a throttle-only build with a heavy rider is closer to 20. At 15 Wh/km, 40 km needs about 600 Wh. On a 48V pack that is 12.5 Ah before margin; with 25 percent for cold and ageing, specify around 16 Ah at 48V, and check that the controller’s peak draw sits inside the BMS rating. Our 48V 31Ah pack covers this duty with room to spare.

Golf cart, 18 holes plus a spare round

A fleet cart covers roughly 1.5 to 2.5 kWh per 18 holes depending on terrain and payload. Two rounds between charges means about 4 kWh usable, and with an 80 percent depth-of-discharge target that is a 5 kWh pack — roughly 100 Ah at 48V. The constraint is not capacity but current: cart motors are 3 to 5 kW, so the pack and BMS must carry well over 100 A continuous. See golf cart lithium batteries for the conversion detail.

Cargo tricycle, one working day

A 2,000 W trike working six hours with stops averages perhaps 700 W, so about 4.2 kWh at the wheel and roughly 5 kWh from the battery after efficiency. On a 72V pack that is about 70 Ah. Payload matters more than distance here: every extra 100 kg raises consumption on stop-start routes, which is why we ask for the load as well as the route. Our electric cargo tricycle ships with a 72V 100Ah pack for exactly this duty.

For stationary loads the method is identical — a portable station for a market stall is watt-hours of appliance load times hours, with the same margin. The PS-2000 power station at 2,048 Wh is sized that way.

Mistakes that cost money

Quoting amp-hours without a voltage, so the supplier cannot price energy
Sizing to average power and ignoring the peak, which trips the BMS on the first hill
Sizing to the requirement exactly, with no allowance for cold or ageing
Choosing a voltage the controller was not designed for
Copying a lead-acid capacity figure, which is quoted at a discharge rate lithium does not use
Forgetting that charging time is part of the duty cycle

Most of these mistakes start with a number taken at face value. Our specification sheet walkthrough shows which lines need test conditions attached before they mean anything.

Frequently asked questions

How do I convert watt-hours to amp-hours?

Divide watt-hours by the nominal pack voltage. 1,200 Wh on 48V is 25 Ah; on 72V it is about 17 Ah. Always quote both numbers, because amp-hours without a voltage does not describe how much energy the pack holds.

Is a bigger pack always better?

Not always. Extra capacity adds weight, cost and charging time, and on a two-wheeler it changes the handling. The right pack covers the duty cycle with margin for cold and ageing, and no more. If capacity is cheap and current capability is the constraint, spend the money on cells with a higher discharge rating instead.

How much margin should I add for winter?

Plan 15 to 25 percent above the calculated requirement if the product will be used below 10 °C, and more if it charges outdoors in freezing conditions. Low temperature reduces usable capacity and, more importantly, charging below 0 °C damages cells permanently — a BMS low-temperature charge cut-off is the fix.

What depth of discharge should I design for?

Around 80 percent for daily commercial duty. Designing to 80 percent depth of discharge roughly doubles cycle life compared with discharging to empty, which usually means buying a somewhat larger pack once instead of a smaller pack twice.

Does a higher voltage give more range?

Not by itself — range comes from watt-hours. Higher voltage delivers the same power at lower current, which reduces losses and heat and allows thinner wiring, but the energy stored is voltage times amp-hours either way.

How long will the pack take to charge?

Divide capacity by charger current and add roughly 15 percent for the constant-voltage stage. A 100 Ah pack charged at 20 A takes a little over five hours. Fast charging is possible but shortens cycle life, so size the charger to the time the machine actually has available.

Get your pack sized

Send the load power, hours per charge, voltage, lowest operating temperature and target market, and we will return a sized pack with the continuous and peak current checked against the cells and the BMS. Start on WhatsApp, or see battery packs, electric two-wheelers and cargo tricycles.