How do we calculate kWh per dollar?

Tue Nov 28

Written by: Casey O.

TL;DR: We divide the price by the battery likely nominal voltage multiplied by its nominal capacity.

To calculate kWh per dollar, multiply a battery’s voltage (V) by its capacity (Ah). Then divide the price by that value.

\frac{$}{V \cdot Ah}

Maybe you did a little napkin math and noticed that our Price per kWh math doesn’t quite add up.

\begin{matrix} 12 V \cdot 100 Ah & = & 1.2 kWh \\ \frac{$ 200}{1.2 kWh} & = & \frac{$ 166.67}{kWh} \end{matrix}

But we listed the ‘Price per kWh’ as $156.25 for the battery.

On their spec sheets, batteries list their ‘nominal voltage’. The listed nominal voltage of that “12V” battery, is 12.8V and that value is what we use to calculate the energy capacity.

\begin{matrix} 12.8 V \cdot 100 Ah & = & 1.28 kWh \\ \frac{$ 200}{1.28 kWh} & = & \frac{$ 156.25}{kWh} \end{matrix}

The nominal voltage of a “24V” LiFePO4 battery is 25.6V, for a “48V” battery, 51.2V, etc

It’s not just us, many retailers also use the nominal voltage to calculate the amount of energy stored in the battery. They aren’t lying, and the number is bigger, which is great for marketing.

Sorry, what does nominal voltage mean? If you connect a multimeter to that “12V” battery, it will always measure 12.8V?

Well, no. It could actually read anywhere from 10V to 14.6V depending on the charge level, as well as other factors. Nominal voltage is the voltage that a battery is ‘supposed’ to run at. It’s a specification, a useful approximation.

Nominal Voltage (V) - The reported or reference voltage of the battery, also sometimes thought of as the “normal” voltage of the battery. Source