When perusing the Juicebox source in a previous post I also noticed how it was calculating the “saved” value. If you are not familiar with the Juicebox saved value: On the premium Juicebox with a display it shows a $ saved value that increases as you use the JB.

What is this value? How is it calculated?

To begin with the Juicebox makes a few assumptions (constants declared in the code):

- Price of gas $3.50
- mpg of car to compare to 25
- Price of electricity per kWh $0.12
- Efficiency of the EV 300 Wh/mile

It then uses those values to come up with a “savings per kWh” value:

gas cost per mile = price of gas / mpg (3.50/25 or $0.14 /mile)

gas cost per kWh = gas cost per mile * 1000 / wH/mile ($0.14 * 1000 / 300 or 0.466 per kWh)

savings per kWh = gas cost per kWh – price of electricity per kWh ($0.466 – $0.12 = $0.346)

The total savings then is this savings per kWh value multiplied by the total kWh the JB has put into the car.

The total kWh is calculated by multiplying the input voltage, output current, and time interval (Watts = Volts * Amps, kWh = Watts over standard time interval–hours).

Much like my previous post this all is pretty straight forward math. The number it reports out may or may not be meaningful to you if your situation closely matches the configuration of the Juicebox (25 mpg prior car, EV burning 300 Wh/mile, etc.).

For those of us with the FFE, the Wh/mile value is more like 240-250 (about 15% more efficient) and, in my case since I drove an ICE Focus before the FFE, the mpg figure would be more like 30 mpg (about 15% more efficient as well). Which, for my case, the two differences pretty much cancel out making the $ saved value fairly accurate–if gas prices remain near $3.50.