So I was merrily riding a bus yesterday on my way to Potsdamer Platz for a Dosa lunch when I noticed this:

And a couple of thoughts hit me: What’s up with the piss stains? 🤷

And something I could actually answer: How long would I need to use this USB port to make up for the **€**3.20 that I paid for this ticket.

Now, a day later, I have a calm Sunday morning to ponder all of life’s most urgent questions so let’s get back to the thought from yesterday.

**Contents**hide

## Power draw from a USB port

The best way to know exactly how much power I can draw from that port would have been to test it.

The next best thing is to try and guess based on some indicators. From the color of the port, it is safe to conclude it is a USB 2.0 port, and not a USB 3.0 port. From Wikipedia, we see that a USB 2.0 port intended for high power devices allows for a current draw of a** minimum** of 0.5 amps. The voltage is also standardized to 5 volts.

Amperes times Voltage gives us the power output of the port, which is 0.5 amps x 5 volts which is 2.5 watts (again, **at a minimum**).

## Measuring power consumption

2.5 watts is the power that can be drawn from the port. To calculate how much energy can be consumed over a given time period, we need to simply multiple the watt number by the time number. Typically, it is measured in watt-hours.

My electricity company bills me for the kilowatt-hours I consume. If I use my TV that’s rated at 100 watts for 10 hours, it will be billed as 1000Wh, or 1kWh of energy consumption.

## Price of electricity in my area

Electricity does not cost the same everywhere, and not even in the same area. Even the same provider might use different prices depending on the time of consumption and a whole list of other factors.

Looking at Vattenfall’s website (an electricity provider here in Berlin), I can see prices range between 25.07 cents / kWh and 33.37 cents / kWh. To make it easy for calculations, I’m going to go with 30 cents / kWh (all **€** cents).

## How much electricity can I buy for the cost of a BVG ticket?

For €3.20, the price of a single BVG ticket at the time of writing, I can consume (3.20 / 0.30) kWh, which is 10.667 kWh (or 10,667 Wh) of energy with my current electricity provide.

## How long do I need to use the USB port to cover the cost of the ticket?

To find the time duration in which our 2.5W port will output 10,667Wh of energy, we simply need to divide the target consumption number by our consumption rate:

10,667 (Wh) / 2.5 (W) = 4,266.8 hours (the Watt unit nicely cancels out giving us the number of hours)

**Which is roughly 177.8 days, or just under 6 months.**

This time can roughly be cut by half if BVG changes the USB ports to 3.0 guaranteeing a minimum current draw of 900mA but I’m not holding my breath.

## How much will a ticket effectively cost if I make full use of my ticket’s validity to charge a device?

Now of course being in a bus for 6 months is going to cost more money for season ticket and I might never recover my full ticket.

But what about the journey that I’ve already paid for? A BVG single ticket is valid for 2 hours. If I make 100% use of the time a ticket is valid for to charge my device, what’s the effective cost of my ticket?

At a minimum of 2.5W for 2 hours, I’ll consume at least 5Wh. At 30 cents / kWh, that’s 0.15 cents worth of electricity.

That’s 0.15 cents or €0.0015 that I can immediately recover from my ticket price, **effectively making my single journey BVG ticket cost not €3.20, but a jaw dropping €3.1985** 😎

## In conclusion

So there you have it. Go grab your 0.047% discount on BVG tickets! Subscribe for more financial tips. Thank you for reading!