Archives For October 2013

Plug Me In

October 8, 2013 — Leave a comment

I’m going to have a bit more fun with this blog post. For this thought experiment, I’d like you to suspend your disbelief. Imagine, for a moment, that someone offered you the chance to “plug” your body into a standard outlet and let yourself “charge.” All of your energy would be gathered from this charging process. You would eat nothing. How long must you remain connected to the outlet? How much will it cost?

Where do we start? There are a few ways to approach this, but I’ll start with the basal metabolic rate for an average adult male. For a 70 kg male, this is typically around 1,600-1,700 Calories (kilocalories). If you would like to do more than just sit against a wall, you will need a bit more energy. Let’s round that up to 2,000 Calories. Converting this to units with which we can work, this comes to 8.36 megajoules (MJ). Like most thought experiments, it is easier to work in orders of magnitude, so we will round this up to 10 MJ.

We now know how much energy we need, but how long will it take to draw this energy from an outlet? Every outlet has a maximum power draw, but very few appliances, if any, reach this maximum value. We denote the amount of power drawn in joules per second as Watts (W). On average, microwaves draw 1,450 W, vacuum cleaners 630 W, computers 240 W (though, as I type this, I am drawing <100 W), and alarm clocks 2 W. In other words, it’s variable. If we were to charge ourselves like a microwave oven, it would take almost 2 hours. However, if we used a computer charger (100 W), it would take 28 hours! A laptop computer charger would thus not suffice, since we would not acquire our necessary daily energy within a given day. All of this energy would be expelled as heat, and you would be a blob of meat plugged into a wall outlet. That’s not a fantastic way to live.

In case you are wondering, architectural engineers model heat production from humans as if they were 100 W light bulbs. This is eerily similar to our 100 W laptop charger that provides just enough energy to get us through a single day!

If you tried all of the above with one of Tesla’s new 10 kW chargers, you’d be ready for your day in only a few minutes!

What about the cost? Two apples provide approximately 200 Calories of energy (note that the energy yield from eating is not 100%, so you will actually receive less than 200 Calories from an apple). The cost of the apple varies based upon season, region, type, and quality of the fruit. Let’s say the two apples cost you $1.00 for ease of comparison. You spend $1.00 for 200 Calories of fresh, delicious apple. How does this compare to energy cost from your wall outlet? In the United States, the average is $0.12 per kWh. The energy from those apples, then, would cost you less than three cents. Over the course of a year, you would spend less than $200 to keep yourself more than fully charged! Imagine spending that much on food in a given year.

Do not try this at home, or while in the Navy.