Prius Plug-In Hybrid: Some Numbers

In April 2012 we took delivery of a new Prius Plug-in Hybrid Vehicle (PHV), replacing a 2005 Prius. Now in mid-2013 I have enough history of utility and gasoline use to make a reasonable comparison of the relative costs of these vehicles.

Unfortunately for simple math, almost immediately after we got the PHV, we took off for six weeks in France. This mid-April to May gap messes up the pattern of utility bills and driving history. I had to choose two slightly mismatched periods of time for comparison: for the old vehicle, April 2011 through March 2012; and for the new one, June 2012 through May 2013.

This means that the electricity usage numbers are not precisely comparable. However, both periods span the darkest months of the year and also span one summer. So both include maximum lighting time and maximum home air conditioning. Nevertheless, all of these numbers should be taken as approximate.

Gasoline Usage

Twelve months of the 2005 Prius: 12048 miles, 251 gallons. That's 48mpg or, a more significant measure, 21 gallons per 1000 miles.

Twelve months of the 2012 PHV: 10790 miles, 170 gallons, yielding 63mpg or 15 gallons per 1000 miles.

Bottom line: given our usage patterns, the PHV saves us 6 gallons per 1000 miles driven. At current prices that's about $25 per Kmile, or given our normal 12K/year distance, about (ta-daa!) $275 per year in fuel costs.

Electricity Rates

Palo Alto Utilities charge for electricity usage based on Kilowatt-Hours (KwH) per month. Up to 10 KwH per day is called Tier 1, and charged at $0.09524 per KwH. Usage from 10 to 20 KwH is Tier 2, charged at $0.1302. Our latest bill, usage for May 2013, showed 487 KwH charged at $60.36. That is presumably based on

31 days * 10 KwH = 310 * 0.09524 =	$28.5244
487 - 310 = 177 KwH * 0.1302 =	$23.0454
total	$51.5698
Actual bill	$60.36

Hmmm. Think I need to have a talk with the Utilities...

Electricity Consumption

For the period May 2011 through April 2012 we consumed 5206 KwH, an average of 433.8 per month.

For the period July 2012 through June 2013 we consumed 6438 KwH, an average of 536.5 per month.

Thus the PHV seems to have added approximately 100 KwH to our electricity usage, or at Tier 2 rates, about $13 to our monthly electric bill.

The Bottom Line

Based on these somewhat approximate numbers, the PHV is saving us $275 per year in gasoline, while it is is costing us $156 in electricity. For a net saving of (ta-ta-daaaa!) $119 per year.

Since the PHV costs $7,800 more than a regular Prius ($32,000 versus $24,200 currently at toyota.com), it should pay for itself in only... 65 years. (Sad trombone: wah-wah-wah-waaa)

Side Issues

Our driving pattern includes a lot of short local trips. That's why we bought the PHV; its 11-mile battery distance means we often go several days without the gas engine coming on, and often the dashboard readout shows us getting over 100mpg well into a tankful. However, on longer trips and freeway driving the PHV does no better than a normal third-generation Prius, about 55mpg. Our battery-powered local hops pull the average up to 63, or about %15 better than a non-plug-in. But I've talked online with a PHV owner who does nothing but commute 6 miles each way, and is averaging over 160mpg.

I should also note that Palo Alto Utilities is pilot-testing a Time of Day Usage program. Under this scheme, electricity used between 11pm and 6am is discounted $0.019 per KwH. That would lower the cost of the PHV's 100 KwH (all Tier-2) from $0.1302 to $0.1112 for a saving of $2/month. Let's see: hiring an electrician to install some kind of timer in the outdoor outlet where we plug in the car would cost what, $250? So that would take even longer to earn out than the car itself.

Update: The PHV has a built-in charge timer! I can set it to charge itself only between 11pm and 6am. Thus we could realize the Time of Day discount without further expense. That would lower the electricity cost from $156 to $132 per year, increase the savings from $119 to $143 per year, and the PHV pays for itself in only 54 years! Yeah! I have registered for the TOD program but the pilot program is currently closed.

Raspberry Pi, episode 1

Okey-dokey, I will repurpose this blog yet again to describe trying to use the Raspberry Pi that Paul gave me for Christmas.

There's this much one can say about the happy group of Brits behind the Pi: they are teaching many, many people how to spell Raspppberry.

The actual thing is basically a single-board Linux computer, remarkable mainly because of its tiny size: about the size of a playing card. Central on the board is a single chip that comprises 512MB of RAM, a 32-bit CPU, and a graphics processor. Pretty much everything else on the tiny board is either connectors or resistors and capacitors.

The Pi all hooked up. Clockwise from top right corner: Power via a micro-USB connector; the SD card; some experimenter I/O pins; a yellow RCA connector for an analog TV signal; an audio jack; some blinky lights; two USB plugs for keyboard and mouse; an ethernet cable; the HDMI cable leading to the digital TV.

When I stop to think about it the Pi can make me rather giddy. I remember configuring my first home computer, with separate, 10-inch-wide cards for a 16-bit CPU and for 64kilobytes of RAM. So here's one fingernail-size chip comprising a CPU and memory, both four orders of magnitude more capable. (64e3 versus 51.2e7, and as for the CPU, forget it. The Pi's graphics processor can do 64 gigaFLOPs. That old 2MHz Z80 isn't in the same galaxy.)

Anyway it needs a keyboard and mouse, so I went to Fry's and found an optical USB mouse for $3.99 and a USB keyboard for $5.99. Later I had to go back and get a 2-metre HDMI cable, which cost more than the other two combined.

Getting the OS

The big hurdle in getting the Pi going is loading its OS. The Pi's only provision for mass storage is a single SD card, 8GB or larger. (I try not to think too much about having 8 or 16 gigabytes of memory on a thumbnail-sized chip.) If the SD card is initialized as a FAT-32 file system and loaded with a disk image downloaded from the Raspberry mothership, the Pi will supposedly boot up from it into a full-blown Linux system.

That's what the quick start guide claims. But the problem is getting the disk image written onto the SD card. The Pi instructions are for Windows users; my tool is a Macbook. Nevertheless, I thought I'd done it correctly, following these instructions at the "Embedded Linux Wiki".

It didn't work; or at least, when the Pi was all hooked up and plugged in, the screen went blank.

So I did it again and this time, the system booted up into a typical Linux startup screen going to a config screen.

It's aliiiiive! The Pi with the $4 mouse, $6 keyboard, and an expensive Samsung TV doing duty as a monitor.

I config'd it a bit, setting the locale and the time zone, although the latter was a bit of a puzzle: is California in Alaskan or Aleutian or Pacific time?

A Desktop!

I thoughtlessly told the configurator to update itself, but no ethernet was plugged in, so that hung. So I popped the power in and out and this time when it came up, it was in a graphical desktop!

The desktop after playing around a bit: terminal, Midori browser, debian doc open.

Fall Down Go Boom

I played with the desktop for a bit. Impressive that the Pi had instantly found the LAN and the browser could access the web without problem. I started apt-get update to update the database of installed software, and after a bit, everything hung solid. The mouse pointer still tracked but nothing responded to it.

So after a few minutes I popped the power plug out and in and when it booted now, it said PANIC.

Oh dear oh dear oh dear...

One More Time Unto the SD Card

I put the SD card in my Macbook and reformatted it and copied the image to it again. Checked that the Pi would boot and started through the configuration screens again.

After setting the timezone, there was a long pause. Then a series of file-system messages about inodes, including "This should not happen!" and "Data will be lost" and other stuff. Not helpful messages. In no way suggestive of what to do or how to recover.

Oh dear, some more. The Life Of Pi is finished, it seems.

And then it didn't respond to anything.

Anyone want a Pi?

I conclude that the Raspberry Pi is indeed a Linux platform of amazingly small dimensions. Loaded up with dedicated device-control software it can no doubt do yoeman duty as a lump of embedded smarts. People are doing amazing things with it.

However it is not a toy. It ain't for kids, at least, for kids who aren't ready to type sudo apt-get update without thinking about it. It needs constant hand-holding by an experienced user of command-lines, and that user better have a lot of patience. The software, combined with the iffy hardware qualities of SD card mass storage, is just not reliable. Or at least, this one example was not. The user experience, even for a very knowledgeable techie, is full of frustration. For a non-nerd, it would be hopeless.