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.