simplified PHEV cost recovery calculation

Hi all,

There have been some really good discussions about calculating the "payback period" for PHEV conversions, so I just wanted to throw out another approach. In my opinion "payback" should not be the bottom line, but for most of us we need to at least have a reasonable idea of what we're going to spend and how much of it we're likely to get back. The difference is the price to play. While previous methods looked solid, many are perhaps over complicated for an up front estimate in my mind.

The numbers are estimates, and may need adjusting, but here is my basic approach.

Etyp: 4 kWh - Typical trip DC battery energy used before recharge, can be battery usable capacity if you expect to regularly deplete, or 100% minus expected final SOC (based on others experience in similar conditions) times usable capacity.

NWtrips: 12 / week - expected number of typical trip / recharge cycles in a week.

NAtrips: NWtrips * Nweeks = 600 - Number of typical trips per year, calculated as typcial trips per week times typcial weeks (50 in this case) per year.

EA: NAtrips * Etyp = 2400kWh/yr - Annual energy used in electric drive

MAequiv: EA / 0.225kWh/mile = 10667 miles/year - equivalent number of all electric miles per year, whether actually all electric or blended. kWh/mile may vary based on driving style, route etc.

GAequiv: MAequiv / 50mpg = 213.3 gal/year - number of gallons of gasoline offset per year, adjust for your typical prius mpg

VA: GAequiv * $/gal = $746.55 - value of offset gasoline per year, in this case for assumed $3.50 per gallon average.

ECA: (EA / Ceff) * $/kWh = $240 - cost of electricity to charge, in this example given 90% charge efficiency and AZ average $0.09/kWh rate

VNA: VA - ECA = $506.55 - estimated net annual production value

Based on this estimated net annual fuel savings, the cost of the system, and its expected lifespan you can then make an estimate of how much of that cost you are likely to recover. As in all cases, you have to make a lot of assumptions about gas and electric prices and your driving that greatly affect the results. You could also include the lost interest "opportunity cost" of the conversion, but in recent years you would have been lucky to get any interest back, or even keep your principle

Rob