Decision point for an electric vehicle and our home's photovoltaic array expansion

I've searched the old threads here, but I still have two questions for a design decision:
1. Roughly how many kilowatt-hours does it take to recharge an EV? I understand this depends on the battery type/size/efficiency and the miles driven, but I'm looking for either a calculator or an estimate.
2. What are the EV options for a 50-mile range that includes some 55 MPH highway driving? I see that GreenCar sells the occasional Hymotion Prius (Green Car Company), which is tempting, but there's no hurry.


Here's the background:
Spouse and I could see ourselves buying a used EV (or another hybrid) in the next five years. We're happy with our Prius for highways & hauling and we don't need a second car, but it'd be convenient. An EV that could hold a 9'0" longboard inside the passenger compartment would be an unexpected bonus.


Oahu is small enough that we could cope with a 50-mile range. Most highways are limited to 55 MPH or have secondary routes, so highway speeds would be desirable but not essential. For the last five years we've driven less than 100 miles per week and most weeks are closer to 75 miles. Gas currently costs about $3.10/gallon but in 2008 it was closer to $4/gal.


Most of Oahu's power is generated by burning oil, and that fuel cost is passed right along to the customer. Electricity costs 23 cents/kilowatt-hour and has been as high as 30. However over 80% of our current consumption is supplied by our grid-tied photovoltaic array. It's almost paid for itself but we've already maxed it out. Adding an EV to our consumption would require a second inverter & more panels. Oahu's grid-tied net-metering rules allow us to roll over our excess production credit for up to 12 months, so we could recharge an EV during the day or at night. All we'd have to figure out is how many kilowatt-hours an EV would add to our consumption.


We've just about maxed out our roof, too. We could barely squeeze another 500 watts of panels on the remaining space. I'm not interested in high-efficiency cells or high-power-density panels or any of the integrated-PV roof tiles-- they're great concepts but way too expensive right now.


However in a separate home-improvement project we're considering building a pergola along our south wall. A pergola roof would offer plenty of space for more panels in a variety of designs. One local firm actually sells a pergola whose roof consists of PV panels.


So today's pergola design might be complicated by the discussion over how much future generating capacity we'd need, which depends on a vehicle that we might be five years from buying. I could just wing it with another 1000-2000 watts of panels, but I'd rather come up with an estimate that would also help us decide what type of EV (or plug-in) we want.


Any "easy" answers to our questions? Are we missing any other issues?

 

4 miles/KWh? 0.25 KWh/mile? Sounds great-- we can work with that. Thanks!

12.5 hours of sun during the summer is no problem, but quality varies dramatically. Our Xantrex inverter is rated at around 95% efficiency, but I don't have to do any more calculations to come up with an estimate. We regularly produce 12-13 KWh/day in the summer months and 8-10 KWh/day during winter. Of course that varies with cloud cover, vog, and rain.

Our array is 3300 watts of used panels-- 1990s-vintage Siemens & Solarex models of 50-75 watts. We also have Evergreen factory seconds (blemishes, some bad cells) rated at 115 watts. I doubt that every panel is producing rated output, and the Xantrex 3 KW inverter manual says 10% over rating is fine. We're not approaching any voltage or current limits.

We bought the panels off eBay & Craigslist for $3-$4/watt. Made our own racks out of scrap aluminum left over from a lanai-screening project. Paid full retail for the inverter, conduit, & wiring. Had a very patient solar installer teach us the mechanical & electrical code requirements, mounted everything on the roof ourselves, strung all our wiring, and paid him $750 to do the grid connection. State & federal tax credits paid for 65% of our $16K cost.

It's always easier to reduce consumption than to raise production, and we're in a good location. No heating or air conditioning. Tradewind cooling with ceiling fans and solar-powered attic exhaust fans. Radiant foil insulation in the attic and most walls. Mostly CFL lighting. EnergyStar appliances. The dishwasher gets all the hot water it needs from the solar water heater. Two old CRT TVs, no big screens or LCDs, Energystar desktop PC/monitor. Power strips for vampire loads. We still use an electric clothes dryer but spouse drew the line on that. We only do 1-2 loads of laundry a week anyway.

We could downsize our 25 cu ft fridge to about 20 cu ft-- even an EnergyStar 25 cu ft fridge is an electricity hog. We use an old MagicChef countertop convection/microwave oven that we're eventually going to upgrade to a more efficient GE Profile of the same technology. Our main oven is a Kenmore convection model with a glass cooktop but that doesn't get much use anymore. We're adding one more solar exhaust fan to an attic void that gets a lot of sun, and we might put some decorative holes in an interior wall to improve airflow. ("Hole" is straightforward engineering, but "decorative" is a challenge.) Our next roof (probably in 20 years) will use whatever's current in reflective solar insulation.

The biggest energy sink in our house is our 17-year-old, but she's leaving for college in just 187 days...

These days the highest component cost of a PV system is the inverter. I should easily be able to find used/blemished PV panels at $3/watt. To recharge an EV under full-use conditions, it looks like we'd want an entire new 3 KW system to have the reserve capacity. I'll have to figure out what dimensions spouse wants for the pergola (that's a décor decision, not engineering design) and then see how many kilowatts the pergola contractor can cram onto it.

You're right about fuel consumption. 55 mpg and $4/gallon gas is about the same as 4 miles/KWh and 30 cents/KWh electricity. At an energy use of ~$7.50/week, even a $25K PV array and 65% tax credits would take over 20 years to pay for itself. And for a trip of over 50 miles, or for hauling a load, we'd still use the Prius. But I guess I'd have to worry about fuel stabilizer in its gas tank...

It's not compellingly cost-effective to install a second PV array, even if energy inflation costs will cut down on the payback time. Of course I enjoy the design & "cool" aspects of this type of project, and a second car would be a convenience.

 

Good point. We were buying panels a few at a time.

Thanks. The Porsche's a bit out of our price range but the RAV looks like it would've more than fit our needs.

Even at 3 miles/KWH we'd probably still go with a 3-4 KW array. I'm going to have to take another look at inverter/panel prices and see how much space the pergola affords. I'd hate to spend top dollar for high-power-density panels and an oversized inverter.

I can see that we'd get to know the location of every plug-in station around town...

 

I've been seeing those at the home shows, and they probably raise reliability while solving some problems of bigger inverters. But I'm not sure that their power optimization is a significant improvement over blocking diodes and a conventional inverter's MPPT algorithms. I'm pretty sure that we're macro-charged for the micro-privilege.


Although we have a variety of panels, their operating voltages are pretty similar. We've arranged the two strings to keep their total voltages within 5% across the day. Performance is "good enough" for our purposes, and I don't know how much better we'd get by paying up for a matched set. I'd love to see an objective study by someone like Consumer Reports.



Anyway if we went with a PV pergola then we'd match the panels because that's how the contractor wants to build it-- with a standard set of dimensions instead of whatever I dredge up from eBay. They also put some effort into hiding the connectors & wires among the pergola's roof framing while maximizing the panel exposure to cast a nice uniform shade effect through the translucent panels, so I don't know how they'd deal with microinverters. But those are décor decisions, not engineering, and we've learned to defer all décor decisions to my spouse...


A local solar contractor does EV conversions as a hobby. His last one was a Geo Metro and he's always casting about for the next project. It'd be tempting to see what he could put together from a typical hatchback but I suspect we'd despair over the lead-acid lifecycle.

 

I've looked into it, but the numbers aren't there for us.

First you have to decide that you want to live in a place with a constant breeze-- it might get very annoying after a while. We have tradewinds blowing across the islands pretty consistently but our yard doesn't see a breeze more than a few hours a day and some neighborhoods hardly see anything at ground level.

Second, you want wind data. A good wind-turbine contractor will set up a data-gathering box at the site and leave it for a month or two. I don't know how common those contractors are. We talked with our local university and eventually located a high-school science project that used a similar system on a home near ours. The conclusion was that our breezes were gusty at times but overall too light & intermittent.

No rush-- after considerable décor discussion (thanks to the feedback in this thread!) I don't think we're ready to do the pergola this year, although we'll re-evaluate when the summer weather heats up. The design is straightforward but the construction will need a lot of yard work.

Anyway we'd wait a few years until new EVs started showing up on Craigslist or Autotrader or similar used-car markets.