By Bill Howard

Gasoline may cost $4 a gallon, but the sun's energy still costs 0 cents per kilowatt hour if you can harness it.

In the near future, solar panels built into car roofs will charge the car when it's parked at shopping malls. And a bit further into the future, they'll even help provide power when you're under way.

Meanwhile, enjoy these accessories.

Solar Battery Charging

Some cars draw so much power from their never-completely-off onboard electronics such as the car alarm, radio or built-in cell phone that the battery draws down in a couple weeks. Plug a solar 12-volt battery charger into the accessory outlet, stick the panel in the front window and you should always have a charged battery that starts the engine. In this case, the accessory outlet puts power into the battery rather than taking it out.

Small panels that provide about 2 watts of power cost $25 to $30 while bigger panels intended more for hikers who want to power or recharge a laptop could top $200. To keep the car battery topped off, make sure the outlet you plug into is always on. If the outlet deactivates when you turn off the ignition, the charger won't damage the electrical system, but it won't charge, either.

In a pinch, you could use a small solar panel to recharge your cell phone by plugging the phone's car accessory cord into a charger. However, if you really needed the phone charged in a hurry, it would be faster to plug it directly into the car battery. This makes sense if you're away from the car. And there are smaller alternatives for cell phones, such as emergency power packs that use two or four disposable batteries ($10 to $20) or buying and carrying a spare battery.

Solar Ventilation

These devices let you clip a fan atop your car's side window when you're parked. It comprises a small array of solar cells, a vent fan and a long, thin vent that exhausts hot air through the tiny window gap you leave open. When you return from shopping, the car will be warm but not quite as hot. It's still not enough ventilation to allow you to leave pets in a car. And it doesn't work well with tinted windows or on cloudy days — but then, on cloudy days, you're not so worried about overheating. You also wouldn't want to leave it attached for days at a time outdoors when you can't check on the car, because even the small gap might be problematic on some cars in a heavy rainstorm. Solar ventilation systems are priced from $10 to $30 and are available from various sources.

Solar Leather Seats

It's not a joke. These are leather seats that won't fry your backside when you hop in the car on a sunny day and the roof is left down. In this case, the solar part refers to a UV-blocking treatment applied to the leather on convertible seats BMWs made in the past year. It reduces leather surface temperatures by 20 degrees, BMW says.

At their introduction last year in the BMW 3 Series convertible, BMW left cars out, top down, in the direct sun of the Florida Keys for 90 minutes. When testers — some wearing shorts — got in the vehicles, the seats were warm, but there were no cries of pain from testers wearing shorts or skirts. The upshot: You'll feel warm sitting down, never scalded.

Integrated Solar Roof Panel (New Cars)

Starting with 2010 models, some cars will incorporate small solar roof panels that provide several watts of charging power to keep accessories going when the car is parked. And they could run exhaust fans to keep the interior cool. They won't directly propel the car.

Solar Hybrid Car (Retrofit)

Coat the roof panels of your hybrid with flexible solar tiles and you extend the gas tank range by 20 to 40 miles, supporters say. Hybrids work off both an internal combustion engine and an electric motor powered by several hundred small batteries like what's in a last-generation laptop. A hybrid's battery/electric motor combo when fully charged is good for a mile or two. City braking regenerates electricity (as does the gas engine) that tops off the battery quickly and then gives you another mile or two of electric-only driving.

On the highway, a solar array adds power to the battery cells. It's like the $30 battery recharger but on a much bigger scale. It costs $2,000 to $4,000 and, says one manufacturer, can pay for itself in two to three years when gasoline costs $4 a gallon. Your mileage may vary but under ideal conditions — a lot of city driving in sunny climates — you could improve fuel economy up to 29 percent, claims Greg Johansen of Solar Electric Systems.

Another company, Solatec, claims its $1,595 (plus installation) roof kit provides 24 watts of power and improves the economy on a Toyota Prius by 10 percent. Critics say the gains are much less than backers report. Down the road, as the price of the solar panels drops and their output climbs, they'll go into cars as original equipment.

Solar Accessories

In the accessories category, not every solar-powered device is a must-have. In addition to the ones mentioned, there's a solar car alarm, solar air purifier, solar Bluetooth hands-free phone kit and solar-assist radar detector (avoid speeding tickets on sunny days?).

Bio: Bill Howard is an experienced automotive writer and the editor of TechnoRide.com, the car technology site produced by Ziff Davis. Previously, he was PC Magazine's executive editor. Howard is also vice president of the International Motor Press Association.