Involved question
Before you ask that, you first need to ask what your needs are. Read up on 12 volt systems. Nigel Calder's book: "Boat Owners Mechanical and Electrical Manual" is a very good resource, also check out www.amplepower.com and www.jackrabbitmarine.com are a couple on-line sources. If you are talking about DC refrigeration you can easily put yourself in the 100 amp per day range. So you have to figure out how to replace 100 amps of power per day. Solar are fine but limited. Take a typical 75 watt panel, maybe about 4' x 2'. Cost about $500. Will put out about 5 amps per hour in direct sunlight. Figure maybe 3-5 hours depending on your latitude. So you'll get about 20 amps per day from a panel -- when its not cloudy. More panels = more amps, but where to put them quickly becomes an issue. Wind generators vary according to make an how windy it is. A typical generator might put out 2-3 amps when its blowing 15 knots. Maybe you get 20-30 amps per day. More when its real windy, none when its not. Cost is about $1000. Best solution for making up the difference is your diesel's alternator. The goal is to minimize engine run time, so people put on high-output alternators (75-150 amps) to charge the batteries quickly. High power alternators run from about $250-$600. Of course, all these devices can fry your batteries if the amount of voltage and amperage is unregulated, so you need to make sure they are hooked up with voltage regulators. Alternator regulators are more expensive because the need to handle more juice. The alternator is generally the primary source for most cruisers -- its always available. Solar and wind are backup methods to reduce the amount you have to run the motor.
