The amount of power needed while cruising depends on many factors including the energy used by appliances and lighting, the output of the engine alternator, the climate and the length of time away from shore power. These and other factors will determine the amount of solar power and thus size of the solar panel(s) you may need. Working with Ed Foster of foster-wills.com, we have developed a worksheet to assist you in inventorying your on board power generation and consumption and estimating you solar power requirement. Print out our Solar Power Calculation Worksheet and use it as a guide to figuring out your power generation requirement. From this you can get an idea of what your solar panel options might be. Hope it helps!