Solar-electric boats can operate during cloudy days by storing excess energy generated during sunny days in batteries, and they can also be charged using electrical grid power. When it's cloudy, the boat can draw power from these batteries to continue operating. Some solar-electric boats also have hybrid systems that can switch to alternative power sources like diesel or electric when needed.

Battery replacement in solar-electric boats depends on the type and quality of batteries used. Typically, batteries in solar-electric boats can last 5–10 years. The battery cells are replaced every 7 years to maintain the efficient performance of the boat. The cost of replacement will vary depending on the market value of the cells.

Solar-electric boats can operate longer hours during daylight by continuously charging their batteries from solar panels. However, their operational hours may be limited at night or on cloudy days unless they have alternative power sources. 

Solar-electric boats are more cost-effective in the long term compared to conventional diesel or petrol boats due to lower fuel and maintenance costs. However, the initial purchase price may be higher. 

Solar-electric boats are feasible for various applications, including leisure, tourism, and certain commercial uses. Now it is being developed to use for extreme operations such as patrolling, defence etc. 

Solar-electric boats come in various sizes, from small recreational boats to larger commercial vessels. There's no strict size constraint, but larger boats may require more extensive battery systems to operate.