A solar water heater's efficiency is evaluated in various ways, making it crucial to know the various metrics before investing in or purchasing one. The primary goal of a solar water heater is to transform sunlight into heat energy, which is then used to warm water for household purposes.

The most widely used method of rating solar water heaters is based on their heat efficiency, which typically falls between 50 to 90 percent. Heat efficiency refers to the system's ability to transform the sun's energy into usable heating. This percentage is usually denoted as a 'collector efficiency' since it gauges how efficiently the solar collector is able to utilize and transfer heat from the sun to water. Some manufacturers provide higher efficiency ratings by focusing on better heat transfer, effective insulation, and reduced heat loss during transfer and circulation methods.

Another critical metric is the overall system efficiency, commonly expressed as a seasonal performance factor, or SPF. This metric serves as a comprehensive and trustworthy method to assess a solar water heater's performance over an entire year by taking into account factors such as insulation quality, storage tanks, heat magnets, and solar collectors' stability. The SPF is an important element to consider while shortlisting solar water heaters, as it offers a complete view of a system's performance after considering various, but varying scenarios, across all seasons.

Additionally, the first cost and lifetime maintenance expenses can considerably impact the final outcome of using a solar water heater. A system with high initial costs but a low operating cost will be more competitive in regions with ample sunlight. However, a system with a lower upfront cost but higher ongoing maintenance costs may not provide the returns desired, making a detailed evaluation necessary before making a decision.

Ultimately, it's vital to consider various metrics such as collector efficiency, overall system efficiency, upfront costs, and running expenses while choosing the most suitable solar water heater that matches your climate, usage, and financial constraints. This careful evaluation will enable you to make an informed decision, ensuring that your investment in a solar water heater yields the optimal possible returns in terms of efficiency, cost-effectiveness, and environmental advantages.