1. Gallons Per Minute Flow Rate
Having an adequate hot water delivery rate makes all the difference in the world if you're trying to enjoy a hot shower or bath. A delivery rate of 5-10 GPM is adequate and provides more than enough hot water for showers and baths. As a matter of fact, the 5-10 GPM water delivery standard gives you enough leeway to keep the hot water on continuously with no breaks for as long as you want.
That means baths that seem to never end and showers that are fit for a king. And when considering outdoor options, look for at least 1 gallon per minute flow rate to give you the hot water you need for an outdoor shower.
2. Light Weight And Easy Portability
Being able to travel with your tankless water heater can make trips to your favorite cabin, barn, or outdoor building something that you actually look forward to. Also, being able to travel with your tankless water heater increases its usefulness and value. With more places to use it and the ability to hook it up nearly anywhere, you'll be sure to get your moneys worth.
3. Outdoor Compatibility
Sometimes the only ability to take a hot shower is through the use of a portable tankless water heater. It's always a great thing, when in the great outdoors, to have an outside portable unit their to supply you with the hot water you need. You should keep in mind, however, that a portable unit designed for the outdoors will have a much lower GPM than an indoor designed model. Also, outdoor versions aren't appropriate for indoor use.
4. Energy Efficiency
In order to save you money on your monthly bills, the tankless water heater you consider purchasing should be rated to be highly energy efficient. This ensures that you can recoup the cost of owning one through what you save over running a traditional water heater every month. You'll be surprised by how fast the cost savings add up.
5. Warranty, Amazon Review Score, and Manufacturer
Getting a good warranty for your tankless water heater, at least 1 year to be safe, is the safest investment you can make. Also, be sure that the tankless water heater you're considering has a good Amazon review score tagged to it. Look for at least 4 stars on any product you're considering.
Lastly, the manufacturer of your product has a lot to do with how long it's going to last. Some good producers of tankless water heaters these days are Ecosmart, Bosch, Rinnai, Rheem, and Stiebel Eltron.