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Sizing a Propane Tank for a Generator

Tanks are Often Undersized for the BTU Demands of the Generator

by Matt Wotton

Tanks are often undersized for the BTU demands of the generator. When installing a propane tank to supply an automatic standby generator there are factors beyond the electrical capacity to keep in mind.

In a power outage situation, the generator will not be able continue to run for extended time frames or in lower temperatures. A propane tank has limitations on the amount of vapor it can produce. As the tank runs lower in product, vaporization capacity becomes limited. As the outside temperature drops, the vaporization ability diminishes as well. A telltale sign of an undersized tank is frost or ice buildup on the outside of the tank.

A generator may run fine in the summer with a full (80%) 420 lb. tank, but when the power drops out in the middle of January this tank may not be the right supply choice. It is important to consider the worst-case scenario in which the generator will need to perform and size the tank and overall system for this situation.

A 420 lb. propane tank will produce 108,000 BTUs at 40% when the outside temperature is 0 degrees. At -10 degrees the same 40% tank produces 54,000 BTUs. At 20% it will be down to 40,500 BTUs. A 14KW generator is burning 47,768 BTUs (3,412 BTUs per KW) per hour.

Depending on delivery schedules, access to the property and additional loads running on this tank, you may be hearing from an unhappy customer and be making an off-route delivery.

Why not do the math and present the end user with the options. Automatic standby generators are an expensive installation. The cost of additional tanks to ensure uninterrupted service will be a worthwhile investment. Charts and tables are readily available. Check with the generator manufacturer or your propane tank wholesalers for capacities and sizing.

The end goal is to always have a happy customer. I have seen many undersized tanks on generators over the past 20 years. We always take the time to size lines and tanks for heating, hot water and cooking applications. Taking a closer look at the generator tank set up will only help to meet the end goals of having a happy customer, making efficient deliveries and promoting future sales.