Singular or plural 'efficiency'

can I use the plural form of 'efficiency' when talking about multiple devices? For example, I have the following question and I need your help:

"Reducing the supply temperature of the heating system increases the efficiency/ies of heat pumps"

I'd appreciate every comment and I would be very thankful for your help.

Solution 1:

In using a plural there is the possibility of interpretation of each heat pump having several efficiencies (for example, as determined in various operating conditions); in my opinion it is preferable to use a singular; this is confirmed by actual usage (ref.); the plural, if used at all, appears to be rare.

• […] have studied the efficiency of mutual fund companies and pension fund companies
• […] explores the efficiency of public spending on health, education, and social protection in the Russian Federation on the general government level
• […] help on improving the efficiency of individual building and property portfolios.
• […] IMPROVING THE EFFICIENCY OF ENGINES FOR LARGE NONFIGHTER
• […] Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters.