NIST, 6.1.5 Unit symbols obtained by multiplication (PDF), in Guide for the Use of the International System of Units (SI), 2008, pp. 12, 13.
«Reference [4: ISO 31-0] suggests that if a space is used to indicate units formed by multiplication, the space may be omitted if it does not cause confusion. This possibility is reflected in the common practice of using the symbol kWh rather than kW·h or kW h for the kilowatt hour. Nevertheless, this Guide takes the position that a half-high dot or a space should always be used to avoid possible confusion; for this same reason, only one of these two allowed forms should be used in any given manuscript.»