(a) Whenever the charge by any electric corporation or gas and electric corporation for electric service, including the minimum charge, is in any way dependent upon the demand of the customer, such demand shall, after January 1, 1939, be determined by a demand measuring device of a type approved by this commission, whenever the demand billed is 5 kw. or more, or whenever the total rated capacity of the customer's current consuming devices is 8 kw. or more, or whenever the customer's consumption has exceeded 800 kw.h. per month in two consecutive months during the preceding 12 months; provided:

1. (1) a minimum charge not exceeding $1.33 per kw. or $1.00 per hp. may be based on connected load, provided it is clearly specified how such connected load is to be determined;
2. (2) the demand of temporary installations or of devices which have a highly fluctuating or large instantaneous demand, such as X-ray apparatus and welders, may be determined by or related to the rated capacity of the apparatus, the capacity of the transformer installation required to supply adequate service to the apparatus, or by other appropriate standards;
3. (3) the demand of installations which have a definitely determinable and unvariable demand, such as electric signs, show windows, or water heaters, may be determined by or related to the rated capacity, or by periodic tests;
4. (4) the demand for breakdown or auxiliary service, may be determined by or related to the capacity contracted for, the transformer capacity required, or other appropriate standards;
5. (5) in other special cases when real merit or necessity is shown, the commission may, on application, permit the use of other reasonable standards.