Cal. Civ. Code § 170-15-2-2

Sec. 2. (a) A landlord or an association calculates the water and sewage service usage portion of a sub-bill using one (1) of the following
methods:
(1) For water service, as follows:
(A) Optional usage calculation using sub-meter data as follows:
(i) The landlord or association divides the total net charge for water service, plus applicable tax, by the total amount of water master metered
to obtain an average cost per unit volume.
(ii) The average water cost per unit volume is multiplied by the actual unit volume consumed by each dwelling unit as measured by a sub-
meter.

(B) Optional usage calculation using other data as follows:
(i) The landlord or association reasonably allocates to each tenant, member, or co-owner a portion of the total net charge for water
service.
(ii) For landlords sub-billing tenants, the landlord deducts the landlord's usage.
(iii) Reasonable allocations may include a charge per dwelling unit, allocated pro-rata, based on:
(AA) square footage;
(BB) the type of dwelling unit;
(CC) the number of individuals residing in the dwelling unit; or
(DD) the estimated volume of water distributed to each dwelling unit.

(2) For sewage disposal service, as follows:
(A) The landlord or association charges a sewage disposal service rate that reasonably allocates to each tenant, member, or co-owner a
portion of the total net charge for sewage disposal service.
(B) For landlords sub-billing tenants, the landlord deducts the landlord's usage.
(C) Reasonable allocations may be based on a charge per dwelling unit, allocated pro-rata, based on:
(i) estimated sewer flow;
(ii) square footage;
(iii) the type of dwelling unit;
(iv) the number of individuals residing in the dwelling unit; or
(v) the estimated or actual volume of water distributed to each dwelling unit.

(b) A landlord or an association may calculate sub-bills using a method other than those specified in subsection (a) if the calculation:
(1) is fair and reasonable; and
(2) results in sub-bills per dwelling unit less than or equal to the amount sub-billed to each dwelling unit under subsection
(a).

(c) In no event shall a landlord or an association issue sub-bills that total more for the usage portion of the water or sewage disposal service
than the landlord or association paid for the same water or sewage disposal service.
(d) Nothing in this section may be construed to eliminate or diminish a contractual right a tenant, member, or co-owner has with respect
to the provision of water or sewage disposal service.