Application Notes: Submetering Tenant Operations

Submetering Tenant Operations

Background information on tenant usage:

Many commercial buildings today have a variety of tenants with different operational requirements and power usage (watts per sq. ft.). Typical examples include computer centers and phone switching systems (or other electrical operations) with 24 hour operations, additional cooling loads and much higher electrical energy usage per square foot. If the building has a single master electrical meter (as is common in most buildings), the building owner is left to attempt to build sufficient lease cost per square foot to cover the additional load requirements. Frequently, this estimated cost does not sufficiently cover the building's electrical bill, leaving the owner with unallocated costs which often translate into higher costs for all the building tenants and an uncompetitive building.

Typical examples of submetering applications include:

High energy use tenants (computing centers, phone switches)
Tenants with extended operating hours (restaurants, retail)
Short term tenants
Apartments and condominiums
Central heating and cooling plants for common systems

Options for recovering these costs have typically been very expensive, with rewiring and installation of a submeter by the utility as the only practical solution to billing energy costs in a fair manner. These installations can run into thousands of dollars and the payback is generally too long to be practical and the owner will choose to simply absorb the unrecovered costs.

In other cases, tenants may request additional operating time (for example on a Saturday) and the building owner needs to determine what the actual additional cost of operating the tenant space and any HVAC systems is to bill the tenant. Ideally, the building owner could submeter every tenant plus the central plant to determine the billing, but unless the usage is frequent enough, this is probably cost-prohibitive. Another option is to establish a fixed cost per square foot for additional tenant operation combined with power metering on the central plant to determine the additional cost of operation for a particular tenant on a particular day.

Theory of operation for electronic power transducers:

Electronic power transducers such as the Veris 6000 and 6010 series provide a cost-effective means of monitoring multi-phase tenant energy usage using an existing building automation system with totalizing and/or analog input capability. The Veris meters use split core, high accuracy current sensors to measure the current flow on each of the phases without the need for expensive rewiring. The voltage on each phase is monitored using input wiring directly into a breaker or other convenient connecting location. The instantaneous current in each phase is multiplied by the instantaneous voltage on the corresponding phase several times per cycle.

The total power is obtained by summing the instantaneous values from each phase and converting to an industry-standard analog output for instantaneous demand (kW). This same calculated value is summed over time to provide a pulse output with a predetermined kWH value per pulse. If one or both of these outputs is connected to a DDC or other automation panel, highly accurate power consumption information is available to the building owner through the operator interface.

The automation system can track both the instantaneous power usage and the total consumption for one or more tenants and provide an output report to indicate the total usage for any time period determined by the owner. This report can be used to generate an additional billing for the tenant based on actual energy usage.

If the owner wants to establish a fixed cost per square foot for off-time usage (weekends, etc.) plus measuring power usage on central plant (chillers, pumps, etc.), then the owner can install Veris meters on the central plant equipment only and measure the actual power usage to establish an additional billing cost for the tenant which accurately reflects cooling load.

Typical submetering installation

Applications are suggestions. Field applications vary; since manufacturer does not install the devices, it is up to the installer to ensure suitability and compliance to appropriate codes.