Quantifying Energy Production in Solar Heating Systems

The influence of power purchase agreements and leases in the PV market illustrates how financing plays a pivotal role in the growth of the US solar industry. A fundamental challenge with implementing these financing models in the solar water heating (SWH) market segment is quantifying the energy a given system produces or offsets. For billing purposes, developers of third-party ownership models and utilities must be able to accurately account for the value of energy delivered by solar heating systems.

The inclusion of solar heating systems in a state’s renewable portfolio standard also creates a need for accurate heat metering. To meet requirements for renewable portfolio standards, utilities use renewable energy credits to quantify renewable energy production. Where they are basing these credits upon actual solar heating system output, a heat meter is required.

Unfortunately, measuring the energy residential and commercial SWH systems produce is not always a straightforward exercise. The cost of heat metering equipment can be prohibitive in small systems, and proper equipment selection and installation are essential to obtaining accurate heat production data. To complicate matters, the US currently lacks a standard for solar heating system metering. Policy makers, utilities and system integrators must rely on European standards that often result in some confusion in the US market.

This article introduces solar heating contractors to the components used in heat metering systems and provides information on component selection and installation. It also includes perspectives from industry stakeholders who oversee utility incentive programs, are active in developing a US standard for heat metering or have experience with these systems as equipment manufacturers or installers.

Solar Heat Meter Subassemblies

Heat meters often require three measurements, including flow rate and two reference temperatures to quantify heat. Current international heat metering standards refer to the equipment used to measure these data as subassemblies. A complete heat meter consists of a calculator subassembly, a temperature-sensor pair subassembly and a flow sensor subassembly.


A calculator utilizes the system flow rate, two temperature readings and the characteristics of the heated fluid to determine the amount of heat a solar heating system produces. The calculator uses the flow rate to determine the volume of the heated fluid, and the density and specific heat to quantify the heat-carrying capacity of this volume of fluid at an observed temperature difference.

This relationship can be described as follows:

Q = V γ c ΔT

where Q is the heat flow in BTU, V is the volume of fluid in gallons, γ is the density of the fluid in pounds per gallon, c is the specific heat of the fluid in BTU per pounds per °F and ΔT is the change in temperature of the fluid in °F.

The calculator must account for the variation of density and specific heat based on the type of fluid in the system and the temperature of the fluid. A database within the heat meter’s calculator subassembly contains these properties.

The calculator also provides an interface for viewing or transmitting system data. Some meters accumulate data and store the information in their internal memory. Others use an SD card or can be networked to allow remote monitoring and data storage. Advanced differential controllers may include a calculator subassembly within their programming and can be connected directly to temperature and flow sensors.


Resistance temperature detectors (RTDs) are commonly used with heat meters due to their accuracy and long-term stability. Two common RTDs used in heat meters are PT100 and PT500 sensors. Silicone and solid-state semiconductor-based sensors may also be used.


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