The Value of Interval Data in Solar Project Analysis

Quality Assurance

Astute solar developers understand the value of interval meter data. They know that these data allow them to perform the highest level of utility rate analysis possible on behalf of their customers. One size does not fit all when it comes to the right sizing of a solar project. Raw meter data enable system analyses that are highly specific to individual customers and their unique load profiles. This article examines some of the specific advantages of working with raw data—such as the ability to find the best utility rate option or determine the optimal solar system size for dollar savings—and illustrates how using these data can give developers a competitive edge.

What Are Interval Meter Data?

Interval meter data are a series of measurements that a digital or smart electric meter captures at predefined intervals throughout the day. Utilities use these data to calculate a customer’s bill. In practice, utilities use many different terms interchangeably to refer to interval meter data, including interval data, meter data, raw data, demand interval data, kilowatt interval data or Green Button data. The format for these data may also vary among utilities.

Units of measurement. The two primary units of measurement for interval meter data are kilowatt-hour (kWh) and kilowatt (kW). The former measures energy consumption over time; the latter measures power demand averaged over a specific interval of time. This basic formula derives one from the other:

energy (kWh) = power (kW) x time (hour)

For example, if the smart meter is averaging kW values over a 15-minute interval, then the corresponding kWh value is one-quarter of the kW value, since there are four 15-minute intervals per hour.

Interval period. Interval data typically come in 60-minute, 30-minute or 15-minute increments. As illustrated in Table 1, the shorter the time interval, the higher the number of data points and the more granular the data resolution. For example, 15-minute interval data contain four times as many data points as 60-minute interval data.

Why Are Interval Data Valuable?

Interval meter data are valuable because the data resolution is more granular than that of monthly summary data. For example, a monthly utility bill for a customer on a time-of-use rate schedule might include only three data points for a given month: on-peak, semi-peak and off-peak kWh consumption values. By comparison, a 15-minute interval data file for the same month would contain roughly 3,000 data points, allowing for a more detailed level of data analysis.

The following are examples of the types of analyses that stakeholders can perform with interval meter data.

Determine the best utility rate option. It is increasingly common for electric utilities to offer customers multiple rate schedule options, some of which are based on time of use. For instance, many California homeowners can choose between a tiered rate structure or a voluntary time-of-use rate plan. Interval data make it possible for project developers and system integrators to precisely quantify the economic performance of proposed PV projects under all of the available rate schedule options. With access limited to a customer’s monthly summary data, it is impossible to compare monetary savings based on different rate structures.

This type of economic analysis is inherently a customer- and PV system–specific exercise. For instance, my colleagues at Energy Toolbase and I pulled interval data for a random sample of 30 existing residential PV system owners in California and used the Energy Toolbase (energytoolbase.com) software to analyze whether these customers would save more money by staying on the default tiered rate schedule or by opting into the voluntary time-of-use plan. When we modeled the economic performance of these PV systems, we discovered that 70% of the customers were better off under the time-of-use rate plan and 5% were better off on the tiered or block plan. The other 25% of these customers had full-offset PV systems and saw no net difference between the rate plans. The average increase in monetary savings for those customers who stood to benefit from a time-of-use rate plan was 36% annually.

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