Author: Mary Prasanna
Distributed Energy Resources (DERs) are small-scale energy systems that generate, store and manage electricity close to where it is consumed, rather than relying on large, centralized power plants and electric grid. These systems are typically connected to the local electric distribution network or situated on the customer’s side of the meter.
Common examples of DERs include:
- Solar photovoltaic (PV) systems
- Battery energy storage systems
- Wind turbines
- Hydroelectric generators
- Combined heat and power (CHP) systems
- Fuel cells
- Electric vehicles (EVs) and their charging infrastructure
- Hybrid systems combining multiple technologies
These are connected to Point of Common Coupling (PCC) which refers to the specific location where a DER system connects to the utility’s distribution network, marking the boundary between the utility’s equipment and the customer’s equipment. These DERs helps in reducing the demand on grid but comes with challenge of visibility, control and accurate forecasting of future demand.
ISONE Forecasting Needs and Challenges
Due to the electrification of heating systems and transportation, it is expected that the regions annual electricity consumption will increase by 11% over next decades.
- Between 1995 and 2005, net annual energy use in New England grew steadily, largely driven by economic expansion and rising use of air conditioning.
- From 2005 through 2024, it declined due to energy efficiency (e.g., advanced HVAC systems, efficient appliances and lighting) and increasing BTM PV adoption.
- The ISO now expects that downward trend to reverse, forecasting steady growth in net energy use through 2034—driven by electrification of heating and transportation under state carbon reduction mandates.
ISONE has seen increased behind-the-meter (BTM) photovoltaic systems, such as rooftop solar panels, for several years now. These are connected to local distribution network instead of regional grid. Their number became large and total capacity from these resources became significant.
The total installed BTM PV nameplate capacity on grid reached 4,542 MW by the end of 2024. It is expected to raise to 11,156 MW by 2045. 2025 Forecast Report of Capacity, Energy, Loads, and Transmission (CELT Report) gave the BTM PV projections. Below table shows that the BTM PV is going to contribute 7.7% of Energy consumption by 2045. The impact of BTM PV on system peak is to be studied carefully.
| Annual Energy Use | Gross (GWh) | Net (GWh) | Reduction from BTM PV (%) from BTM PV (%) |
| 2024 (Actual) | 122,412 | 116,813 | 4.6 |
| 2045 (Forecast) | 182,786 | 168,789 | 7.7 |
The peak demand is the time when electricity usage from grid reaches its maximum. The peak demand on the grid is typically in the early evening but the solar production is strong at midday. The impact of increased capacity of BTM PV on system peak is considerably low. In summer 2024 only 6% (1520 MW) of reduction is observed in estimated peak due to BTM PV. It is estimated that over next 10 years the forecasted summer peak reduction because of BTM PV will remain at the same level. But this trend may not be same afterwards because of the electrified vehicles which may bring summer peaks to nighttime. It is observed that BTM PV has no impact of 2024/2025 winter peaks. It is forecasted that winter peaks may shift towards morning in 2030s and BTM PV may impact its peak.
Most of the state-sponsored distributed PV is not participating in wholesale markets but it considerably reducing the system load. ISO is planning to accurately account for PV in long-term planning. PV forecast will be categorized as:
- PV as a capacity resource in the Forward Capacity Market (FCM)
- Qualified for the FCM and have acquired capacity supply obligations
- Size and location identified and visible to the ISO
- May be supply or demand-side resources
- Non-FCM Energy Only Resources (EOR) and Generators
- ISO collects energy output
- Participate only in the energy market
- Behing-the-meter PV (BTM-PV)
- Not in ISO Market
- Reduces system load
- ISO has an incomplete set of information on generator characteristics
- ISO does not collect energy meter data, but can estimate it using other available data
ISO-NE’s Forecasting Tools
It is forecasted that solar power capacity is going to nearly double in next 10 years which reduces the dependency on fossil fuel-fired energy during sunny days. Distributed solar PV installations growth projections are forecasted in CELT report.
More enhanced forecasting methods are used from 2024 CELT report. Some of the features include:
- A new hourly forecasting approach which accurately accounts for emerging patterns of energy production and consumption. Hourly projections over the CELT’s 10-year horizon and beyond are developed. (Previously the forecast depended primarily on annual energy and seasonal peak projections).
- Increased level of geographic details.
- The base demand forecast is broken down into eight load zones, a smaller subset of New England’s six states.
- Projections related to distributed energy resources, heat pumps, and electric vehicles have been generated for each of the region’s 67 counties.
For PV forecast National Renewable Energy Laboratory’s Distributed Generation Market Demand (dGenTM) model. The model simulates customer adoption of solar technology based on economic and other considerations and allows for a more finely-tuned solar forecast than previous years. It will help in predicting PV installations with nameplate capacity less than 1 MW using residential and commercial dGenTM. For 1-5MW systems policy-based approach is used.
PV Forecast Inputs
The ISO continuously refines its methods, developing sophisticated modeling techniques to project how BTM PV will reduce grid demand in the future.
ISONE developed 2025–2034 Forecast Report of Capacity, Energy, Loads, and Transmission (2025 CELT Report), with the help of:
- Expected economic growth, economic indicators
- Historical weather patterns, climate-adjusted weather data
- Projected adoption of distributes solar photovoltaics (PV)
- Use of Electric vehicles (EVs) and air source heat pumps
- State-level carbon reduction goals, state energy policies
- End-use behavior
2025 CELT report has four key components regarding energy and peak forecasts.
- Base forecast -Historical demand patterns and expected future conditions, including the impact of energy efficiency programs.
- Heat pump forecast - Expected demand growth from electrification of the heating sector.
- Electric vehicle forecast - Expected demand growth from electrification of the transportation sector.
- BTM PV forecast -Expected impact of DERs, mainly BTM PV, that reduce demand for grid electricity.
The sum of first three components will give gross long-term forecast while subtracting BTM PV from gross demand will give net long term forecast.
Final 2025 PV Forecast – 10 Year Horizon
Regional PV Nameplate Capacity Growth
Components of Peak Demand
Planning Implications:
In 2024 ISO-NE implemented Planning Procedure No. 12, which mandates, distribution companies to provide information about BTM PV and any other equipment connected to their systems that generates or stores electricity – DER’s. The basic data from these facilities includes:
- Size
- Physical location
- Electrical location
- Type
- In-service date
- Other characteristics
DER data needs, locational precision, and coordination with utilities. The collection of DER data shall occur three times per year, according to the following approximate dates. Dates may be adjusted as needed to avoid conflicts such as weekends and holidays.
All DERs regardless of their size, type, or other characteristics, must be included in the annual data submission to ISO New England. Facilities that are already modeled generator assets within ISONE’s Energy Management System and market systems are not required to be included, though the Distribution Provider may choose to include them voluntarily.
This data is being used in:
- Long and short-term load forecasts
- Power system modeling
- Transmission planning and transmission service studies
- Operational studies
- The Energy Management System
Availability of this data leads to more accuracy in planning and operating the transmission system, as well as more efficient outcomes of these processes.
Conclusion
In coming years, energy usage patterns are expected to change. This is because by mid 2030s, the annual peak demand is forecasted to be in winter than in summer because of increase in electrification of heating systems. Daily peaks are also expected to move from evening to morning. Demand on electricity from grid will become more variable due to increased installation of BTM PV and Energy storage systems. ISO is continually developing enhanced higher-resolution forecasting methods to develop more efficient models.
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Resources:
- Detailed report of 2025 Photovoltaic (PV) Forecast, by Distributed Generation Forecast Working Group (DGFWG)
