DER Types and Scale

The US grid was planned around a few hundred large power plants. By 2035, it may need to coordinate hundreds of millions of small devices with combined capacity rivaling the entire coal fleet.

Distributed energy resources (DERs) are grid-connected devices located on the customer side of the meter that can generate, store, or shift electricity, distinguishing them from centralized plants by their small individual scale and massive collective number.

There are four functional categories:

• Distributed generation: rooftop solar, small wind, backup generators, and fuel cells that produce power at or near the point of use
• Distributed storage: behind-the-meter batteries (like home Powerwalls or commercial battery systems) that can absorb or release energy on demand
• Flexible demand: smart thermostats, water heaters, and industrial loads that can shift consumption in time without losing their service function
• Electric vehicles: mobile batteries that charge from the grid and, in vehicle-to-grid (V2G) configurations, can discharge back into it

The policy relevance of this taxonomy is scale. Individually, a 6 kW rooftop solar system is trivial. Aggregated across tens of millions of homes, the combined US DER capacity is projected to reach 200-350 GW by 2035 (Rocky Mountain Institute, 2023). The entire US coal fleet is about 200 GW. A resource class of that magnitude cannot be treated as a rounding error in grid planning.

One feeder, four device types

Consider a single distribution feeder serving 500 customers in a suburban neighborhood in 2030. Spread across those customers: 120 rooftop solar installations (avg. 8 kW each), 40 home batteries (avg. 10 kWh), 180 EVs, and 300 smart thermostats enrolled in a demand response program.

What is the combined generation and storage potential on just this one feeder?

Roughly 960 kW of solar generation capacity, 400 kWh of battery storage, up to 1.3 MW of charging load if every EV plugs in at once (or discharge in V2G), and 300+ thermostats that can shift load by 30-90 minutes. A single feeder designed in 1985 to push power in one direction is now hosting a small virtual power plant, one that no central dispatcher fully controls.

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