DER Sizing Methods

A practical guide to right-sizing distributed energy resources for performance, resiliency, and long-term flexibility.

Why DER Sizing Matters

DER sizing is not just a calculation—it is a system performance decision that directly affects reliability, operating stability, and lifecycle economics.

Incorrect sizing commonly drives avoidable outages, unnecessary capital cost, control instability, and faster battery wear—especially during islanded operation and mode transitions.

What incorrect sizing often leads to

Inability to sustain critical loads during outages.

Overbuilt systems that exceed budget without improving outcomes.

Instability during islanded operation or mode transitions.

Excessive battery cycling and accelerated degradation.

Missed peak shaving, fuel savings, or renewable utilization targets.

Limited flexibility as loads, policies, or facility needs evolve.

DER Sizing Workflow (Step-by-Step)

A successful sizing process follows a clear workflow—so decisions remain transparent, traceable, and defensible across stakeholders.

Flowchart • Step-by-step • Dropdown details

Define System Objectives

Start with the outcomes the microgrid must achieve.

Critical load continuity (hours/days)
Islanding capability and transition performance
Fuel reduction targets
Renewable utilization goals
Demand charge management
Grid service participation (if applicable)

Establish Load & Criticality Profiles

Confirm load inputs before sizing DERs.

Total load vs critical load
Priority tiers (must-run, mission critical, discretionary)
Peak demand and ramp rates
Expected growth and expansion phases

Confirm Operating Modes

Define how the microgrid must operate.

Grid-connected normal operation
Islanded operation during outages
Transition and resynchronization behavior
Black start requirements and restoration sequencing

Size for Power Requirements (kW)

Power sizing ensures the system can handle:

Peak load and transients
Motor starts and inrush events
Contingencies and reserve margin
Stable islanded operation

Size for Energy Requirements (kWh)

Energy sizing determines endurance:

Backup duration targets
Renewable intermittency coverage
Load shifting capability
Battery usable energy (DoD limits + reserves)

Validate With Studies & Scenarios

Confirm sizing works across real conditions:

Normal and worst-case scenarios
Cloud events / low solar days
DER outages or maintenance states
High-load / low-renewable combinations
Protection and coordination sensitivity impacts

Optimize and Document

Finalize sizing with clear deliverables:

DER sizing summary (kW/kWh)
Assumptions and design constraints
Single-line diagrams and system boundaries
Control and operating philosophy alignment
Utility compliance notes and next-step studies

Tip: Lock assumptions first, then size (kW → kWh), then validate across scenarios before final documentation.

DER Type	Primary Inputs	Key Outputs
Load / Facility	Load profile Peak demand Critical loads Growth	kW peak kWh consumption Critical load tiers
Solar PV	Available area Irradiance Export limits Energy targets	PV kWdc/kWac Expected production (kWh) Curtailment risk
BESS	Backup duration Peak shaving needs Cycling limits Inverter role	Battery kW (power) kWh (energy) Reserve margins
Dispatchable Gen	Reliability target Islanded needs Fuel strategy	Generator kW rating Redundancy (N-1) Runtime planning
Hybrid Portfolio	Economics + resiliency requirements Controls constraints	Optimized DER mix Performance validation scenarios

Common Sizing Pitfalls to Avoid

Frequent DER sizing mistakes include:

Sizing PV based only on annual energy totals (ignoring hourly load patterns and time-of-day mismatch).

This can create major coverage gaps during evenings or early mornings—when the load is present but PV output is low.

Oversizing PV without considering curtailment risk, interconnection limits, export restrictions, or net metering caps.

The result is wasted generation and a lower ROI because the system can’t export or use the energy it produces.

Undersizing battery power (kW) while focusing only on energy capacity (kWh).

You end up with stored energy that can’t be delivered quickly enough during peak demand or outages.

Assuming full battery capacity is usable (ignoring DoD limits, reserves, degradation, and required backup margins).

Usable energy is typically less than nameplate due to operating constraints and long-term performance protection.

Oversizing generators, forcing low-load operation (inefficiency, wet stacking, higher maintenance, reduced lifespan).

Generators perform poorly at low load and can accumulate unburned fuel deposits—raising O&M costs and shortening life.

Ignoring system losses and thermal limitations (inverter, wiring, transformer losses, derating, round-trip efficiency).

Losses reduce delivered power/energy; thermal derates can cut output precisely when the system is under stress.

Failing to account for seasonal variation (winter solar dips, summer peak cooling loads, wind resource changes).

A design that “works on average” can fail during seasonal extremes—where reliability requirements are tested.

Skipping scenario testing for transitions and contingencies (load spikes, black start, startup delays, islanding, extreme weather).

The “hard moments” reveal control gaps—validation prevents commissioning surprises and redesign cycles.

Not aligning sizing with the operational strategy (peak shaving, resilience backup, demand charge reduction, full islanding).

A well-sized system is sized “for the job”—objectives should drive kW/kWh, controls, and study scope.

✅ Good sizing avoids these issues early—reducing commissioning delays, preventing costly redesigns, improving system performance, and ensuring long-term reliability and operability.

Validation Requirements

DER sizing is inherently project-specific and must be validated through engineering analysis. This page provides general educational guidance only. Final sizing decisions must be confirmed through:

Required Engineering Validation Checks

Load profile analysis and power quality review
Production modeling and renewable variability assessment
Dynamic modeling and simulation
Operating mode and transition scenario testing
Reliability and critical load continuity requirements
Utility constraints and interconnection review
Economic assessment (CapEx/OpEx/fuel savings/value stacking)

Final sizing decisions should always be developed and reviewed by qualified professionals.