Chapter 3 of 4 · What changes when a customer makes power too?

DER & Microgrid Field Guide

A hands-on look at how Distributed Energy Resources (DERs) — solar, batteries, and the equipment that connects them — interconnect with the distribution grid. Worked example: a municipal site with 250 kW AC solar + 750 kW AC battery storage (an even 1,000 kW total DER nameplate). Using the rule of thumb that 1 MW ≈ 500 average homes' worth of instantaneous demand, that's roughly 500 households' worth of capacity tying into one feeder.

Site One-Line: Grid ↔ DER Interconnection

Utility primary (~12 kV) Secondary (site voltage) DC side of an inverter Data + control Who owns it Solar · battery swatch

Click any piece of equipment for a plain-language explanation.

Select a component above to see what it does, who owns it, and roughly what it costs.

DER & Microgrid Glossary

Power-Flow Playground

Adjust solar, load, and the battery — then flip the grid outage switch and watch what the DER Gateway actually does. This uses real interconnection logic, not just a visual toggle.

Adjust the controls to begin.

Solar output 150 kW

Site load 220 kW

Battery state of chargeiState of charge is stored energy (how full the tank is), not power. This battery holds 1,500 kWh when full, so each 10% is 150 kWh. The battery can push its full 750 kW rating from 20% SOC all the way up to 100%. Only below 20% SOC does the max power taper — in a straight line down to 0 kW at 0% SOC — the way real packs throttle back to protect their cells near empty. (Same taper mirrored near 100% for charging.) That ceiling only limits output when the site actually asks for more than it allows. 50%

Max available: 750 kW discharge · 0 kW charge

Battery strategy

Utility export limit at the PCC 500 kW

Grid status

Intentional islanding installed (microgrid controller + transfer switch)

Planning-Level Costs — 1,000 kW DER Interconnection Example

Two separate cost buckets: what the utility installs on its side of the meter to accept the DER safely, and what the customer/developer installs on-site to generate and store the power. Figures are illustrative planning-level ranges, not a bid.

Utility-side interconnection equipment Utility owned

Equipment	Role	Approx. cost
1 MVA distribution transformer	Steps feeder voltage down to service voltage sized for the combined 1,000 kW DER + site load	$100,000
Pole-mounted recloser	Utility-side fault protection; isolates the DER's tap point from the rest of the feeder during a fault	$70,000
DER Gateway / interconnection meter & comms	Utility visibility & control point — monitors export, can signal curtailment or trip	$3,000
Utility-side subtotal		$173,000

Customer-side DER equipment Customer / developer owned

Equipment	Typical unit cost	This site	Approx. cost
PV array + racking + string inverters (250 kW AC)	~$1.50–$2.20/W AC	250,000 W	$375,000–$550,000
BESS — battery, PCS/inverter, enclosure, HVAC, fire suppression (750 kW / ~1,500 kWh, 2-hr duration)	~$350–$500/kWh	1,500,000 Wh	$525,000–$750,000
Microgrid controller + automatic transfer switch (optional — only needed for intentional islanding)	flat	—	$150,000–$400,000
Customer-side subtotal (with islanding)			≈ $1.05M–$1.70M

Ranges are planning-level and vary heavily by region, labor rates, site conditions, interconnection study results, and whether islanding capability is included. The utility-side figures come from a real interconnection cost estimate for this exact 1,000 kW example; the customer-side figures are broad industry ranges (NREL/EIA cost benchmarks) meant to show relative scale, not a quote. Note the utility equipment ($173K) is a small fraction of total project cost — its purpose isn't to handle bulk power, it's to make the DER safe and visible to the grid.