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Microgrid Sizing Calculator

Size a microgrid for 1-5 buildings. A microgrid can disconnect from AND reconnect to the grid — that islanding capability is what separates it from simple off-grid solar.

Load profile
zones
kWh/zone
%
Infrastructure
kW
days
PSH
Microgrid sizing result
40.0 kW solar + 240 kWh battery + 30 kW inverter
Total daily load (2 zones)160 kWh/day
Critical load (battery-backed)96 kWh/day
Solar array40.0 kW (100 × 400W panels)
Battery bank240 kWh LFP (at 80% DoD)
Inverter/charger30 kW
Transfer/control switchAutomatic Transfer Switch (ATS)
Backup generator30 kW (est. $18,000 installed)
Microgrid controller cost$7,500 (the microgrid premium)
Est. total system cost$274,500
Simple off-grid (no reconnect)$267,000 (saves ~$7,500 vs microgrid)
Grid-tied with backup$167,500 (less autonomy, no islanding)
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How to Use This Calculator

Define your zones and load

A microgrid serves one or more buildings or load zones from a shared power source. Enter the number of zones (1-5) and average daily kWh per zone. If zones have very different loads, use the average. For accurate data, check 3-6 months of utility bills or download interval data from your smart meter. Commercial buildings typically use 80-200 kWh/day; residences 20-50 kWh/day.

Set critical load percentage

The critical load is the portion that must stay on during a grid outage: refrigeration, medical equipment, communications, security, essential lighting. This is what the battery bank is sized to serve. Non-critical loads (EV charging, air conditioning, water heating) can be shed during outages. A typical home might have 30-40% critical load; a hospital 80-100%.

Choose islanding capability

The defining feature of a microgrid (vs. simple off-grid) is the ability to disconnect from the utility grid, operate independently ("island"), and then reconnect when the grid restores — automatically and safely. This requires a microgrid controller ($3,000-8,000) in addition to the solar, battery, and inverter equipment. The calculator shows cost comparisons for all three approaches.

The Formula

Total Daily Load = Zones × kWh Per Zone Critical Daily Load = Total Load × Critical Load % Solar Array (kW) = Total Daily Load ÷ PSH ÷ 0.80 Battery (kWh) = Critical Daily Load × Autonomy Days ÷ 0.80 (DoD) Inverter (kW) = (Total Load ÷ 8 hrs) × 1.5 safety factor → round to 5 kW System Cost = Solar + Battery + Inverter + Controller/Switch + Generator Microgrid Premium = Microgrid Controller ($3,000-8,000) + ATS ($2,500) Off-Grid Savings = Skip Controller + Skip ATS (no reconnect capability)

The key cost driver that differentiates a microgrid from simple off-grid is the microgrid controller — typically $3,000-8,000 — plus a compatible automatic transfer switch. This controller manages the seamless handoff between grid-connected and islanded operation, monitors grid voltage/frequency for reconnection, and coordinates multiple power sources (solar + battery + generator). Without it, you have off-grid solar with manual disconnection — cheaper but without automatic islanding.

Example

Small business campus — 2 zones in Dallas

A small office campus with 2 buildings uses 80 kWh/day per building. Critical load is 60% (servers, security, refrigeration). They want 2-day battery autonomy and grid islanding capability, with a 30 kW backup generator.

Zones2 buildings × 80 kWh/day = 160 kWh/day total
Critical load60% = 96 kWh/day battery load
Autonomy2 days
Location PSH5.0

Result

Solar array40 kW (100 × 400W panels)
Battery bank240 kWh LFP
Inverter/charger30 kW
Microgrid controller + ATS~$7,500
Estimated total cost~$265,000

The microgrid controller and ATS add ~$7,500 to the cost versus simple off-grid, but provide automatic seamless islanding — the system disconnects from the grid during outages without any manual intervention and reconnects when grid power restores. For a business campus, this automatic operation is worth the premium.

FAQ

Off-grid solar operates permanently disconnected from the utility grid — there is no grid connection at all. A microgrid is normally grid-connected but can disconnect (island) from the grid during outages and then reconnect when grid power restores — automatically and safely. The reconnection is the hard part: it requires synchronizing voltage, frequency, and phase with the utility before reconnecting, which is what the microgrid controller manages. Microgrids give you the cost benefits of normal grid connection (smaller battery needed, no need to size for worst-case winter) plus resilience during outages.
A microgrid controller is the brain of the system. It continuously monitors grid voltage and frequency; manages the transition from grid-connected to islanded mode when the grid fails; optimizes how solar, battery, and generator work together; controls load shedding (turning off non-critical loads); and manages reconnection to the grid when it restores. Leading products include Schneider Electric's EcoStruxure Microgrid, ABB Ability, and Caterpillar's Energy Management System. Costs range from $3,000-8,000 for small systems up to $50,000+ for larger commercial installations. Budget approximately $5,000 as a planning figure.
For grid-tied microgrids in areas with reliable utilities: 1-2 days of critical load autonomy is typically sufficient — most outages are resolved within 24-48 hours. For areas with unreliable grids (rural, storm-prone): 3-4 days. For fully off-grid remote sites in low-sun locations: 5-7 days. Adding a backup generator dramatically reduces the required battery size — a generator can recharge batteries during extended cloudy periods, allowing you to use 2-3 day batteries instead of 7-day banks at much lower cost.
Some existing systems can be upgraded to microgrid capability. If you have a hybrid inverter (like a Victron MultiPlus, Schneider XW+, or SMA Sunny Island) with automatic transfer switching already installed, adding a microgrid controller software layer is often possible. However, basic grid-tied systems with standard string inverters cannot be upgraded — they require anti-islanding protection that deliberately shuts them down during grid outages for lineworker safety. An energy installer can assess your existing system and quote an upgrade path. The key is having islanding-capable hardware from the start.
Microgrid permitting is more complex than standard grid-tied solar. You typically need: electrical permit (local AHJ), interconnection agreement with the utility (if grid-tied), possibly a Public Utility Commission review for larger systems (>100 kW in many states), and IEEE 1547-2018 compliance for grid interconnection (covers voltage/frequency ride-through and reconnection protocols). The utility must agree to the islanding scheme — some utilities actively resist islanding capability. Work with an experienced energy engineer and check your state's microgrid-specific incentive programs, as several states now fast-track microgrid permits.

Related Calculators

🏙 Off-Grid Solar Calculator
Size a complete off-grid solar system — no grid connection required.
🔋 Battery Backup Calculator
How much battery capacity you need for critical loads during outages.
⚡ Solar Generator Calculator
Size a solar generator for portable or backup power needs.
📅 Days of Autonomy Calculator
Calculate how many days your battery bank powers your loads without solar.

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