Off-Grid Microgrid Sizing for Remote Health Clinic in Northern Maine

Scenario

A new off-grid primary care clinic in Aroostook County, Maine (latitude 47.3°N), required reliable solar generation year-round. Key constraints: (1) extreme seasonal irradiance variation (GHI drops to ~1.2 kWh/m²/day in December), (2) snow cover risk limiting optimal winter tilt, and (3) strict reliability requirement — no more than 48 hours of battery backup depletion during worst-month conditions.

Given Data

• GHI: 3.4 kWh/m²/day (annual average, but design based on December value: 1.2)
• DHI: 1.0 kWh/m²/day (December)
• DNI: 0.4 kWh/m²/day (December — low due to frequent cloud cover)
• Tilt angle: 55° (optimized for winter sun elevation; matches latitude +15° rule)
• Azimuth angle: −10° (slight west-of-south offset to capture higher afternoon insolation and reduce morning snow melt interference)

Calculation

Using the same POA model with December inputs and adjusted geometry:

• θ ≈ 48.5° at solar noon (cos θ ≈ 0.662)
• β = 55° → cos(β) ≈ 0.574
• ρ = 0.7 (fresh snow albedo)

DNI component = 0.4 × 0.662 = 0.265 DHI component = 1.0 × [(1 + 0.574)/2] = 1.0 × 0.787 = 0.787 Reflected component = 1.2 × 0.7 × [(1 − 0.574)/2] = 0.84 × 0.213 = 0.179

POA = 0.265 + 0.787 + 0.179 = 1.231 kWh/m²/day

The Solar Irradiance Calculator yields 1.23 kWh/m²/day, matching manual calculation to two decimals.

Result and Decision

The December POA (1.23 kWh/m²/day) was used to size the 18 kW PV array (with 40°–60° adjustable tilt mounts) and 120 kWh lithium-iron-phosphate battery bank. The design met the 48-hour autonomy target even with 15% soiling loss and 10% system derating. The team rejected fixed-tilt alternatives (<45°) after modeling showed <0.95 kWh/m²/day in December — insufficient for critical loads.

Lesson

In high-latitude, snow-prone locations, maximizing winter POA requires aggressive tilt angles and accounting for elevated albedo — neglecting snow reflection can underestimate POA by up to 20%, risking undersized off-grid systems.