Power Output Calculations

Estimated solar production for 1,720W array in Rutland, VT (43.6°N, 72.97°W)

System Parameters

Array	4 × QCells Q.TRON BLK M-G2+ 430W = 1,720W STC
Tilt / Azimuth	26.57° fixed (6:12 pitch) / 180° (due south)
Inverter	EG4 6000XP (DC input 100–480V, 2 MPPT, 99% MPPT eff.)
Inverter efficiency	96% (CEC weighted)
DC wiring losses	2%
Soiling losses	2%
Year 1 degradation	1.5%
Combined system loss factor	89.9%

1. Monthly Production Estimate

Production formula per day:

E_daily = P_stc × POA × F_temp × F_system

where:
  P_stc    = 1.72 kW (array STC rating)
  POA      = plane-of-array irradiance (kWh/m²/day) at 26.57° tilt
  F_temp   = 1 + (−0.30%/°C) × (T_cell − 25°C)
  T_cell   = T_ambient + (NOCT − 20) = T_ambient + 23°C
  F_system = (1 − 0.015) × 0.99 × 0.96 × (1 − 0.02) × (1 − 0.02)
           = 89.9%

Month	POA Irrad. kWh/m²/d	GHI kWh/m²/d	Amb. Temp °C (°F)	Cell Temp °C	Temp Derate	Daily kWh	Monthly kWh
January	2.85	1.70	-7.5 (19°F)	15.5	102.8%	4.53	140.5
February	3.70	2.36	-6.4 (20°F)	16.6	102.5%	5.87	164.2
March	4.80	3.43	-1.4 (29°F)	21.6	101.0%	7.50	232.5
April	5.12	4.86	6.1 (43°F)	29.1	98.8%	7.82	234.6
May	5.08	5.40	12.8 (55°F)	35.8	96.8%	7.60	235.6
June	5.43	5.66	17.5 (64°F)	40.5	95.3%	8.01	240.2
July	5.96	5.73	20.0 (68°F)	43.0	94.6%	8.72	270.3
August	5.61	5.08	18.9 (66°F)	41.9	94.9%	8.24	255.3
September	5.20	4.04	14.7 (58°F)	37.7	96.2%	7.73	232.0
October	3.31	2.57	8.1 (47°F)	31.1	98.2%	5.02	155.8
November	2.45	1.65	2.2 (36°F)	25.2	99.9%	3.79	113.6
December	2.16	1.37	-3.6 (26°F)	19.4	101.7%	3.40	105.3

2. Annual Summary

Annual Production2380kWh/year

Average Daily6.5kWh/day

Capacity Factor15.8%

Specific Yield1384kWh/kWp

Cross-check with NREL PVWatts v8:PVWatts estimates 2,063 kWh/year AC output for this system (1.72 kW, 26.57° tilt, south-facing, premium module type, 14% losses). Our calculation yields 2380 kWh/year, which is 15.4% higher than PVWatts. The difference is attributable to PVWatts' premium module assumptions (19% eff., −0.35%/°C) versus our actual panel specs (22% eff., −0.30%/°C), and our explicit loss model versus PVWatts' aggregate 14% system loss.

3. String Configuration Analysis

The EG4 6000XP accepts 100–480V DC input per MPPT channel. We must ensure the string voltage stays within this range across all operating temperatures.

Option A: 4 Panels in Series (4S1P)

Recommended

Vmp (STC)	32.94 × 4 = 131.76V
Voc (STC)	39.32 × 4 = 157.28V
Imp	13.05A
Isc	13.74A

Temperature-Adjusted Voltage Range

Cold extreme (−25°C)	Voc = 39.32 × [1 + (−0.0024)(−25 − 25)] × 4 = 176.2V
Hot extreme (35°C amb)	Vmp = 32.94 × [1 + (−0.0024)(58 − 25)] × 4 = 121.3V

Verdict: Voc at coldest design temp = 176.2V (under 480V limit). Vmp at hottest = 121.3V (above 100V minimum). This configuration works safely within the EG4's 100–480V input window. All 4 panels connect to a single MPPT input, leaving the second MPPT channel free for future expansion.

Option B: 2 Strings of 2 in Parallel (2S2P)

Not Recommended

Vmp (STC)	32.94 × 2 = 65.88V
Voc (STC)	39.32 × 2 = 78.64V
Imp	13.05 × 2 = 26.10A
Isc	13.74 × 2 = 27.48A

Temperature-Adjusted Voltage Range

Cold extreme (−25°C)	Voc = 39.32 × [1 + (−0.0024)(−25 − 25)] × 2 = 88.1V
Hot extreme (35°C amb)	Vmp = 32.94 × [1 + (−0.0024)(58 − 25)] × 2 = 60.7V

Verdict: Vmp at hot temps = 60.7V and even STC Vmp = 65.88V. Both are below the EG4's 100V minimum MPPT start voltage. The inverter will not track maximum power reliably, and may fail to start MPPT entirely in warm weather. This configuration is not viable.

Recommendation

Use Option A (4S1P). Wire all 4 panels in a single series string connected to one MPPT input of the EG4 6000XP. The voltage range (121V hot to 176V cold) fits comfortably within the 100–480V window with margin on both ends. The second MPPT channel remains available for future array expansion (up to 4 more panels in another 4S string).

4. Methodology & Sources

Calculation Method

Monthly AC energy is computed using the standard PV performance model:

E_monthly = P_stc × POA × F_temp × F_system × days

F_temp = 1 + γ_Pmax × (T_cell − 25°C)
T_cell = T_ambient + (NOCT − 20°C)

F_system = (1 − degradation) × η_MPPT × η_inverter × (1 − wiring) × (1 − soiling)
         = (1 − 0.015) × 0.99 × 0.96 × (1 − 0.02) × (1 − 0.02)
         = 89.9%

This approach follows the methodology described in the NREL PVWatts Version 5 Manual with explicit loss factors rather than a single aggregate loss percentage.

Loss Factors

Loss Category	Value	Rationale
Year 1 degradation	1.5%	QCells datasheet spec (LID + initial stabilization)
MPPT efficiency	99%	EG4 6000XP datasheet
Inverter efficiency	96%	CEC weighted efficiency (typical for transformer-based)
DC wiring losses	2%	Industry standard for residential roof-mount
Soiling	2%	Conservative for Vermont (rain-washed, low dust)
Temperature	Varies	−0.30%/°C from STC (25°C), per QCells datasheet

Note: Snow losses are not modeled separately. At 26.57° tilt (6:12 pitch), snow generally slides off within a day or two. Any persistent coverage is partially captured in the POA irradiance data (which reflects actual historical cloud/weather patterns) and the soiling factor. Actual winter production may be 10–20% lower than shown if heavy snow events persist.

Raw Irradiance Data

Plane-of-array irradiance values from NREL PVWatts v8 API for 26.57° tilt (6:12 pitch), 180° azimuth:

Month	GHI (kWh/m²/d)	POA @ 26.57° (kWh/m²/d)	Tilt gain
January	1.70	2.85	+68%
February	2.36	3.70	+57%
March	3.43	4.80	+40%
April	4.86	5.12	+5%
May	5.40	5.08	-6%
June	5.66	5.43	-4%
July	5.73	5.96	+4%
August	5.08	5.61	+10%
September	4.04	5.20	+29%
October	2.57	3.31	+29%
November	1.65	2.45	+48%
December	1.37	2.16	+58%

The 26.57° tilt (6:12 roof pitch) boosts winter irradiance (up to +68% in January) by better aligning with the low winter sun angle. Summer POA is slightly below GHI because the sun passes overhead at angles steeper than the panel tilt.

Data Sources

1. Solar irradiance (POA) — NREL PVWatts v8 API (TMY data from NSRDB, Perez model for plane-of-array decomposition). Query: lat=43.6, lon=−72.97, tilt=26.57, azimuth=180, array_type=0 (fixed).2. Global Horizontal Irradiance — NREL Solar Resource API v1 (Perez-SUNY/NREL, 2012). Verified against Solar Energy Local (ZIP 05777).3. Temperature data — US Climate Data, Rutland VT and Weather-US, Rutland VT. Monthly averages computed as mean of daily high and low.4. Panel specifications — QCells Q.TRON BLK M-G2+ 430W datasheet.5. Inverter specifications — EG4 6000XP product documentation.6. PV performance methodology — Dobos, A. (2014). PVWatts Version 5 Manual. NREL/TP-6A20-62641.

Last updated March 2026. All estimates are for Year 1 production. Subsequent years: degrade at 0.33%/year per QCells warranty.