Shopping for solar panels means encountering a dizzying array of numbers and specifications. Watts, voltage, amps, temperature coefficients — what do they all mean, and which ones actually matter when choosing panels for your home?
Wattage (Power Rating)
The wattage rating is the most commonly cited specification. It tells you how much power a panel produces under Standard Test Conditions (STC): 1,000 W/m² of sunlight at 25°C cell temperature.
Residential panels typically range from 350W to 450W. A higher wattage means more power per panel, so you’ll need fewer panels for a given system size. A 400W panel produces about 1.6 kWh per day in average conditions.
Efficiency Rating
Efficiency tells you what percentage of sunlight hitting the panel gets converted to electricity. Consumer panels range from 17% to 23%. Higher efficiency means more power per square foot of panel area — critical if your roof space is limited.
Temperature Coefficient
This often-overlooked spec tells you how much efficiency drops as temperature rises above 25°C. It’s expressed as a percentage per degree Celsius, typically -0.3% to -0.5%/°C.
A panel with a temperature coefficient of -0.35%/°C will lose 3.5% of its output when the cell temperature rises 10°C above 25°C. On a hot summer day, panel surface temperatures can reach 60–70°C, making this spec important for warm climates.
Voltage Ratings
Open circuit voltage (Voc) is the maximum voltage the panel produces with no load connected. Maximum power voltage (Vmp) is the voltage at which the panel produces maximum power. These matter for inverter compatibility and system design but are mainly the installer’s concern.
Current Ratings
Short circuit current (Isc) is the maximum current the panel can produce. Maximum power current (Imp) is the current at maximum power output. Again, these are important for system design and wire sizing.
NMOT vs STC
Some manufacturers also list performance under Nominal Module Operating Temperature (NMOT) conditions, which are more realistic than STC. NMOT assumes 800 W/m² irradiance, 20°C ambient temperature, and 1 m/s wind speed. NMOT ratings are typically 20–25% lower than STC ratings and give you a more realistic picture of everyday performance.
Degradation Rate
This tells you how quickly the panel loses output over time. Premium panels degrade at 0.25–0.3% per year, while budget panels may degrade at 0.5–0.8% per year. Over 25 years, this difference adds up significantly.
Certifications to Look For
IEC 61215: International standard for crystalline silicon panel design qualification and type approval.
IEC 61730: Safety certification for panel construction.
UL 1703 / UL 61730: U.S. safety listing required for grid-connected installations.
What Matters Most
For most homeowners, focus on wattage, efficiency, temperature coefficient, warranty terms, and the manufacturer’s reputation. Don’t get bogged down in voltage and current specs — your installer handles those. A quality panel from a reputable manufacturer with a strong warranty is more important than chasing the absolute highest numbers on a spec sheet.
