How to Buy Solar Panels
Understand solar panels before buying
Learn about solar panel ratings before buying. Many people are confused by the difference between the various power ratings, or the kW and kWh. The following is a practical guide to remember what each rating means.
Solar Panels have 3 Ratings
Each solar panel is rated for perfromance using three environmental scenarios for solar intensity, temperature and wind. These scenarios are standard in the industry, and given the acronyms STC, PTC and NOCT. A solar panel is capable of all three ratings. It is important to understand that the performance of any module will fluctuate throughout the day, and the ratings offer a guide for what to expect.
STC MAX watts at Factory
STC Peak Watts are the FACTORY RATED MAXIMUM DC POWER generated under Standard Test Conditions for sunlight and temperature using a "flash" testing methodology. STC irradiance is at 1000 W per square meter, cell temperature at 77F (25C), air mass is AM 1.5. STC requires a high level of solar radiation at a very low temperature to produce the rated amount of power. That means their production values are infrequent, occurring on only a few mild, clear spring or summer days.
PTC MAX watts Real World
PTC Peak Watts are the REAL-WORLD RATED MAXIMUM DC POWER generated under PVUSA Test Conditions for sunlight and temperature. PTC irradiance at 1000 W per square meter, cell temperature at 68F (20C), air mass AM 1.5. This is more realistic rating of maximum power. PTC ratings are determined by third-party testing labs, and then published by the California Energy Commission (CEC).
NOCT watts Everyday
Nominal Watts are the AVERAGE CONTINUOUS DC POWER generated in EVERYDAY conditions for sunlight and temperature. NOCT means Nominal Operating Cell Temperature. NOCT irradiance at 800 W per square meter, ambient temperature at 68F (20C), solar cell temperature at 113F (45C), wind speed 2 mph. NOCT ratings are typically reported in the manufacturer's specification or datasheet. Unless reported on the panel spec sheet, generally calculated here as PTC watts x 0.82%.
Next, let's understand how these panel ratings translate to a complete system that will deliver energy to your home or business.
What is a kW?
A kW is a kilo-watt.
1 kW is one kilo-watt or one thousand watts.
In other words, if a solar panel has an STC DC rating of 500 watts, then two solar panels would make 1,000 watts or 1 kW DC power.
kW = POWER
Think of kW like a car speedometer that is capable of going up to 100.
kWh = ENERGY
Think of kWh as a car that travels 300 miles at an average speed of 30 MPH over time on a tank of gas (or electricity for an EV).
What is a kWh?
kWh is a kilo-watt hour.
1 kWh is one kilo-watt hour, or one thousand watts for an hour. kWh are measured in AC ratings.
Your utility bill is measured in AC kWh consumed every month. The average home uses 30 kWh per day or 11,000 kWh per year.
Check your utility bills to see how much energy (kWH) you use every year.
How much POWER does your home need?
Most homes can accept from 24,000 watts to 48,000 watts of AC power from the utility at any moment.
If your home has a 100 Amp electrical panel that operates at 240 Volts, then the house can accept up to 24,000 watts (100A * 240Vac) of power from the utility at any moment.
That is a lot of power. For example, if you have a 1,000 watt microwave oven, then you could have enough power for 24 microwaves running at the same time. Most homes never use that much power.
In other words, that is just the MAX power rating; it does NOT mean that you need 24kW or 48kW of solar to power your home or business. Instead, the solar array will be sized to generate the energy needed to offset the desired kWh consumption for the project. If at any moment, more power is needed than the solar array can produce, then the utility can deliver that power in concert with the solar.
Solar kits are ranked by kW or Power
Any given solar kit can generate AC kWh of energy over time.
For example, a 10 kW DC solar kit generates 13,000 AC kWh per year in cloudy Cleveland, while that same 10kW generates 18,600 kWh per year in sunny San Diego.
The average home in the USA consumes 11,000 kWh per year. That generally translates to requiring 7 kW in a climate like California, but could be 10 kW in less sunny environments
Once you check your utility bills and know your kWh energy consumption per year, then go to Calculate How Much Solar to determine what size solar kit is needed.
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