Panel comparison

[skellator]

This might be a dumb question but I am scratching my head looking at these numbers.

Which solar setup will produce more power, how much more, and why?

Setup #1

- Panel: https://www.renogy.com/300-watt-24-volt-monocrystalline-solar-panel/
- Charge Controller: https://www.renogy.com/rover-li-40-amp-mppt-solar-charge-controller/
- Battery: https://www.renogy.com/12v-100ah-smart-lithium-iron-phosphate-battery/

Setup #2
- Panel: https://www.renogy.com/175-watt-monocrystalline-solar-panel/
- Charge Controller: https://www.renogy.com/rover-li-40-amp-mppt-solar-charge-controller/
- Battery: https://www.renogy.com/12v-100ah-smart-lithium-iron-phosphate-battery/

Only difference in the two is the panel. One is a 24v panel the other is a 12v.

Thanks!

 

[DThames]

Power (in this case) is measured in watts. 300 watts is more than 175 watts. Notice also in the specifications of the panels, the 300w panel is larger, catches more sun, makes more power.

 

[skellator]

At the end of the day isn't it all about the amps it produces? Or do I have this all backwards? Thank you for you reply

 

[DThames]

Power (in this case) is measured in watts. 300 watts is more than 175 watts. Notice also in the specifications of the panels, the 300w panel is larger, catches more sun, makes more power.

Amps going into the battery, yes...all about amps. Power, is different. 10 amps at 40v is double the power of 10 amps at 20v (random example). The MPPT charger can take 10 amps off of a 300w, 24v type panel and make about 20 amps going into a 12v battery. The 175w, 12v type panel will output about 9 amps, and about 9 amps will go to the battery. Power in watts is what is produced. More power, faster charging. Also more power, more power available in less than ideal sun conditions.

 

[hankcurt]

I think DThames probably answered the question you were asking, but I wanted to add some other ways to look at panel efficiency.
People are usually trying to get either the most watts per dollar spent or the most watts per square foot (if they have limited space). Also, the output of the panels drops when they are on a hot roof, so I like to do the Normal Operating Condition calculations and use those power numbers instead of the laboratory power numbers that are advertised.

As a demonstration, I calculated those things for each of the panels below.

175 Watt panel (standard test conditions, 25°C)

• Surface Area 9.53 ft²
• Renogy website price $233.99
• Normal Operating Condition Temperature (NOCT) 47°C
• Maximum Power Point (Pmax) temperature coefficient -0.42% per °C
• Power output lost under hot conditions
  • 175w x -0.42% x (47°C - 25°C) = -16.2 watts so...
  • 175w - 16.2w = 158.8 watts output under hot conditions
• Power per Square Foot
  • 158.8 watts / 9.53 ft² = 16.66 watts per ft²
• Power per Dollar (based on Renogy website price)
  • 158.8 watts / $233.99 = 0.678 watts per $

300 Watt panel (standard test conditions, 25°C)

• Surface Area 17.71 ft²
• Renogy website price $306.99
• Normal Operating Condition Temperature (NOCT) 47°C
• Maximum Power Point (Pmax) temperature coefficient -0.47% per °C
• Power output lost under hot conditions
  • 300w x -0.47% x (47°C - 25°C) = -31 watts so ...
  • 300w - 31w = 269 watts output under hot conditions
• Power per Square Foot
  • 269 watts / 17.71 ft² = 15.19 watts per ft²
• Power per Dollar (based on Renogy website price)
  • 269 watts / $306.99 = 0.876 watts per $

So the result of all this is that if you are space constrained, the 175 watt panels will give you more power in a limited space, but if you are trying to get the most power for your money, the 300 watt panels are the better value (but you can probably find better prices and should recalculate the cost per watt with those prices).

 

[JRG]

hankcurt,

Where did you this this spec from?:

Maximum Power Point (Pmax) temperature coefficient -0.42% per °C

Also, is the NOCT always 47C?

 

[hankcurt]

I got the spec from the datasheets on Renogy's website, 175w here, and 300w here.

The coefficients between the two panels were different, -0.42% for the 175w and -0.47% for the 300w, so the 175 watt panels are marginally better at handling the heat.

I used the NOCT from the datasheet. It is reasonably close to a warm day roof temperature, and I assume that is the temperature that they tested the panels at. If you know that your operating conditions are different, I would just substitute your temperature in the calculation.

 

[JRG]

Perfect, thanks!

 

[Fallingaway]

Thank you for sharing that! Very useful to be able to calculate the watt/$!

 

[chrisski]

I want to add to this thread that there’s other panels out there as good or better, but much cheaper than the the Renogy Panels. I have nothing against Renogy. I bought two of theire 175 watt flexible panels today because they fit in the puzzle of solar panels I have made my RV roof into. Had I went with a Standard glass monocrstalline panel, I would have went with something off SanTanSOlar Which is local to me.

When looking at any panels, you should compare the claimed watts versus the dimensions. I was ready to order a 200 watt panel from an unbranded company that was much cheaper than a 175 watt panel I was buying until I saw it was also much smaller. So there’s no way it would perform to specs.

 

[Fallingaway]

So true! I saw a 300ah lifepo4 on amazon today and it was only 100$ more than their 200ah version and it was the same size as the 200ah. Needs to be consequences for shady sellers online, it’s getting really bad especially on eBay.