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What does the "W" number mean for a solar panel? For example 400W ?

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JohnnyDangerDude

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What does the "W" number mean for a solar panel? For example 400W ?

Does it mean if you have optimal condition, sun, angle, you can theoretically get 400W of power?

Or does it mean you can accumulate 400W in 1 hour of full sun?

Does it mean its 400W in 1 day of sun?

Does it means you have 400W X .20 of power?

What exactly does the 400W solar panel mean?

Ebay seller is selling me that a 500W panel will consume 500W in a day. I thought they were suppose to produce power not consume it?

Thanks.
 
It's the theoretical maximum power (Watts) that your panel can generate under standard test conditions (IIRC 25C and 1,000W/m2 of sunshine).

In real life you don't get near that figure very often or for very long.

Watts are an instantaneous measure of power. Energy is in joules or, more commonly Watt-hours (Wh) or kilowatt-hours (kWh) which is what your electricity supplier uses in order to bill you.

EDIT For a real-world example, here in sunny Thailand our 330W panels generate, on average, 1.2kWh per day, of course this is very much dependent upon the weather.

EDIT 2 Fixed Watt/hours to be Watt-hours to avoid confusion same with kilowatt/hours.
 
Last edited:
JohnnyDangerDude said:
Ebay seller is selling me that a 500W panel will consume 500W in a day. I thought they were suppose to produce power not consume it?
Click to expand...
Scamity scam! At best it'll be a 100w panel that can produce 500w in perfect conditions in a lab if you round up in a day.

The W in 400w would be how much power it can produce in STC test conditions where it's exactly 25c and getting 1000w of solar per square meter.
 
Ebay is full of these scam listings, they are not proper 400w panels capable of a peak output of 400 watts but a panel that over a full day will output 400wh (0.4Kwh) with a peak of 100 watts or 50 watts depending on how scammy it is.
 
JohnnyDangerDude said:
seller is selling me that a 500W panel will consume 500W in a day.
Click to expand...
Something is lost in translation.
A 400 watt panel is large, a 500 watt larger still.
A typical 400 watt panel will be around 20 Square feet area, 70 inch x 42 inch . The typical maximum output will be 35 volts at 11.5 amps. It's volts times amps that gives the watts, 35 x 11.5 = 400 .
You can confirm the panels power from the maximum volts and current rating , or from the area , about 17 to 20 watts per square foot.
 
crossy said:
It's the theoretical maximum power (Watts) that your panel can generate under standard test conditions (IIRC 25C and 1,000W/m2 of sunshine).

In real life you don't get near that figure very often or for very long.

Watts are an instantaneous measure of power. Energy is in joules or, more commonly Watt/hours (Wh) or kilowatt/hours (kWh) which is what your electricity supplier uses in order to bill you.

EDIT For a real-world example, here in sunny Thailand our 330W panels generate, on average, 1.2kWh per day, of course this is very much dependent upon the weather.
Click to expand...

The 400W panel is 400W as in a snapshot if you were to measure the power at its peak performance under best conditions right? It's not how many watts it produced over a period of time such as a hour or day right?

They Ebay scammers are trying to twist the meaning of everything to justify sending you a toy 20W solar panel and pretend its a 500W panel.

They are like " well, it will give you 500W if you run it 25 hours a day and add up all the electric"
 
mikefitz said:
Something is lost in translation.
A 400 watt panel is large, a 500 watt larger still.
A typical 400 watt panel will be around 20 Square feet area, 70 inch x 42 inch . The typical maximum output will be 35 volts at 11.5 amps. It's volts times amps that gives the watts, 35 x 11.5 = 400 .
You can confirm the panels power from the maximum volts and current rating , or from the area , about 17 to 20 watts per square foot.
Click to expand...

Well this "500W" panel is just about 1 square foot. I wonder if they do the same scam locally in China or if this is just something they do to Americans.
 
kommando said:
Ebay is full of these scam listings, they are not proper 400w panels capable of a peak output of 400 watts but a panel that over a full day will output 400wh (0.4Kwh) with a peak of 100 watts or 50 watts depending on how scammy it is.
Click to expand...

Ebay seems like they are willfully ignorant of this. I reported the listing and almost immediately I got 2 emails. One say would look into the issue and another one saying that no action will be taken. There is no way a person would look into this issue that quickly. Probably all bots, AI and 3rd party contractors.

This is the fake flash memory scam all over again. You Order a 1024GB flash memory and you get 1GB being labeled as 1024GB but no ability to store more then 1GB. When the seller like they are completely innocent and you must of just had corrupted data or something.

I think they rely on the fact that its hard to test these solar panels or flash memory out of the box without special tools or software.

The visual inspection on the size of the panel seems to be the best way to spot a counterfeit at first glance.

I order one of those meters to get an actual W value but it will take a while to arrive.
 
I have a small 50W panel sold as a 50W panel and it works as expected. But when buying it I came across all the scammy listings for the 400/500w postage stamp sized panels and that was 5 years ago.
 
JohnnyDangerDude said:
They Ebay scammers are trying to twist the meaning of everything to justify sending you a toy 20W solar panel and pretend its a 500W panel.

They are like " well, it will give you 500W if you run it 25 hours a day and add up all the electric"
Click to expand...
But it's worse than that. It's not twisting the meaning, it is just plain wrong. Watts don't add up.

It would be like advertising a car, claiming it can travel at 1000 mph, just because it you can get it to 50 mph on twenty separate occasions 😡
 
They're (purposely) mixing their units, confusing watts (instantaneous power) with watt-HOURS (power over time).
 
SeaGal said:
But it's worse than that. It's not twisting the meaning, it is just plain wrong. Watts don't add up.

It would be like advertising a car, claiming it can travel at 1000 mph, just because it you can get it to 50 mph on twenty separate occasions 😡
Click to expand...
W is not the same as Wh. It's a scam. There was another thread where someone else got scammed in a similar way.
 
crossy said:
It's the theoretical maximum power (Watts) that your panel can generate under standard test conditions (IIRC 25C and 1,000W/m2 of sunshine).

In real life you don't get near that figure very often or for very long.

Watts are an instantaneous measure of power. Energy is in joules or, more commonly Watt/hours (Wh) or kilowatt/hours (kWh) which is what your electricity supplier uses in order to bill you.

EDIT For a real-world example, here in sunny Thailand our 330W panels generate, on average, 1.2kWh per day, of course this is very much dependent upon the weather.
Click to expand...

Point of clarification. It can be confusing to describe energy as "power over time", because the wording ("over") implies the math operation of division. Properly stated, energy is an amount of power supplied for a certain duration of time.

Watt/hours and kilowatt/hours is incorrect. (But the post has been corrected.)

A kilowatt (kW) is a measure of instantaneous power. Energy is typically written (here) as an amount of power (in kW) delivered for an hour (h), and the two quantities (power and time) are *multiplied* to get the numerical value of energy.

For example. 4 kilowatts of power delivered for 3 hours yields 12 kWh of energy (4kW x 3h). Likewise, 6kW of power delivered for 2 hours also yields 12kWh of energy (6kW x 2h).

Note: There are several common quantities used for power and energy. For the purposes of this solar forum, kilowatts (kW) is used for power, and kilowatt-hours (kWh) is used for energy.

<end pedantic rant> :geek:
 
Last edited:
SeaGal said:
(y) Exactly - we should be describing power as "energy over time" instead. Power (in kW) = kWh/h ;)
Click to expand...

Calling the units moderator 😎

l00semarble

Thread 'Units moderator. There should be a units moderator assigned.'

I'm not volunteering.

I have to say that I fairly often view threads and then just close them and move on when the units used are all incorrect.

The point isn't to be a stickler and pain the the ass and not help people that don't understand. If someone is asking for help and they have their units all screwed up then help is less likely. The poster needs some help understanding the units used in this trade/hobby and getting them right and understanding them IS helping them.

If there are a variety of moderators maybe one can be tasked to nuke posts with incorrect units and help...
 
SeaGal said:
Power (in kW) = kWh/h ;)
Click to expand...
You joke, but I honestly think this would make more intuitive sense to people that aren't used to dealing with technical units.
The fact that the 'base' unit in electrical systems, the Watt, is a rate (equal to 1 Joule per second) seems really hard for people to grasp.
 
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