Question re solar cells, amps and volts and watts

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HopePressler1
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Question re solar cells, amps and volts and watts

Post by HopePressler1 »

My 12 year old daughter is working on a science fair project using solar cells. I’m trying to help her understand how amps and volts work. She understands the concept that amps are like the amount of water going through a hose and volts are the pressure of water going through a hose.

The question is about watts. If you have 12 volts and 6 amps which equals 72 watts is that the same as 6 volts and 12 amps which also equals 72 watts? You end up with the same amount of watts in both cases, but is it the same?

She is wiring together solar panels and trying to charge lithium batteries in an rc plane while it’s flying so it could fly indefinitely or more likely to at least extend flight time. I understand the difficulty of the solar plane scenario, but she is very excited, and I think going through the process is a worthwhile experience.

My other question is why do some solar panels have 7.2 volts and only 100mA and other panels have .6 volts and 6 amps? How are solar panels made with such a variation in volts vs. amps?

Thank you for any help.
Hope
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Re: Question re solar cells, amps and volts and watts

Post by bfinio »

Hi Hope,

I'm glad your daughter is already familiar with the "water analogy" because it helps here (but you have to be careful, because electricity does not behave EXACTLY like water, so you can't apply the analogy too literally).

To answer your question - in both the cases you mentioned, yes, the total power is the same. As you've already discovered, the equation for electrical power is

Power [watts] = current [amps] x voltage [volts]

Due to conservation of energy, you cannot magically get more total power out of a power source. In this case, we're getting power from the sun, so there is a maximum amount of power from the sun that falls per square meter of surface. This is called solar irradiance (https://en.wikipedia.org/wiki/Solar_irradiance), and it varies a bit depending on where/how you measure it (e.g. at the top of the atmosphere vs at the surface of the earth, perpendicular to the sun vs sunlight hitting at an angle). But the point is there's some maximum amount of power we can get from the sun and we can't increase that.

What we CAN do, however, is create a trade-off between current and voltage. As you've discovered when looking at solar panels, some might have high voltage and low current or vice versa. Going back to the water analogy - think of the difference between a wide, slow-flowing river (low pressure, but high current due to the large amount of water) vs a pressure washer (very high pressure, but actually a very low amount of water flow).

Every electrical load (spinning a motor, charging a battery, lighting up LEDs, etc) will have different voltage and current requirements. Going back to the water analogy again...you wouldn't want to take a shower with a pressure washer, and you wouldn't want to use your shower head to pressure wash something. The same applies for electrical loads. So this allows you to choose panels appropriate for the load. As it sounds like you've already discovered, you can also wire solar panels together in series (to increase voltage) or in parallel (to increase current).

I'm not sure what the required specifications will be for the solar plane scenario, so that will require more research on your daughter's part. But, for example, there would probably be a difference between choosing panels that can drive the motor directly with no batteries (motors require a lot of current), vs charging a battery over a longer period of time (for example while the plane is on the ground), which can be done with lower current.

Hope that helps!

Ben
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