Schoolboy Physics Question (Watts/Amps/Volts)

peterb26

peterb26

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(I wish I had paid more attention at physics as school......:mad:)

I am now happily comparing a number of solar panels - but can't convert "watts" to "amp hours".

Thought it was watts = amps x volts ??

If that is the case, then will a 90 watt panel give me 7.5 amps on a 12v circuit under perfect conditions?? Just seems too good to be true thats all....

Thanks in advance:)
 
Yes, and if run for an hour will give you 7.5 Ah

Ignoring inefficiencies, which is a big ignore! (ie your battery won't have anywhere near 7.5 Ah more in it after an hour)

Also when charging the voltage will rise to about 14V so you need to re-do you calc with 14V in instead of 12V or more like 6.5, again iIgnoring inefficiencies.
 
peterb26 said:
(I wish I had paid more attention at physics as school......:mad:)

I am now happily comparing a number of solar panels - but can't convert "watts" to "amp hours".

Thought it was watts = amps x volts ??

If that is the case, then will a 90 watt panel give me 7.5 amps on a 12v circuit under perfect conditions?? Just seems too good to be true thats all....

Thanks in advance:)
Click to expand...

Firstly a fully charged 12V battery should read 12.7/12.8V, the '12V' is a nominal figure. Secondly in order to 'force' a charge into the battery the charge voltage needs to be higher still, most alternators and regulators will be set at 14.2/14.4V. So under perfect conditions a 90W panel will give a tad over 6A, but only if pointed directly at the sun, no cloud and nothing to cast even the tiniest shadow.

We had two panels on our last boat. The 75W one (mounted flat on a gantry) would peak at around 4.5A at midday and the 120W one at around 7.5A at the same time. These were summer figures in UK/French waters.

I read once that a good average daily input from a 60W panel in southern UK in June/July/August would be about 20Ah, so pro-rata giving 30Ah from a 90W panel.
 
Robin said:
We had two panels on our last boat. The 75W one (mounted flat on a gantry) would peak at around 4.5A at midday and the 120W one at around 7.5A at the same time. These were summer figures in UK/French waters.

I read once that a good average daily input from a 60W panel in southern UK in June/July/August would be about 20Ah, so pro-rata giving 30Ah from a 90W panel.
Click to expand...

Out of interest, how big is a (modern) solar panel producing 75W or 120W? And how much would you expect to pay for one?

Thinking of fitting a battery monitor and wondering whether, at the same time, it would be worth fitting a solar panel just to add a bit of a boost when at anchor for a day or two.

Cheers.
 
Angele said:
Out of interest, how big is a (modern) solar panel producing 75W or 120W? And how much would you expect to pay for one?

Thinking of fitting a battery monitor and wondering whether, at the same time, it would be worth fitting a solar panel just to add a bit of a boost when at anchor for a day or two.

Cheers.
Click to expand...

I bought the 120W one over 10 years ago (still have it, it will go with us to the USA when we eventually escape) and it cost about £500 back then. The 75W one was already fitted to the boat when we bought her and was sold with her. My guess is they are probably 15% or so higher now?

These are not cheap items but if you anchor a lot (we do) and like to be self sufficient powerwise without running the engine except on passage if required they are excellent. We have arrived at an anchorage with 60% full batteries, stayed 6 days and had to switch one panel off periodically to avoid overcharging. We were running a large (210L) fridge 24/7 plus all the other usual stuff. We didn't have regulators on our panels but they could be switched off manually and I monitored them carefully using a digital voltmeter and individual ammeters on each panel.
 
Robin said:
We didn't have regulators on our panels but they could be switched off manually and I monitored them carefully using a digital voltmeter and individual ammeters on each panel.
Click to expand...
I thought breaking the output circuit of an active solar panel was bad practice, where does the 75 watts go when it is switched off?
 
Angele said:
Out of interest, how big is a (modern) solar panel producing 75W or 120W? And how much would you expect to pay for one?

Thinking of fitting a battery monitor and wondering whether, at the same time, it would be worth fitting a solar panel just to add a bit of a boost when at anchor for a day or two.

Cheers.
Click to expand...

I have a total of 125 Watts from two panels, one about 500 x 600 mm and the other 1000 x 600 mm. They are on an arch at the stern of the boat, fairly well out of shadow. Originally they were intended to rotate to maximise sunshine but it was just not worth the effort so they are now horizontal. At midday, midsummer (like now) they are producing 9 Amps in Greece. They begin to produce a little at around 0700 and still produce something at 2000. Yesterday at 1750 on a cloudy day they were producing 2.7 Amps.

For costs see midsummerenergyl who are competitive and reliable. My panels were bought from them but seem to be unlisted at present.
 
I thought breaking the output circuit of an active solar panel was bad practice, where does the 75 watts go when it is switched off?
Click to expand...
No where!

watts = volt x amps.

switch it off and the amps becomes zero. volts x 0 = 0. ie no watts to go anywhere :D

Inductive devices like wind generators and alternators are a different kettle of fish. Interrupting the current causes a big rise in the volts and it is the voltage surge that causes the problems like blowing the diodes.
 
VicS said:
switch it off and the amps becomes zero. volts x 0 = 0. ie no watts to go anywhere
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Energy cannot be created or destroyed it just changes form. If the photo-electric process cannot dispatch energy in the form of electrical current where does the energy go?
 
jonjo5 said:
Thank you. Is that bad for the solar panel?
Click to expand...

Well the energy that would have been used to create electricity will simply be absorbed and raise the temp of the panel.

Silicon is fine up to well over 200 deg C so it shouldn't harm it - but the panel may be made of a plastic that doesn't cope as well. I'd have thought the panel manufacturer would give acceptable temp ranges for the panel.
 
pteron said:
Heat.
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... or is electricity not actually produced in the first place if there isn't a battery connected to complete the circuit? Not an expert, so just throwing that into the debate to see if anyone can confirm or deny.

Obviously (even when the panel is "on") the sun's energy falling on the panel that is not converted to electricity is converted to heat.
 
Angele said:
... or is electricity not actually produced in the first place if there isn't a battery connected to complete the circuit? Not an expert, so just throwing that into the debate to see if anyone can confirm or deny.

Obviously (even when the panel is "on") the sun's energy falling on the panel that is not converted to electricity is converted to heat.
Click to expand...

Essentially, the light causes carriers to flow in one direction, but by doing so an opposite voltage is created which causes the intrinsic carriers to drift in the other direction balancing it out. So there is an internal current flow in the photodiode.

But heat is the easy answer - all energy ends up as heat eventually :)
 
I suspect the question has been done to death but what is the relative performance of solar panels and wind gennies in UK / French waters?

I need to do something - the fridge and lights reduce the two new 110 amp domestics to 12.2 v in 48 hours so I end up having to run the engine. I have tried a 60 watt flexible solar panel on a previous boat, but came to the conclusion that it was fit only for keeping batteries topped up on an unoccupied / switched off boat. On the other hand a brief flirt with an old Ampair convinced me it was too noisy to tolerate.

But all that was 10 years ago. How is the comparison now?
 
Bosun Higgs said:
Doesnt go anywhere. Just like a battery that isnt connected.
Click to expand...

correct. The unconnected panel still produces a voltage. this is called the open circuit voltage. If you measured it, you'll find it higher than the nominal 12V dc output produced when the panel is connected, but limited to a maximum by the panel size/no. cells/cell technology).

no load connected means no curent flow. no current flow means no power draw. Just an open circuit voltage.

Another problem with disconnecting wind generators (DC permanent magnet types) is that no current draw means no back EMF being generated in the output coils. This can cause the generater to "runaway" - i.e. rotate at high & potientially destructive speeds. unecessary bearing wear etc. also.
 
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