Simple numpty based electrical question

[Loggo]

Electricity has always puzzled me and still does.

Could someone answer me this please. If I have a solar panel of 100 w I believe it should be around 8 amps ( watts being voltage x amps)

All other things being equal in good (alright perfect) conditions over a 10 hour period can I take it that the battery will receive an additional 80 amps of charge ?

Or is there more to it than that ?

 

[jwilson]

The 100 watt output (correct - about 8 amps) will almost certainly be based on absolutely perfect conditions: midday sun on a clear day, with the panel exactly square on to the sun and not a trace of shade on any part of the panel. In real life you are very unlikely to get 80 amp-hours of charge in 10 hours from a 100 watt panel.

 

[JumbleDuck]

Could someone answer me this please. If I have a solar panel of 100 w I believe it should be around 8 amps ( watts being voltage x amps)

Yup, spot on. In practice, though you are unlikely to get 8A, because (a) the panel won;t be facing directly at the sun and (b) the sun won;t be at full summer midday brightness. But your calculation is fine.

All other things being equal in good (alright perfect) conditions over a 10 hour period can I take it that the battery will receive an additional 80 amps of charge ?

Almost, Amperes measure current, which is the rate at which electricity is flowing round the circuit. The cumulative effect of current over time is to transfer charge. The scientific unit of charge is the coloumb, which is 1 ampere second (ie one amp flowing for one second) but that's a fiddly small quantity, so for practical purposes it's normally replaced by ampere hours. So if you had 8A for 10 hours you would get 80 ampere hours of charge.

Or is there more to it than that ?

Nope - I think you're there.

 

[Tempus]

The maths are correct, but in practice any form of energy transfer or storage is not 100% efficient.

your solar cell will only deliver its rated output when directly facing strong sunshine. As the angle changes, or the light intensity falls the charge will fall quite quickly. Assume a 60-80% max efficeiency.

The charging efficiency of batteries also falls as they approach full charge, so below half charge they charge at around 90% efficiency, but this can fall to as low as 60% when the battery is above 80% full.

Net result is that on a sunny day, into a largely discharged battery you could get around 70% of the rated charge into the batteries, in your example of 80ah you'd get around 56ah. However topping up a reasonably charged battery on a normal sunny day could easily be at below 50% and possibly below 40% giving around 40ah.

 

[VicS]

Electricity has always puzzled me and still does.

Could someone answer me this please. If I have a solar panel of 100 w I believe it should be around 8 amps ( watts being voltage x amps)

All other things being equal in good (alright perfect) conditions over a 10 hour period can I take it that the battery will receive an additional 80 amps of charge ?

Or is there more to it than that ?

100 watts is the maximum power you will get under standard conditions of 1000watts/ sq.metre of sunlight at 25C.

The specification of the panel should tell you the terminal volts at which this can be achieved. It is usually in the 15 to 18 volt range. for a 12 volt panel intended for battery charging
The spec should also tell you the open circuit ( no load ) volts, which is likely to be in to 20 to 22 volt area, and the maximum current

It may also give a typical daily output in ampere hours ( Ah)
In practice in the UK you will only get at 4 hours at maximum power daily and typically in in summer you will get around 30 to 35 Ah per day put back into your battery if you use an MPPT controller. 10 %, or more, less with a PWM controller

In winter you may only get around 8Ah per day


Although I dont think it covers this subject a good source of general info for electrical numpties is the TB-Training website http://www.tb-training.co.uk/

 

[xeitosaphil]

I have a new 100w solar panel fitted about two months ago, its open circuit voltage is about 21v but it is rated at 18v and also states its max output is 5.56A. ( as do most others of this wattage)
My understanding right or wrong is, the solar panel voltage is pulled down to battery voltage 12+ once it is connected to the battery,and that the rated output is also pulled down by the same percentage difference beween its rated output at 18v and the battery voltage.
I can honestly say that since having the panel in some of the perfect conditions available the last few in South Devon , with the panel south facing, I have not had more than 4.6A from it.
I think most panel outputs are based on about 6hrs of charge in the UK, so your 80Ah maybe a little optimistic.

Phil

Yup, spot on. In practice, though you are unlikely to get 8A, because (a) the panel won;t be facing directly at the sun and (b) the sun won;t be at full summer midday brightness. But your calculation is fine.



Almost, Amperes measure current, which is the rate at which electricity is flowing round the circuit. The cumulative effect of current over time is to transfer charge. The scientific unit of charge is the coloumb, which is 1 ampere second (ie one amp flowing for one second) but that's a fiddly small quantity, so for practical purposes it's normally replaced by ampere hours. So if you had 8A for 10 hours you would get 80 ampere hours of charge.



Nope - I think you're there.

 

[[3889]]

To give a practical example, the highest ampage I've ever seen from my 80W panel ( Theoretical max =6.6) , angled directly at the sun in a cloudless sky in July, S. France, was 5.1A, measured at the panel terminals.

 

[pvb]

I think solar panels used in the UK are likely to be a disappointment.

 

[lw395]

A nominal 12V panel will be putting out at least 14V to charge a battery, so there is one error.
But a nominal 10W panel will usually give out more than 10W under lab conditions when new, because they degrade with age and tend to be rated around their output when a few years old.
Solar panels in the UK need not be a disappointment if you have reasonable expectations.
I have a small one charging a battery for lights in my lock up, a boat I sail on has one to trickle charge the batteries. both of these have done what's needed and given good value for money.
If you want to run fridges and inverters 24/7 you may need to spend a lot and give a lot of boat space to them.

 

[Stemar]

Jissel has 40w of solar panels. In the grotty spring we had, that meant that, after a weekend on board with no other charging, the batteries were fully charged when I came down the following weekend. This summer, as long as we didn't run the cold box much, they kept up with our needs for lights, water pump and an hour or two of CD/radio.

Someone suggested that, as an average, I could expect 25% of your wattage in amp-hours per day, so my 40w should give around 10AH a day, more in summer, lots more if it's sunny, less in winter, lots less on those dull grey days. So far that feels about right.

 

[Loggo]

Thanks to everyone for the replies. I think I will try a 200w, and see how things go - just to extend my mooring up time next summer.

 

[rob2]

I use a rule of thumb that I calculate electrical inputs and outputs and then assume 50% efficiency at best... Works for me.

Rob.

 

[macd]

All other things being equal in good (alright perfect) conditions over a 10 hour period can I take it that the battery will receive an additional 80 amps of charge ?

Or is there more to it than that ?

As others have suggested, in order to produce anything close to peak output, panels need to be close to perpendicular to the sun. (They'll produce over 90% if within 20 degrees or so of that). Unless you're going to spend all day pivoting them about, this isn't going to happen, even in the tropics. But on a midsummer day at your latitude, a horizontally-mouted panel will be only 30 degrees from the ideal...but that's only at solar noon. The further the sun is from its zenith, the less the output of a fixed panel. Shade effects, as mentioned, also have a huge effect -- even from thin things like halyards, stays and aerials.

That said, in my experience they provide a useful contribution to your energy budget, even at UK latitudes. Just don't expect anything like sustained peak output.

 

[William_H]

Thanks to everyone for the replies. I think I will try a 200w, and see how things go - just to extend my mooring up time next summer.

Solar panel size is usually dictated by space on the boat for the panels. I think that in the long run cost is not really the limiting factor. So if you reckon you can mount 200watts then go for it. A hint here that the panels sold for house PV systems are often much cheaper than boat ones, they about 200 w but 40 v. This is not however a problem with a MPPT regulator for a 12v system. good luck olewill

 

[duncan99210]

Rather than a single 200w panel, use smaller panels. They're easier to find places to fit than one big panel and a shadow across one panel will only affect the output from that panel, not all of them.

 

[Stemar]

Rather than a single 200w panel, use smaller panels. They're easier to find places to fit than one big panel and a shadow across one panel will only affect the output from that panel, not all of them.

+1

The only place for my panels is on the hatch garage, where there's a risk of shading from the boom. Because of that, I got 2, 20w panels. Research suggested that I'd get more umph out of one 20w in full sun and one shaded than a single 40w partially shaded. Seems to work like that so far, though I've never actually measured the output.