Amps from 120w PV panel??

G'day Wombat

Theoretically 120/12=10 A.
Realistically consider about 80% efficiency i.e. 8 amps

May be more, may be less but I would work along those lines as a best case scenario.
More competent persons may offer a more accurate answer

What controller do you have?
In theory you might see up to nearly 9A (at, say, 13.5V) but that's assuming a very good mppt controller and ideal conditions for the battery you're charging.

Poor/cheap mppt controller or PWM and you're looking at whatever the max is for the panel(s) so probably in the 6.5A range but check your panel specs.

What starts out as a simple question so often has some complexities hidden in it!

As Garrycol and Ross M have said, panels are rated at their maximum power point, which occurs at about 17-18 volts. So 120 watts divided by 17 volts tells us the panels will deliver about 7 amps under ideal conditions.

But what is delivered to the battery? Connected directly to the battery, the battery will initially load the panels so the panel voltage drops and the panels will deliver their 7 amps until the battery is fully charged and can't accept any more current. When that point is reached, the current drops and the panel voltage rises and can damage the battery. A controller is used to prevent this occurring. The simplest controllers recognise that the battery is fully charged and disconnect it from the panels. MPPT controllers allow the panels to operate at their Maximum Power Point, 17V and deliver their maximum current, 7A. The battery needs about 14-14.5V to charge it; the controller effectively swaps the excess panel voltage for some extra charging current. The better MPPT controllers are pretty efficient and provide a significant increase (around 15+%) in charging current. The not-so-good ones don't do so well.

So - the straight answer is that the panels can provide about 7A. How much reaches the battery? Depends on the controller, but 7 to 8.5A.

You might find Electricity for Camping a useful read.

Cheers

John

Wombat,

I purchased two "cheeeeep" units on ebay. They are rated at 120 watts. This is at the maximum voltage of 18.6 volts. I have checked all the specs of lots of panels and the same applies if you spend $300 or $800 per panel.

P=EI so P (power) 120w = E (volts) x I (current or amps)

My system runs up to 120w at the maximum voltage of 18.6 volts so.... 120=18.6 x 6.45 amps
In fact it runs at 12 to 14 volts depending on the amount of sun and gives me between 4 and 6 amps per panel. (each panel is giving me realistically between 60 and 80 watts at 12 volts)

As my two fridges draw 3.5 amps each when running, one did not give me enough, so I now have two panels and everything is great for 3 weeks or more camped in the one spot

Hope this helps
Peter

Depends on the quality of the panel. Some panels are rated more conservatively than others and can produce more than their rated output. I have a couple of Sharp 130 watt 22volt panels on a cheap PWM controller – these (2 in parallel) blow 15 amp fuses in ideal conditions (up to approx. 8.5 amps each). Had to change the fuses to 20 amp fuses.

Good to see so many mathematical equations.... and most are sort of there.

One important thing people are forgetting is on most panels the rated output is with a panel temperature of 25 Deg C at maximum sunlight..... on an average day this temperature can increase to 45+ Deg C...... as the panel temperature increases the voltage decreases as does the amps.

The output of the panels can be up to 20% less at 45 Deg C compared to 25 Deg C.

So take into account the average 10-20% lost in general efficiency, wiring and sunlight input plus another 20% due to temperature you can be looking at an overall efficiency of between 60-70% of the advertised output.

On a 120w panel I would expect an output of around 80w.

The cheap solar kits are a a great buy, but you do need to upgrade the wiring and connectors to Anderson plugs ( If you use connectors) as they are way to restrictive. Remove the controller from the back of the panels and shift to the battery/ies.

http://www.ecoonline.com.au/product-information/solar-pv-panels/solar-wire-sizing-calculator