Max amps possible from scam panel

paul12345

Asking for a friend - true! I cannot understand a panel of only 1170 * 540 mm (46 * 21 inch) has a power rating of 400 W. So I searched & found a package the drop-shipper claimed  was 800 watt (two of the 400),  and one critical reviewer writes:
"...
so the panel has 80W not 800. I give two stars since the quality and cables looks good and the packaging was ok. I measured during good sunshine with direct inclination and both (2) panels connected in series gave 41Volt with 4Amps = 160W so thats 80W per panel. Sure I didn't expect 400W per panel (that is not possible with this size) but I hoped for 150W per panel considering the size and the price..."

If so would then one 80W going into a 12V battery be at 7 amp (max) or would max amps be much lower?

BB.

As an example, a good quality crystalline silicon panel is around 15-17% efficient. And solar noon is around 1,000 Watts per sq meter:

• 1.16 meter * 0.54 meter = 0.6264 sqm
• 0.6264 sqm * 1,000 Watt/sqm * 0.15 efficiency = 94 Watts 
• 94 Watts Pmp@std test conditions * 0.77 panel+controller temperature/deratings = 72 Watts typical (warm summer day near solar noon)

So 80-90 Watts (rated) for this size panel--Seems about right.

More or less, 140 Watt or smaller panels can ship "best way" (UPS, Bus, Post Office, etc.). Panels >~140 Watts typically have to be placed on a pallet (packed/protected) and ship by truck (depot to depot, or truck with lift gate--Depending on how many panels).

Shipping "large format" panels can be expensive for packaging+shipping+insurance... And that can exceed the price of the panel if shipping one or two panels.

When shopping around, make sure you the full costs (panels+shipping) to make sure you are not surprised later.

Panels have a Vmp and Imp rating under standard test conditions (solar "flash" test @ room temperature). More or less, you can estimate the Vmp-panel by counting the number of cells... Each solar cell is pretty close to 0.5 Volts per cell.

• 36 cells in series = 18 volts Vmp (typical "12 volt panel")
• 60 cells in series = 30 volts Vmp (a typical Grid Tied panel)
• 72 cells in series = 37 volts Vmp (another typical GT panel, or "24 volt" panel for battery systems)

The current output of a solar panel--These are (more or less) "current sources" (batteries are voltage sources). The amount of current a panel output is (roughly) based on the amount of sun hitting the panel. For example a 18 Vmp @ 5 Imp panel will output (roughly) under full noontime sun 5 amps from zero volts to 18 volts Vmp. Above Vmp, the current begins to drop.

If you have 80 Watt panels and Vmp=18 volts, then Imp:

• 80 Watts Pmp / 18 Vmp = 4.44 Amps (under full noontime sun)

At 50% sunlight, then 4.44 * 0.50 = 2.22 amps under those conditions.

Note there is connecting the panel directly to the battery bank. And there is Panel=>PWM controller=>battery... And for PWM controllers, maximum current output from array = maximum current input to battery (assuming battery is discharged, needs full charging current).

For MPPT controllers, the math is a bit different (for larger wattage arrays/systems, MPPT has other advantages). But usually for "smaller systems", PWM controllers are just fine.

Is this making any sense to you? It is answering your questions?

-Bill

paul12345

Thanks, I was calculating the Pmp by battery system. Should be Vmp.