# Solar panel watts&amp;amp output

Registered Users Posts: 3
edited January 2017 #1
Hi,
I have 2 130 watt panels on my RV for a total potential of 15amp output per hour if my battery is low in the morning.
A friend recently installed one 260 watt panel on her van that is rated for about 8amps output per hour.
Can you explain this?
Edited.

It is a bit more complicated that that... Assumign your friend has a MPPT type solar charge controller--they are doing just fine.

You probably have a PWM charge controller--Which is fine for your installation

Power = Volts * Amps.

A PWM controller "lets through Amps"... I.e., (forgetting losses), 15 amps in is 15 amps out.

A MPPT charge controller is a "down converter"... A DC switching (buck mode) power supply. Again, ignoring losses, the MPPT controller takes Watts in and outputs the same Watts out:
• Power = Voltage * Current; Current = Power / Voltage
• 260 Watt / 14.5 volts charging = 17.9 amps out
• Including losses: 260 Watts * 0.77 panel+controller deraiting * 1/14.5 volts = 13.8 amps typical maximum current
A PWM controller:
• 260 Watts * 1/18 volts Vmp = ~14.4 volts Imp (current maximum power)
Realistically, if both installation are properly designed and installed, they have about the same output.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 3
Thanks for that explanation. I had been faked out by the 8 amp rating on the label while misunderstanding the different types of controllers. I had kind of thought the panel might have been 24v but anyway... last thing I needed to know. heh.
• Registered Users Posts: 3
Hi,
Well, I found out now that her controller is a PWM Sunsaver 20 by Morningstar. So, she is getting only the 8amp rating to the battery?
• Solar Expert Posts: 1,589 ✭✭✭✭
Yes, and that 260 watt panel is putting, at best 115 watts into her batteries.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.