Solar Panel Output Question

[cdcooper]

Hello, my name is Chris and this is my first post. I am a relative solar novice. I have used the resources on the Mobile Solar Power website and this forum to build my latest system. Quick back round. A few years back I installed a rudimentary solar concept system to run lights in an off grid cabin. I used a 3 - 15w solar panel system from Harbor Freight, a VMAX SLR125 12 Volt 125ah AGM Solar Battery, a ProStar-15 solar charge controller and a cheap 200w inverter. We used the system to run some DC lights and charge phones, etc. Fast forward to 2022 and we were overdue for a system upgrade. Using the information from Mobile Solar Power website and this forum I designed and built a new all-in-one system using Sungoldpower's 3KVA Solar Inverter 24V 110V Parallel 80A Mppt Solar Charge Controller, a SOK 24V 100Ah LiFePO4 Battery and 2 Mega 200 Watt 24 Volt Solar Panels wired in series and mounted on the roof of the cabin facing South.

Here is my question. During full sun (say 12 noon) the solar inverter shows PV input voltage somewhere in the 82-83 volts range, it shows the amps between 2-3 amps. But the wattage never goes above 132 watts. Is this normal? The voltage seems accurate (open circuit voltage of each panel is 45.3v, maximum power voltage 37.6v) so given the fact the panels are wired in series the voltage should be roughly double the single panel voltage. But I was expecting the PV wattage to be closer to 200 watts. The panels are clean and free from obstructed light at that time of day. I know I'm missing a piece of the equation, but don't know what piece I am missing and would appreciate someone pointing me to the right resource so I can continue to learn.

Thank you.
Chris

 

[sunshine_eggo]

If our batteries are near full or in absorption phase, they will only accept the power needed to maintain voltage.

Try hitting it with a > 400W load and see if your PV increases.

Also, that unit is going to consume:

45W * 24h = 1.1kWh of energy just being on. That's 43% of your battery capacity.

With only 400W of solar, you may only harvest about 2kWh in good sun.

 

[cdcooper]

Very interesting. That makes sense now. When I checked the battery it was around 26.7V. I will try to generate a 400W load to observe what happens. Thank you for pointing out the inverter power consumption. I don't think I accounted for that much consumption and may eventually have to add a 2nd battery if I add a water pump. I considered to leaving the inverter powered on when we are not there as to power an outside motion activated light, but given the power consumption of the inverter, I am glad I decided to leave it off, as it should ensure a fully charged battery.

Thank you for providing such a clear & concise explanation.

 

[sunshine_eggo]

Very interesting. That makes sense now. When I checked the battery it was around 26.7V. I will try to generate a 400W load to observe what happens. Thank you for pointing out the inverter power consumption. I don't think I accounted for that much consumption and may eventually have to add a 2nd battery if I add a water pump. I considered to leaving the inverter powered on when we are not there as to power an outside motion activated light, but given the power consumption of the inverter, I am glad I decided to leave it off, as it should ensure a fully charged battery.

Thank you for providing such a clear & concise explanation.

Make sure your water pump does not have excessive surge. Your unit does not have a meaningful surge rating.

I can't say specifically with this unit, but many similar models will permit charging from AC and PV when in the OFF position. In this situation, you could turn the inverter off but still get charging. If you can install a 24V or 12V motion sensitive light, you could power it directly from the battery (you'd need a 24V-12V converter for 12V light).

 

[cdcooper]

I can confirm that this model solar inverter will allow PV charging while the inverter is switched off. Currently the motion sensitive light is wired into the cabin wiring. I like your suggestion and will look into running a separate wire and changing out the current light for a DC version at some point.

Thank you for the additional advice regarding the water pump. My current thinking is to set up a gravity feed to a holding tank behind the cabin, then use a relatively low power pump to provide water up to the faucet. That said I will now be cognoscente of the pump's surge rating before purchasing.

The only other additional (luxury) load we are considering is an AC drip coffee maker.

 

[sunshine_eggo]

AC water pumps tend to have 5X surge. Pumps may publish a surge or LRA (locked rotor amps), and that's your maximum surge.

A 1/2 hp pump will probably have a 30A or 3600W surge. I suspect your unit will be able to handle that.

By "holding tank" I assume you mean a tank that just holds water. If you're not already planning it, a pressure tank would be very beneficial as well. This will supply water pressure via a pressurized bladder in the tank. The water pump would only come on when the tank pressure drops below the set point. Without a pressure tank, water pumps can hammer on and off due to surges in pressure.