# Efficiency Problem

Registered Users Posts: 2
edited October 2015 #1
I installed three 300W Solar panels in parallel With the following electrical ratings for each
Vmp= 36 V
Imp= 8.33A
Voc=43.2
Isc=9.85A
with PWM Charge Controller to Charge two 12V 200A Batteries in Series.

Does (Vpm=36V) Mean that I need to add one more battery to the system to gain the optimum output current
if So will an MPPT charge controller solve my problem without the need to add one more battery and replace my 24V inverter ?

Thanks

• Solar Expert Posts: 494 ✭✭✭
Panels in parallel connection, right?  Under some conditions there will be some "waste" of the panel capacity. That is just the way it is. The Vmp needs to be higher than the battery bank by an amount sufficient to compensate for the drop in Vmp in hot weather. You also need sufficient Vmp to have a high enough voltage to do an EQ.   An MPPT controller might make better use of the available PV output under some conditions. However, some of that gain can be lost to the internal losses of the MPPT controller.

Some of us use MPPT controllers because of the distance between our best PV panel location and the CC and batteries. For example, our PV is 325 feet of wire distant. Using higher voltages from the PV in series saved money on wire size.
Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
• Registered Users Posts: 2
Thank you for the quick response

currently I am getting 24Ah Max instead of 900/24=37.5Ah (At solar noon)
its like I am wasting 1/3 of the panels power

I mean will an MPPT CC make that big different?

• Solar Expert Posts: 3,741 ✭✭✭✭
Hazmak said:
currently I am getting 24Ah Max instead of 900/24=37.5Ah (At solar noon)
its like I am wasting 1/3 of the panels power

I mean will an MPPT CC make that big different?
There is the issue of STC vs NOCT output.  The 300 watt ratings of your panels is at STC (standard temperature and conditions).   In the real world your panels will have a NOCT rating which accounts for the panels being hot, and other factors.   My point being: don't blame your controller for losing 1/3 of your power... the power wasn't all there to begin with.

You asked about MPPT vs PWM controllers.   You would probably gain a bit with an MPPT controller, especially in the winter.  Whether the increase in harvest is worth the cost depends on many factors.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 494 ✭✭✭
edited October 2015 #5
Do your system batteries achive a full charge daily? As measured by a hydrometer, not a voltage reading or some figure coming from some electronic device. If the batteries reach full sp gr, reach float charge every day, things are working. Don't worry about PWM vs MPPT. If your system does not reach float every average day, then you should definitly worry and do something.

Does the present CC have capacity to handle more panels? If you needed more charge power that could be cheaper than a new MPPT CC.

How long has the system been in operation? Have you been through a winter, the time of least PV power?
Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
• Solar Expert Posts: 5,803 ✭✭✭✭✭
I tried to figure out your which panels you had from your spec's but google had nothing. NOCT values (Normal Operating Cell Temperature) tend to be about 75% of your panels rating so a 300 watt panel would normally produce about 225  watt at solar noon.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Super Moderators, Administrators Posts: 32,596 admin
For a PWM charge controller, your math is not correct:
currently I am getting 24Ah Max instead of 900/24=37.5Ah (At solar noon)
its like I am wasting 1/3 of the panels power
The Math for a PWM controller looks more like:
• Watts Array / Vmp = Imp-array
• 900 Watts / 36 Volts Vmp = 25 Amps "high noon" current
Note, the Imp-array will be (mostly) affected by the amount of sun light hitting the panel. Battery Voltage will not matter near as much (between 12 volts and ~30 volts, the Imp-array will be about the same current).

For MPPT controllers, the work on the Pmp=Vmp*Imp principle... So, on cold weather Vmp is high (Imp is about the same over temperature for our needs), Pmp will rise. In normal warm weather, the Vmp will fall by almost 20% (plus you have another ~5% loss in the charge controller).... More or less, the typical (no winter) maximum power production would be:
• 900 Watts * 0.77 panel+controller derating * 1/29 volts battery charging voltage = ~24 amps typical max output (MPPT)
• 900 Watts * 0.77 panel+controller derating * 1/24 volts battery charging voltage = ~29 amps typical max with discharged battery
A MPPT controller can give you ~10% to 15% more current in sub-freezing weather--But that is not, usually, the reason for getting MPPT controllers. The usual reason is you can get "GT solar panels" (less expensive solar panels, but more expensive MPPT type charge controller) and put the panels farther away from the battery bank+charge controller (longer wire runs)--and get very cost effective system (reduce copper wiring costs).

For larger systems (say over ~800 Watt), an MPPT type system + GT panels is usually a better choice. For smaller systems (typically less than ~400 Watts), a PWM system can be a better (more cost effective) system.

Does that make sense?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset