Overall Efficiency of MPPT Charge Controller vs. Inverter vs. Combo Units (Off-Grid)

sunnyboy

Is there a general consensus on how to build the most efficient off grid system?  I see these 90+% efficiency specs, but I am not sure how to compare them to determine the best option:

1) MPPT Charge Controller + Inverter as separate units tied together via the battery bank (do I multiply their efficiency percentages) vs.

2) A combo MPPT Charge Controller - Inverter unit.  Does the efficiency spec mean solar output to 110V AC efficiency?

Also, is there a general rule that combo units are more efficient?  I am also wondering if there is difference in efficiency in using the AC electricity as it is being generated in the day vs the conversion to battery energy then back to AC via the inverter at night.  I suspect that the roundtrip to the battery probably wastes some energy.

Photowhit

sunnyboy said:

 I am also wondering if there is difference in efficiency in using the AC electricity as it is being generated in the day vs the conversion to battery energy then back to AC via the inverter at night.  I suspect that the roundtrip to the battery probably wastes some energy.

Solar panels ONLY generate DC energy. Using AC during the day when Solar is producing energy will be more efficient than using from a battery bank. Depending on the type of battery, They can be 35% more efficient than using AC from a Nickel Iron battery bank, or around 5% when using a Lithium battery bank. Lead acid falls somewhere in between around 85% for flooded lead acid more efficient for some AGM's.

Estragon

IMHO, there should be no difference in efficiency between separate components and combo per se, all else held equal (design, component and build quality). In both cases, there are losses in charge controller, battery, and inverter (which do multiply).

Using power during the day is not only not incurring battery losses, it likely increases battery life expectancy.

BB.

I think photowhit has a typo... Lithium batteries have less than 5% loss (95 to near 100% charging efficiency).

AGM batteries are probably in the 95 to 98%+ efficiency range.

In general, focus on your loads and location (hours of sun per day, any shading, etc.). Some that full of grid solar will cost you around $1 to $2+ per kwh (or 5-10x typical utility bill cost).

Understanding your load and focusing on conservation (most efficient loads, turn stuff when not used, lots of insulation and good building design for local weather conditions) is almost always cheaper than making a larger off grid power system.

All in one units or separate units... Usually a matter of personal choice and needs (ease of repair, integration, remote control & status).

Start the design based on loads and sun first. Once you have sized the system, you can better choose what hardware will work best for you.

Bill

Photowhit

BB. said:

I think photowhit has a typo... Lithium batteries have less than 5% loss (95 to near 100% charging efficiency).

No, I've been reading about 5% for lithium(LiFePO4 used for home storage). Here's Victron Energy stating 92%

https://www.victronenergy.com/upload/documents/Datasheet-12,8-Volt-lithium-iron-phosphate-batteries-EN.pdf

Here's a Science journal suggesting 95%;

https://scitechdaily.com/lithium-iron-phosphate-batteries-could-lead-to-cheaper-more-efficient-solar-energy/

So using energy from the array vs using energy from stored battery sources saves...

35% with Nickel Iron,
15-20% Flooded Lead acid,
5% with Some AGM and LiFePo4

Raj174

This is very likely the case for LFP battery banks with balancing BMS's installed because they bleed off power at absorb voltage to balance the cells. This may constitute the lesser efficiency. I would think that most end users would purchase this type of battery bank. However, banks without this type of BMS, those that only monitor and implement low and high battery protection, are 99+% efficient. 

Rick

Dave Angelini

Raj174 said:

This is very likely the case for LFP battery banks with balancing BMS's installed because they bleed off power at absorb voltage to balance the cells. This may constitute the lesser efficiency. I would think that most end users would purchase this type of battery bank. However, banks without this type of BMS, those that only monitor and implement low and high battery protection, are 99+% efficient. 

Rick

Definitely Rick ! Also often misquoted in this efficiency number is the CC conversion loss when charging and the inversion loss when discharging running AC loads.

The round trip will vary but in a balanced system with plenty of charge current and reasonable discharge current the system is pretty amazing to someone who is use to all the loss of Lead Acid. The different chemistries of Lithium used for offgrid are getting even better with all this new interest in storage.  I see that LG Chem has 2 new home batteries out soon, One an AC coupled and one DC coupled.

BB.

As an example of why I suggest an overall conversion efficiency of an off grid AC power system is ~52% (charge during day, use power at night/during bad weather):

• 81% derating of Vmp*Imp=Pmp for "warm to hot solar panels" (Vmp falls as panels get hot in sun)
• 0.95 MPPT charge controller efficiency
• 0.80 Flooded Cell Lead Acid battery Efficiency (if not FLA, use one of above numbers from Rick)
• 0.85 AC inverter efficiency

Multiply this all together:

• 0.81 * 0.95 * 0.80 * 0.85 = 0.52 = 52% end to end Solar Panel to 120/240 VAC system efficiency

Note that the above makes lots of estimates... If you live in a cold climate (sub freezing), your panels can approach 100% of rated output (high mountains, less air, etc.).

For MPPT controllers, they can range in efficiency by a fair amount (large controller, morning sun, less efficient). Also the derating math formulas for MPPT vs PWM controllers is entirely different. However, the end results of 0.95 MPPT eff * 0.81 solar panel derating = 0.77 panel+controller deratings is "close enough" to use for either MPPT or PWM controllers (note that "cold solar panels" do not produce more power in sub freezing weather for PWM controllers).

AC inverter efficiency is ~90-95% if the inverter is operated in "middle of power ratings"... However, lightly loaded or heavily loaded inverters are less efficient--Use 85% as "conservative" derating.

And, unless you manage your loads every day based on how much sun you have (by seasons, weather conditions), for a system that has both base loads (refrigerator, lighting, etc) and optional loads (clothes washing, well pump, etc.)... For base loads you should only use ~50% to 65% to 75% of "predicted average daily output. When in sunny weather, run the optional loads. During poor weather/winter, do no run the optional loads unless you plan on running the genset too.

-Bill