Off-Grid Inverter Selection

trevorawalton

I am an electrician in Oregon but I have never worked a residential solar project. I want to install a system for a new home I am building but am unfamiliar with the products available. 

I want an 6-10kw inverter that can handle a few wind turbines, solar and battery backup. The kicker is that I do not want it to be tied in to the electric grid. Most of my experience with inverters require a Line side connection from the grid. There has got to be other options. 

Any help or questions for clarification will be greatly appreciated. 

Thank you!

Dave Angelini

Look at Outback Power systems or Schneider Electric Solar. In my opinion it is best to make the whole system the same brand.
https://solar.schneider-electric.com/product/conext-xw-na-solar-inverter/
http://www.outbackpower.com/outback-products/make-the-power

My wife and I are starting our 26th year offgrid full time. You are welcome to e-mail or look at the picture link on my webpage below.

Good Luck!

BB.

Trevor,

Why do you want to be off grid? If it is because grid power would cost you $40,000 to run to your home--Even then, it is a close call.

If it is to save money, off grid power is generally more expensive than utility power--Typically around $1 to $2+ per kWH, or something like 5-10x the cost of utility power.

Also it depends on where you live--North west Oregon, you may not get much sun (especially during winter)--And are left to genset or (possibly) other power source (wind, water, gassification, etc.). Pretty much all of which have higher maintenance than solar or utility power.

Say you are near Bend Or. Fixed array :
http://www.solarelectricityhandbook.com/solar-irradiance.html

Bend
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 46° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.82	3.70	4.36	4.89	5.17	5.69
Jul	Aug	Sep	Oct	Nov	Dec
6.40	6.40	5.86	4.47	2.82	2.39

Say you want to use an North American average of 500-1,000 kWH per day--Or pick 750 kWH per month. The size of battery bank based on 2 days of "no sun" and 50% maximum discharge for longer battery life:

• 750,000 WH per day / 30 days per month = 25,000 WH per day
• 25,000 WH per day * 1/0.85 AC inverter eff * 1/48 volt battery bank * 2 days of storage * 1/0.50 max discharge = 2,451 AH @ 48 volt battery bank

To charge such a battery bank, recommend 5% to 13% rate of charge. 5% is good for weekend/sunny seasons (non-winter), and 10%+ is better for full time off grid:

• 2,451 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 9,390 Watt array minimum
  
• 2,451 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 18,780 Watt array nominal
  
• 2,451 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 24,415 Watt array typical "cost effective" maximum
  

And say you toss the bottom three months, assuming generator assitance during poor winter weather, gives us a nominal 3.70 hours of sun for "February break even" month:

• 25,000 * 1/0.52 off grid system eff * 1/3.70 hours of sun (Feb avg) = 12,994 Watt array minimum (Feb)
  

And for a 24,415 AH @ 48 volt typical flooded cell lead acid battery bank, the maximum AC inverter (and maximum recommended charging sources) would be ~24.4 kWatts (~1kWH per 100 AH @ 48 volt battery bank capacity).

As a starting point for off grid homes... Typically ~3.3 kWH per day (3,300 WH) per day is good for a smaller, very efficient home with a "near normal" electric existence (LED lighting, refrigerator, TV, Laptop, Washing Machine, central heat, well pump). Use propane or other energy source for heating/hot water/cooking.

And roughly 10 kWH per day (300 kWH per month) would be the maximum I would suggest for a "do it yourself" system. Over that size (like 25kWH per day), that is a big system for a first project without the help of an engineer or solar supplier with experience in larger systems.

Energy usage is a highly personal set of choices--I am not predicting your needs--But needed to "assume something" to give you an idea of what "an average" home off grid system would look like (if you did not plan on a lot of energy conservation).

The above system would supply, on average (battery, mid February):

• 25,000 WH per day / 24 hours per day = 1.042 Watts continuous load over 24 hours
  
• 25,000 WH per day / 8 hours per day = 3,125 Watts continuous load over 8 hours (say 3 hours in morning, 5 hours in evening)
  

A 2,451 AH battery @ 48 volt battery bank would supply:

• 2,451 AH * 48 volts * 1/20 hour discharge rate * 0.85 inverter eff = 5,000 Watts (for 10 hours, or 5 hours per day)
• 2,451 AH * 48 volts * 1/8 hour discharge rate * 0.85 inverter eff = 12,500 Watts (for ~4 hour or a bit less)
  
• 2,451 AH * 48 volts * 1/5 hour discharge rate * 0.85 inverter eff = 20,000 Watts (for an hour or so)
  
• 2,451 AH * 48 volts * 1/2.5 hour discharge rate * 0.85 inverter eff = 40,000 Watts (for a few seconds, like starting a pump)

Typically, for this size battery bank, I would suggest around:

• 2.451 AH (1 kW per 100 AH) / 2 = 12.25 kWatt "comfortable max" inverter
  
• 2.451 AH (1 kW per 100 AH) / 1 = 24.5 kWatt max inverter

Note that most quality inverters will surge around ~2x rated power for a few seconds for starting motors and such.

Anyway--Lots of guessing here. More of a stake in the ground for an education. The battery bank is the heart of an off grid system. Knowing your loads sizes the battery bank. The battery bank (and loads) size your charging source(s).

Your thoughts?

-Bill

mike95490

Wind power will seldom be a usable option, unless you have to tie your hat onto your head every time you go outside