Off the grid in the desert is where many of the facts of life converge.  If you wish to live simply the desert may allow you to do that.  If you ever add even small complications, the desert will punish you for the transgression. 

When I moved into my desert home it had a simple rain water roof collection system with gutters emptying into a rusted 1000 gal. tank located on a raised pedestal for gravity fed water indoors, propane to run a refrigerator and a cookstove, and a few oil lamps for light in the dark of night.

Needing electricity for the myriad of gadgets that make up a digital studio, I got busy and added eight 80 watt solar panels and eight AGM 85 Ah batteries, the requisite solar charge controller, and a 4000 W inverter. 

I was under the impression that I could not leave flooded cell batteries for 3-4 months without maintenance or I would have bought eight Trojan L-16H 6v, 420ah batteries instead of the AGM batteries as they give you more watt-hours for the dollar.  Since then I have been assured by Trojan I can do just that.  When it comes time for replacement or I need to add to the system, I will convert.  I think of the addition of solar panels and inverter as a property cost.  It is very likely they will not have to be replaced and land here on the grid is $3000-$7000 per acre while land off the grid is $300 per acre.  Parcels of 10-40 acres allow about a $50,000 investment in solar equipment to make up the difference.  Batteries, on the other hand, need to be replaced.  Even the best buys will cost you $.24 per Kwhr, twice as much as grid use.   My cost of electricity is mainly the cost of battery replacement: $2240/7 = $300/year for 1,460 kwhrs or $0.20/kwhr plus the cost of capital (Trojan L-16H will last 7 years).  In the West Texas Desert the cost of installing a solar system is much less than the cost to hook up to electricity or the additional cost of land near electricity, so there is no additional capital cost to going solar.  Additionally, the cost of power here is very high. $0.14+/kwhr .  The cost of electric power is rising rapidly, but so is the cost of lead.  The average monthly electric bill in this area is $210 @$0.14/kwh or 1500 kwhr/30days = 50kwhr/day compared o the 3 or 4 I use.  Most of the total cost for electricity for home owners here is air conditioning in the summer; something you will have to forgo off grid unless you are really going to spend that $50,000. 

Batteries:

Currently there are eight LifeLine group 27 AGM (absorbed glass mat) batteries; about 85 ah x 8 = 690 ah @12v = 8280 whr. At 50% discharge = 4140 whr. The conventional design point is storage for three days of overcast.  For this part of the country, that may be too much safety margin as we VERY rarely see that.   The use of a generator to carry the system over conditions beyond design points is cost effective.  I have a 2.5 kw propane fueled Onan generator which needs to be run very seldom.  Even so, it is good to have twice the battery storage that you plan on using as they will charge faster. The current power use is 2850 WHrs per day, but I would like to have ability to use 3985.  I currently can store 4140 WHrs.  4140/2850 = 1.45 days of capacity, but only about one day (4140/3985) to meet desired power use.  It's clear that I should at least double the capacity.  Eight Trojan L-16H 6v, 420ah batteries provide 20,160 whrs; or about 10kwhrs @ 50% discharge.

This gives me 2.5 days with the desired use.

Inverter and Solar controller.

I use a Trace SW 4024 which is a fine unit.  I have never needed to press its 4kw capacity and it charges even the AGM batteries with precision.  In addition, it will handle all of the automation when I need to leave for extended periods of exercising the generator and equalizing batteries.

I recently bought a Blue Sky point tracking solar controller which adds a little to the efficiency of solar tracking. It is capable of 50A @ 24v = 1020w.  The maximum I can produce is 940 and I doubt I will ever see over 625. 

I have thought of using the old AGM cells as a separate system to provide 12v to the house for new 12v LED lighting and whatever else I can think of.  I have a spare 12v Xantrix inverter/charger and a spare Trace C-40 controller and two spare small PV cells which will not be used otherwise.  The addition of a 12v system would not be cost-effective otherwise and even with free equipment will not pay for the wire and lights.  If I can sell the equipment used, it would be better financially.   Currently it will only relieve the 110v system of 144whrs/day or about 450wh of storage = 900 whr of battery.  That's about $15 a year in batteries.  It will take seven years to pay for the wire.  It won't pay for the LED lighting in my lifetime.  L-16H can give about $30/kwhr of storage per year ($2240 for 10 kwhrs with a 7 year life).

I will continue to add to this page.  Right now there is little background information on design and no tutorial.  All things happen in time.