May have to do Off-Grid system.

Re: May have to do Off-Grid system.

I will take a shot at giving you some answers--But be aware that these are pure guesswork and may be of limited use until you know what your true power budget is (or what you are willing to pay)... So, first the boiler plate answers:

1. Conservation: Off Grid power is expensive--It probably works out to around $1-$2+ per kWHr--vs the $0.12 to $0.25 per kWH you are probably paying today for your home. Almost anything you do towards conservation will be a better investment than throwing Solar PV or Generator Fuel at your power needs.
2. Lots of insulation, double pane windows, low power laptop instead of big desktop computers, turning printers/computers/entertainment centers off when not in use, etc... All will save you big bucks in building out your power system.
3. Really push the issue with SSVEC and APS--At your current level of power usage (30kW a day without A/C)--It would be a huge change in lifestyle for a 1,200 sqft home to work on 3 kWH per day (a nice round number for enough power to run your computer, some lighting, a ceiling fan, water pressurization pump, etc.). Not to day you will opt. to pay for Grid Power--but it will give you something to compare and contrast against when you look at your off-grid power costs.
4. Plan out your off-grid system carefully before spending money. In general, it is difficult to build out a cost effective system and do much more than double its capacity without ending up ripping out the old installation and installing 90% new parts for the larger system.
5. If you do decide to bring utility power to your homestead--You can still use solar to reduce power power bill and/or provide emergency backup power (storms, unreliable / last to be fixed at the end of the utility line type service, etc.). There is pure grid tie system (solar panels + inverter connected to the utility grid--cheapest solar and simplest to run and maintain). There are also hybrid systems which do grid tied and off-grid for emergency power (add batteries and a genset). And full off-grid (by the way, the hybrid systems can do full off grid very nicely too and connect to utility power later if it ever comes).

The above is just my personal opinion--So feel free to question or disagree.

So, lets look at PV Watts for Tuscon AZ. Plan on off grid system with 0.52 derating and a fixed array. Use 1kW (1,000 watts) of solar panels for an even number:

"Station Identification"
"City:","Tucson"
"State:","Arizona"
"Lat (deg N):", 32.12
"Long (deg W):", 110.93
"Elev (m): ", 779
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 32.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.70, 85, 7.23
2, 6.11, 80, 6.80
3, 7.03, 103, 8.76
4, 7.50, 104, 8.84
5, 7.29, 100, 8.50
6, 7.15, 94, 7.99
7, 6.44, 87, 7.40
8, 6.85, 93, 7.91
9, 7.06, 95, 8.07
10, 6.72, 96, 8.16
11, 5.99, 86, 7.31
12, 5.27, 80, 6.80
"Year", 6.59, 1105, 93.92

And lets plan that your system will supply your needed power for 9 months of the year, and the other 3 months you will use a genset to make for low winter production/cloudy/stormy season.

In your case, there is not a huge difference between winter and summer production, and the 4th lowest month is July at 87 kWH per month per 1,000 watts of panels or:

• 87 kWH per month * 1/30 days per month = 2.9 kWH per day for July per 1,000 watts of panels

Now, we will look at two systems one that does 3.3 kWH per day (100 kWH per month) and another that does 33 kWH per day (1,000 kWH per month) based on July production:

• 3.3 kWH per day * 1/2.9 kWH per day per 1,000 watts of panels = 1,138 Watts of solar panels for "small" system
• 33 kWH per day * 1/2.9 kWH per day per 1,000 watts of panels = 11,380 Watts of solar panels for "large" system

So, just from a solar panel point of view--That is the difference between $3,000 worth of solar panels vs $30,000 worth of solar panels (note the above numbers include solar panel derating, charge controller losses, flooded cell battery losses, and inverter losses).

Next major cost is the battery bank... Basically, we recommend as a starting point to plan for 3 days of no sun and 50% maximum discharge for longer battery life. We also take into account that AC inverters are about 85% efficient:

• 3,300 WH per day * 3 days of no sun * 1/0.50 max discharge * 1/0.85 inverter efficiency =23,400 WH of battery storage for small system
• 33,000 WH per day * 3 days of no sun * 1/0.50 max discharge * 1/0.85 inverter efficiency =234,000 WH of battery storage for large system

Now--We need to choose a battery bank voltage... For your small system (if the well pumping is kept small and efficient) you might get away with a DC pump and even a 300 Watt TSW inverter on a 12 volt battery bank.

For the large system, you are most likely going to a 48 volt battery bank (voltage drop and high currents are very difficult to deal with on a 12 volt battery bank system).

So, battery size in AH (remember, the Large battery bank is still 10x the size/cost of the small battery bank system--The AH rating does not show the 12 vs 48 volt bank capacity difference):

• 23,400 WH / 12 volts = 1,941 AH of 12 volt battery bank
• 234,000 WH / 48 volts = 4,875 AH of 48volt battery bank

Using the middle of the road cost/quality Trojan batteries:

Trojan L16RE-2V 1110 AH 2-Volt Deep Cycle Battery Price: $330.00

For the 1,941 AH @ 12 volt bank... We are looking at two 6 cell series string (12 batteries) overall for $3,960 for the set (2,220 AH at 12 volt bank).

For the 48 volt bank, looking at 4 strings of 24 cells (96 batteries) at $31,680 (4,440 AH at 48 volts).

The above are just suggested starting points for costing/designing your system... Batteries are pretty much linear in pricing--10x larger battery bank costs about 10x more.

Same with solar panels (although, buy enough, you may get a volume discount/save on shipping by the pre-packaged pallet full, etc.).

Solar panels--Generally, the > 100 watt size are less expensive ($$$ per watt).

>100 per watt solar panels also, generally, have Vmp that is not directly compatible with PWM type (less expensive) charge controllers... MorningStar, Xantrex, Outback, all make nice 60 Amp (Outback also make an 80 amp) 12/24/48 volt charge controller... The nice think with 48 volt battery banks, a single charge controller can handle 4x the array size at 48 volts vs 12 volts (fewer charge controllers needed for a large system).

The above is estimated to support 3.3 or 33 kWH per day--If you really wanted to "go small", you can do it on ~1 kWH per day too (obviously, no A/C, minimal electric use, etc.).

Inverters--there is also a wide choice here... We try to talk people into the much more expensive pure sine wave types... But there are are people here who are very happy with MSW or both (small True Sine Wave for computer, small appliances; and a large Modified Square Wave to run the well pump).

Plan on replacing your battery bank every 4-8 years (some people get more, a few people get less). And the inverters/charge controllers are pretty much obsolete after 10 years (and will need repairs/replacements every 10-15 years or so).

Power use is a very personal matter--There are folks who live off-grid that are at both ends of the spectrum here that can give you real life experiences (including at least one person whose generator caught fire and burned down his entire solar+genset station ).

Doing lots of planning up front can help save you uncounted issues and money in the future.

Questions? Suggestions? The above is intended to be educational--Not a hard and fixed design for your system/needs. There are still many decisions to be made.

-Bill