Small off grid system - does this sound right

Re: Small off grid system - does this sound right

A good place to start is the PV Watts program.

Let's assume Flagstaff is close enough in location/weather. Choose 1 kW of solar panels (smallest the program lets you use). Derating of 0.52 (includes flooded cell batteries 80% efficient and 85% efficient inverter). Fixed array (no tracking) tilted to latitude.

"Station Identification"
"City:","Flagstaff"
"State:","Arizona"
"Lat (deg N):", 35.13
"Long (deg W):", 111.67
"Elev (m): ", 2135
"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:"," 35.1"
"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, 5.19, 84, 7.14
2, 5.92, 86, 7.31
3, 6.27, 99, 8.42
4, 6.44, 96, 8.16
5, 6.56, 99, 8.42
6, 6.61, 92, 7.82
7, 5.95, 85, 7.23
8, 5.54, 79, 6.71
9, 6.59, 94, 7.99
10, 6.19, 93, 7.91
11, 5.47, 83, 7.05
12, 5.07, 82, 6.97
"Year", 5.98, 1072, 91.12

So, usable 120 VAC power from a 1kW system would average around 79-99 kWhrs per month... Divide by 30 days per month = 2.6 to 3.3 kWhrs per day (depending on season)...

Knowing your loads is important. Measuring your real loads with a kWhr meter is a good step.

• 155 watts * 10 hours = 1,550 Watt*Hours = 1.55 kWhrs per weekend day

Assuming you will have the same load 24 hours per day, 7 days per week makes the system much more expensive:

• 155 watts * 24 hours = 3,720 Watt*Hours = 3.72 kWhrs per day

If you are there only on weekends... You can assume that you will have at least 5 days to recharge for your two days of use (note: we assume a 3 day no-sun on the battery bank with 50% maximum discharge and then you will want to recharge back above 75% for best battery life quickly--assume 5 day cycle for most cost effective battery life).

For cost effective use--you will probably want to design for "average" power usage and use the genset for cloudy weather (and get a discharged battery bank above 80% if no sun is predicted for the next few days).

Of course, you observation need clear weather--so you will unlikely to be using the equipment much during predicted bad weather (when solar production would be low).

To size the array:

• 1.55 kWhr per day * 1/2.6 kWhrs per 1 kW array (winter months) = 0.596 kWatts (exactly your planned system)

To size the battery bank (assuming 85% inverter on a 12 volt battery bank):

• 1,550 Watt*Hours per day * 3 days no sun * 1/0.50 min batt * 1/0.85 inv eff * 1/12 battery bank = 911 Amp*Hours (at 12 volts)

Now, those are the average rules of thumb. If you want more solar panels, you should think about larger battery bank. If you want less solar panels, you will have to make up with a genset (typical is to start the genset in the morning, make coffee/toast/microwave/vacuum and charge--let the solar panels finish the charging after a couple hours of genset run time).

Your battery bank, I have sized for 3 days of no-sun and 50% maximum discharge. If you are only there when the sun shines and/or plan on using the genset more if the sun does not shine, you could cut the bank size by 1/2 or 2/3 pretty easily. The bank will not last quite as long--but your initial battery costs are much less too (realistically, whether you replace 3 batteries every 3 years vs replacing 9 batteries every 10 years--it comes out to be a wash in costs many times--then you are looking at the issue of how much pain to change out batteries every three years vs every 10 or so).

Inverter wise, the Morning Star 12 True Sine Wave 300 Watt inverter would be very nice for your system (very low standby power usage too).

I also suggested a 12 volt system because your loads are small (less than 1,200 watt peak) and you can use 12 volt appliances directly. However, watch out when charging. A flooded cell deep cycle battery is charged between 14.5 and 15.5 VDC and discharge can go down to 10.5 volts (dead battery + wiring drop)... Many 12 volt car adapters assume 13-14.2 volts and a few people have found that the higher/lower voltages can cause problems (or even failures) with standard adapters (some reports of laptop 12 volt adapters failing in solar RE systems).

Using 120 VAC gives stable voltage, ability to use extension cords for long distances, and lessens the needs for adapters and issues with wide range 12 volt power from the battery bank.

Regarding the drink cooler--typically for weekend use, it may be more cost effective to use an RV style Propane refrigerator/freezer. Solar really only makes economic sense if you are using it 9+ months out of the year (capturing most of the available sun energy). A weekend type use, you only pay for the propane when you need the cold--and there are no costs when the fridge is turned off (you probably need propane for cooking anyway).

Lastly, if you are looking for a genset--Highly recommend that Honda eu2000i for your application. Very fuel efficient down to 400 watts (or less) loads, quiet, and reliable (reports of 2,000-3,000+ hours of life--rebuilding probably not worth the effort).

"Typical" gensets use the same fuel flow at 50% of rated power as at 1% of rated power--whereas the eu2000i uses about ~25% at 1/4 rated power.

-Bill