Proposed small system

Re: Proposed small system

ScottBarMtn wrote: »

Typical power needs, on a daily basis. If I need more power than this, I would plan to run a generator:
- Two 55 watt 12 volt halogen light fixtures running 6-8 hours per day (no inverter needed for these...)
- Occasional use of small tools (drill, etc.) during the day. Maximum 7 amp 120 volt motor for one hour per day. I would turn on the inverter only when I use a tool.

Lets call it:

• 2x55 watts * 8 hours per day + 120 volts * 7 amps * 1 hour per day = 1,720 Watt*Hours per day

That is actually a fair sized system for an off-grid cabin. If possible, I would suggest looking for alternatives for the Halogen lamps.

Anyhow--Assuming you are sort of near Medford Oregon (weather wise inland a bit), using PV Watts website, 1kW panel (even number), 0.52 system efficiency, fixed array:

"Station Identification"
"City:","Medford"
"State:","Oregon"
"Lat (deg N):", 42.37
"Long (deg W):", 122.87
"Elev (m): ", 396
"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:"," 42.2"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.33, 34, 2.45
2, 3.43, 47, 3.38
3, 4.99, 74, 5.33
4, 5.53, 78, 5.62
5, 6.35, 92, 6.62
6, 6.42, 87, 6.26
7, 7.25, 99, 7.13
8, 7.14, 98, 7.06
9, 6.34, 86, 6.19
10, 4.91, 72, 5.18
11, 2.44, 34, 2.45
12, 1.92, 28, 2.02
"Year", 4.93, 828, 59.62

Assume that you want at 9 months of the year solar power only--and the other 3 months of the year you will use a backup genset... That makes February at 47 kWH per month:

• 47 kWH per month / 30 days per month = 1.567 kWH = 1,567 WH per day
• 1,720 WH per day loads / 1,567 WH per day per 1,000 Watt of panels = 1,098 watts of panels

So that will meet your load for ~9 months of the year.

Battery bank wise, we recommend 3 days of no sun, and a maximum of 50% discharge--For a work shed (not used every day)--You might go as low as 1 day of no-sun and 50% maximum battery discharge (assume 12 volt battery bank):

• 1,720 WH * 1/12 volt battery bank * 1/0.85 inverter efficiency * 1 day of no sun * 1/0.50 max battery discharge = 337 AH minimum @ 12 volts
• 1,720 WH * 1/12 volt battery bank * 1/0.85 inverter efficiency * 3 day of no sun * 1/0.50 max battery discharge = 1,012 AH maximum @ 12 volts

If much of your power is not using the inverter and using most of your power during the day (when the sun is up)--you could get away with less battery and a bit less solar panels (again, just a starting rule of thumb).

Proposed system:
- Solar Panel: One of Kyocera KD135SX-UPU - 135 Watt, 12 Volt Solar Panel (Junction Box)

Probably not big enough solar panel... Assuming February and 3.43 hours of "full sun" average, with 0.52 end to end efficiency:

• 135 Watt panel * 3.43 Hours of sun * 0.52 system eff = 241 WH per day (February)

So, that would only be 241/1,720 = 1/7th of your daily loads (February).

- Charge Controller: Xantrex 12 Amp, 12 Volt Charge Controller
- Batteries: Two of Trojan T105-RE 6 Volt, 225 AH Deep Cycle Battery
- Inverter: Samlex 1500 Watt Pure Sine Wave Inverter 115 volt AC output, 12 volt DC input
- Charger: Xantrex Truecharge2 12-Volt, 20-Amp Battery Charger
- Battery sensor: Xantrex Battery Temperature Sensor, 15 foot cable

May change depending on how much power you will need.

- One kill-a-watt meter for AC
- Two DC amp-hour/watt-hour meter (one between panel and battery, one between battery and inverter)

For the DC Amp-Hour Meters--I would suggest one Battery Monitor instead. Bi-directional measurement of AH/WH into/out of the battery bank. I think it will be easier to use and give you more information on the health of the battery bank.

- Generator: Existing Dayton 3000 watt generator (may buy a generator/welder as backup)

Take a look at a Honda eu2000i or eu1000i for your solar assist loads (charging battery bank, running small tools--The TrueCharger should run fine on the smaller eu1000i if you want a smaller genset). Tend to be much more efficient (fuel wise) vs the average 3-5 KWwatt gensets (the Honda eu2000i runs almost 10 hours on 1.1 gallons of gasoline).

- Estimated cost: A little less than $2000

Depending on what your actual sun availability and power needs (I may be 20% on the conservative side)--Your panels may cost you $2-$3+ per watt--so that would be $2-$3k+ just for solar panels. Obviously, for ~7 months of the year, a system with 1,100 watts of solar panels will generate way more than 1,098 WH per day.

Questions
- Do you see any weak links in this configuration?
- What is the difference between an electronic-based inverter like the Samlex, and a transformer-based inverter like the Outback? Will I regret buying the Samlex?
- Could I get by with a 1000 watt inverter?
- Any other suggestions? Cost is a consideration...

Revisit loads and power needs. Set against the amount of sun/season that you will be working out there (if summer only, only need 1/2 the size of solar array).

Not sure which inverter to recommend yet--Need to understand AC loads more. Certainly, you are comparing less expensive against more expensive inverters--It sounds like the less expensive inverter would meet your needs.

-Bill