newbe question

Re: newbe question

Based on what you asked for:

• 40 watts * 3 hours per day = 120 Watt*Hours per day
• 12 volts * 0.170 amps * 8 hours per day =16.32 WH per day
• 120 + 16 = 136 WH per day

For the battery, we usually recommend 3 day of "no sun" and 50% maximum discharge. This is a small system, so you probably will go with a 12 volt battery bank and you will be using an 85% efficient inverter for running the lights:

• 136 WH per day * 1/12 volts * 1/0.85 eff inverter * 3 days * 1/0.50 max disch = 80 AH @ 12 volt battery

For the amount of solar panels, I will guess you are from around Richmond Virginia. I will use PV Watts website for the amount of sun for your area, 1,000 watts (1 kW) of solar panels for starting point, 0.52 for overall efficiency (77% panel+charge controller efficiency + assuming you are running the lights with an 85% efficient inverter), and an 80% efficient flooded cell battery bank):

"Station Identification"
"City:","Richmond"
"State:","Virginia"
"Lat (deg N):", 37.50
"Long (deg W):", 77.33
"Elev (m): ", 50
"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:"," 37.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.0 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.99, 64, 5.12
2, 4.37, 62, 4.96
3, 4.96, 76, 6.08
4, 5.32, 75, 6.00
5, 5.49, 79, 6.32
6, 5.54, 74, 5.92
7, 5.55, 76, 6.08
8, 5.31, 74, 5.92
9, 5.30, 73, 5.84
10, 4.65, 68, 5.44
11, 3.95, 58, 4.64
12, 3.51, 55, 4.40
"Year", 4.83, 834, 66.72

Your worst month of production (this is a 20 year average of solar for your area) is 55 kWH per month for an off-grid system (52% efficient):

• 55 kWH per month / 30 days per month = 1.833 kWH per day for December per 1 kW of panels
• 136 WH per day * 1 / (1,833 kWH per day per 1,000 watts of panels) = 74 watts of solar panels

So, an 80 AH 12 volt deep cycle battery (not a automotive or marine battery) + 74 watt solar panel would give you enough power, on average, to power your needs 12 months of the year.

As always with solar, you will have a week or stretches at a time where you have bad weather and will either need to cut back on the power usage or use a back up genset (or take the battery back to the house) to recharge.

The above is just a starting point--you or I can adjust the assumptions, if needed, to give you a more cost effective system for your needs.

You may also want to look at your loads and decide if you want to add more panel (or possibly battery bank) to support additional loads (a few power tools, radio, etc.). If this were a cabin--many people would double the battery and double or 4x the amount of solar panel to allow for addition loads (which always seem to grow over time)...

Solar off-grid power is expensive... Looking at your lights, is 40 watts the best you can do, or should you look at CFL's or possibly LED based lighting. With solar, conservation is your friend.

Also, now that you are looking at somewhere around $1,000 (just for sake of argument) worth of time and labor to put an off-grid system together--Is there power close enough that you can spend a $1,000 trench a cable to the barn and have 120/240 VAC utility power at the barn. Now you have power 24/7 and can power any major tools/lighting/appliances you may need.

The solar system will only be generating, roughly, $0.30-$0.40 of electricity per month--Solar may be a good solution for you--or not. Utility power (if anywhere close) is usually hard to beat.

-Bill