solar powered garden

Re: solar powered garden

I will assume you are south of San Francisco California (actually right next to me).

We need to know your electrical needs--I will make some guesses for an example...

• Your location (near San Francisco)
• Use pure solar 9 month of the year
• Battery (need to run 24 hours x 7 days--sun or fog)
• You will have backup power (utility or genset) or simply not run when no sun.
• 110 watts * 2 hours per day

From PV Watts website for SF California, the number of hours of sun per day by month (I just left the Hours of Sun data--there is more, but I stripped it out to keep it simple):

"Station Identification"
"City:","San_Francisco"
"State:","California"
"Lat (deg N):", 37.62
"Long (deg W):", 122.38
"Elev (m): ", 5
"PV System Specifications"
"Array Type: Fixed Tilt"
"Array Tilt:"," 37.6"
"Array Azimuth:","180.0"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.62,
2, 4.59,
3, 5.22,
4, 6.11,
5, 6.36,
6, 6.47,
7, 7.01,
8, 6.67,
9, 6.62,
10, 5.41,
11, 3.87,
12, 3.35,
"Year", 5.45

Please note that numbers are estimates an rules of thumb... I give all the digits so you can reproduce my math... In solar +/- 20% is as close as it gets for estimates.

Size the battery bank. Assume AC pump with Inverter, flooded cell battery bank, and ~0.52 system efficiency.

February through October operation, 4.59 hours of sun minimum per day (Feb). Battery should be 1-3 days of no-sun operation, and 50% maximum battery discharge for long life and 85% efficient inverter, 12 volt battery bank:

• 110 watts * 1/12 volt battery * 2 hours per day * 1/0.85 eff * 1 day no sun * 1/50% max discharge = 43 AH minimum battery bank @ 12 volts
• 110 watts * 1/12 volt battery * 2 hours per day * 1/0.85 eff * 3 day no sun * 1/50% max discharge =129 AH maximum battery bank @ 12 volts

For the amount of solar panels (in February) based on 4.59 hours of sun minimum:

• 110 watts * 2 hours runtime * 1/0.52 system derating * 1/4.59 hours of sun per day = 92 watts of solar panel minimum

If you are using a 12 volt inverter, a 300 watt TSW inverter from MorningStar is a very nice unit.

If you are using a DC pump, you can skip the inverter.

Just as a quick guess, you are looking at ~$1,000 or so for the equipment--but before we pick the parts and price them--I wanted to see if my assumptions are correct and if the $1,000 price range is what you expect or not.

By the way, will this be an an attended or automated system? Batteries do not like to be deep cycled until "dead"--batteries will usually quickly fail (weeks/month) in such usage. Turning off pump/load when the battery is low is necessary.

-Bill