Peter Brenner: Help before I buy system

Re: Peter Brenner: Help before I buy system

Solar panels are pretty impressive from a longevity point of view--25-40 years of power without any maintenance (other than a quick washing--if you want). Price around $10 per watt a decade ago--now better panels are probably in the $2.50 per watt or less.

A 6kW/12kW surge Hybrid inverter for $3,400--that is not bad either.

So, just to pick some numbers for a back of the envelope calculation (I like to do this for a sanity check). Using PV Watts, Tulsa OK, 10kW of solar panels, 0.52 derating (off-grid, battery, inverter, relatively conservative assumptions):

"Station Identification"
"City:","Tulsa"
"State:","Oklahoma"
"Lat (deg N):", 36.20
"Long (deg W):", 95.90
"Elev (m): ", 206
"PV System Specifications"
"DC Rating:"," 10.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 5.2 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 36.2"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.01, 643, 49.51
2, 4.46, 627, 48.28
3, 5.33, 808, 62.22
4, 5.88, 848, 65.30
5, 5.61, 804, 61.91
6, 5.77, 776, 59.75
7, 6.06, 830, 63.91
8, 5.95, 823, 63.37
9, 4.99, 688, 52.98
10, 5.41, 808, 62.22
11, 4.11, 605, 46.59
12, 3.70, 580, 44.66
"Year", 5.11, 8841, 680.76

Assume $5,000 for electronics, $2.50 a watt for 10kW of panels, $1,000 for used batteries every 5 years. Over 20 year "life of system":

• ($5,000+$25,000+4*$1,000) / (20 years * 8,841 kWhrs per year) = $0.19 per kWhr

Not a bad price--"free batteries" do reduce the costs. Note, the above assumes you use 100% of your power (only true for Grid Tied systems). Also, the inverter would probably need replacement after 15 years +/-. Does not assume genset use.

Looking at an efficient gasoline genset (efficient at less than 50% loading)... The Honda uses 1.1 gallons over 15 hours to generate 400 watts. Assume $2.50 per gallon (cheaper than $3.00+ per gallon in California):

• 15 hours * 0.4 kW / 1.1 gallons of gas = 5.45 kWH / galllon
• $2.50 per gallon * 1/5.45 kWhrs per gallon = $0.48 per kWhr (generator)

Add the cost of generator+maintenance will drive the kWH price higher. Running the genset to charge battery bank--more losses. Run genset at less than 50% or so of capacity, higher fuel flow, higher costs...

You could choose a larger genset, possibly diesel, and get quite a bit better kWhrs per gallon--but larger gensets mean larger fuel flows and higher minimum load requirements.

If this was a grid tied system, the government will "give you" a 30% Federal Tax Credit for the solar system, reducing its costs a bit.

Depending on what you are looking for (and how much cash you can justify)--the Solar seems to have a significant long term cost savings over a genset--if you are preparing for an off-grid world.

Note that the above calculations are pretty much the minimum cost for self generated electricity. Personally, I would plan on a cost cushion of 2x to allow for unplanned failures, future cost increases, less than 100% usage of power, etc...

-Bill