Laptop Power

Re: Laptop Power

Assuming it is on or near somewhere on the PV Watts website, you can use it to estimate the output of a solar PV system.

Some basics, assume a location (Learmonth, Aus), assume that you are using AC power and a ordinary lead acid deep cycle batter with AC inverter gives you deratings of:

0.77 charge controller and solar panel derating
0.80 flooded cell lead acid derating
0.85 reasonably efficient AC inverter (or DC adapter for laptop)

0.77*0.80*0.85= 0.52 end to end efficiency

From the PV Watts website (fixed solar panel, use 1 kW as the panel size (sorry, minimum allowed by the program:

"Station Identification"
"City:","Learmonth"
"State:","AUS"
"Lat (deg N):", -22.23
"Long (deg W):", 114.08
"Elev (m): ", 6
"Weather Data:","IWEC"

"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:"," 22.2"
"Array Azimuth:"," 0.0"

"Energy Specifications"
"Cost of Electricity:"," 0.1 A$/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (A$)"
1, 7.64, 105, 0.09
2, 7.37, 92, 0.08
3, 7.49, 104, 0.09
4, 6.52, 89, 0.08
5, 5.71, 82, 0.07
6, 5.20, 73, 0.07
7, 5.53, 82, 0.07
8, 6.58, 95, 0.09
9, 7.42, 104, 0.09
10, 7.88, 112, 0.10
11, 7.79, 105, 0.09
12, 7.68, 106, 0.10
"Year", 6.90, 1151, 1.04

That gives you around 73-112 kWhrs per month of "AC Power" per 1,000 watts of panels... Or:

• 73kWhr per month / 30 days per month = 2.433 kWhrs per day min or 2,433 Watt*Hours per day minimum per 1,000 watts of panels

Now, lets say you use the smallest netbook laptop out there that runs around 20 Watts average from the AC plug.

• 24 hours per day * 20 watts = 480 Watt*Hours per day

The minimum power was 2,433 Watt*Hours per day from 1,000 watts of panels

• 480 Watt*hours per day * 1/2,433 WH per day per 1,000 watt of panel = 197 watts of solar panel

So, you need to do similar measurements on your intended loads... Find the Australian (or where ever you are) version of the Kill-a-Watt meter to measure your AC loads, and you can also get a DC version of an Amp*Hour/Watt*Hour meter to measure your DC loads.

Battery sizing, normally, we recommend 3 days of no-sun and 50% maximum discharge--so that works out to a battery bank 6x your expected daily load for best life and optimum costs.

• 480 Watt*Hours * 3 days * 1/0.50 discharge * 1/0.85 invtr eff * 1/12 volt battery bank =282 Amp*Hours at 12 volts

A "typical" 6 volt golf cart battery is 225 Amp*Hours -- so two of those connected in series for 12 volts would have 225 Amp*Hour capacity (20 hour discharge rate). So, they are still a bit small for this mythical application.

Remember the PV Watts is a 20 year average sun/weather based observation... Yours may be better or worse.

You will probably still need a small genset to charge the battery bank for bad weather... Ideally, size the generator for the charger/load needs. Most gensets are pretty efficient at 50-100% loading. Below 50% of rated load, they still consume 50% fuel flow (even down to zero Watt loading).

Some gensets (like the Honda euX000i inverter/generator line) are very quiet and efficient down to ~25% loading--but may be a bit pricey for your area.

Anyway, that is where I would start.

-Bill