Laptop Power

Art

Hi Guys,
Not exactly a beginner when it comes to electronics, but new to much of the math involved.

I'm looking to solar power a laptop computer in a very remote location,
and would like as much up time as possible for the laptop.

I thought about using a generator to charge a car battery, but don't think I'd like the noise.

It looks like I'm going to fork out for a larger panel. What size panel do you guys think I'd need?
I plan on charging a seperate car battery, and I can charge/power the laptop from that.
I understand the pitfalls with using a car battery in place of deep cycle battery.
Cheers, Art.

RCinFLA

Re: Laptop Power

Laptops and their chargers have quite a range of power consumption.

Newer the better, dependent on programs running, etc.

As to worse case, take a look at charger output voltage and current rating. This would be the maximum. (something like 20vdc, 3.5 amps)

icarus

Re: Laptop Power

Beware most of the "lap top solar kits" that are out there. They over promise, under deliver and overcharge on a per watt/hour basis.

General draw for a lap top might be ~50-75 watts. ( Confirm with a kill-a-watt meter ~$20)

Another quick easy calculation is to take the name plate rating of a Pv panel, divide it by 2 to account for system loses, multiply that number by 4 which is the average number of hours of good sun one might expect. (Might be less, but is seldom more). This number give you the average number of watt/hours you can expect per day.

So let's say you have a lap top that draws 50 watts and you wish to run it 8 hours
8x50=400 watt/hours.

To get ~400/wh/day you would need say 200 watts of PV/2x4=400

That allows no reserve for cloudy days etc. Also if you are charging a system battery (deep cycle type) and then plugging your lap top into that system there would be additional loses.

Play with the numbers and see how it works out.

Tony

Art

Re: Laptop Power

I don't plan on using a commercial laptop solar kit or anything like that,
but you're saying even a large 100 Watt panel would hardly suffice?

BB.

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

BB.

Re: Laptop Power

By the way, Tony has a typo, that is 50 watts * 8 hours = 400 Watt*Hours of load.

-Bill

icarus

Re: Laptop Power

Not any more! LOL

Thnx Bill