calculate watts from volts and max amps

[pierhogunn]

like the title says, I would like to calculate max watts based on volts and amps.

reason being is that I would like to figure out how many watts of PV I will need to completely run my laptop in the field...

19.0 V 3.8Amp is what the power supply sends to my laptop

I would like to figure something out so that I can use this thing when I am at highland games and other out door events where my battery life will not quite go the distance

[crokett]

This page for DC circuits says watts = amps * voltage for D/C. does your power adapter put out A/C or D/C?

http://www.elec-toolbox.com/Formulas...l/formulas.htm

[eccentrictinkerer]

The rating on the power supply is 19 volts@3.8 amps = 72.2 watts. This the power available to the computer.

Generally the computer does not consume 100% of the available power.

Check your owners manual specification page for the actual power consumption.

A 100 watt - 12 volt inverter will generally power all but the very old laptops. A more elegant solution is a "cigar lighter" power supply to laptop cord from the same company as your laptop. These cords are usually a bit more expensive than an inverter.

I use a 100 inverter to power my laptop on the road. My wife uses the inverter to power a heating pad to keep my MIL's tush warm on winter trips to the doctor.

[maxparot]

It's as simple as stated above. The only catch is some laptops will not charge from power supplies it doesn't recognize. This isn't a problem if your battery is full and the supply is sized well enough to keep up with demand. With your requirements I'd shoot for 80 watts or more. What may be a safer way to deal with your requirements is to have a 120 watt PV charging a battery and a 100 watt modified or true sinewav inverter power your AC power supply. In this way the computer's power supply filters the power for the computer and keeps the laptops battery fresh. If clouds go by you have the reserve of the battery supplying the inverter and then the laptops battery before you are out of juice.

[LCHIEN]

19V is what the power supply sends to the computer. 3.8A is the MAximum current it can send at any time when plugged into AC line. However, 3.8A is not what the computer needs all the time, the current will vary greatly depending upon
1) is the battery is being charged
2) is the computer is running
and 3) what clock speed the computer is running at
4) if the computer and its peripherals are in a power save mode or hibernating
5) what brightness level the LCD is set
whether the Hard disk is spinning
whether the CD/DVD is spinning, reading and or writing
if the wifi, bluetooth etc. is on whether the sound is playing and if you have any USB accessories plugged in etc.

So the max watts that can be used is 3.8 x 19V or around 70 watts but it will not be using 70 watts all the time. But that's the only good news.

The max current or power the spec sheet says the PV will deliver is based upon direct, unshaded sun at exactly 90 degrees incident angle which means the PV array must be pointed straight at the sun in both elevation and azimuth with no clouds. On a cloudy day and the sun low in the horizon and the PV array roughly aimed south about 45 degrees you're lucky to get about 25% of what the PV array claims. You need to pick it allowing loss factors based on the latitude and longtitude and time of year, the time of the day it needs to operate and weather conditions. Also is the area dusty and or is someone going to clean it frequently?
The voltage is not constant and so you'll need to furnish a regulator and up converter circuit.
It's not simple to size PV arrays...

[pierhogunn]

this is just some speculation relating to making a small thing that I can stick in the backpack with the laptop, unfold it and stash is on the sunny side of the tent during the day