Trying to calculate imputs and loads for my computer

Hi to all,

The good news is that I have my first 4   100 watt solar panels, the nucleus of my intended mobile power station.

The bad news is that Im struggling with calculating loads when I see something like this:  These are the power requirements for my desktop computer.

Input Voltage 90-264 Vac
Input current (maximum) 1.7/1.8
Wattage 90W / 130 W NOTE: System with 65 W CPU is shipped with 130W PSU.

If my 400 watt panels generate 70% efficiency for an arbitrary 8 hours of sunny skies...that's 2240 watts? Where do I go from here? Volts x Watts = Amps therefore....

If my 400 watt panels generate 70% efficiency for an arbitrary 8 hours of sunny skies...that's 2240 watts?

Not quite, its 2240 watt-hours. You gotta keep track of your units; you multiplied watts by hours, afterall!

So basically, take the wattage of your computer and multiply it by how many hours you want to run it and you will have the start of your energy budget. Assuming you have the 130 W power supply (you can run a laptop on much less power, by the way - closer to 40 watts) you would burn 130 watt-hours every hour. To leave it running all day mining cryptocurrency, or whatever you are using a desktop for in 2021, would draw 3120 watt-hours.

Dont forget that running an inverter also uses some power, so you need to either do an efficiency calculation, or measure the standby power, and treat your inverter like its own load. Mine, for example, pulls 20 watts just to be turned on, so I know I need to budget 480wh every day just to have AC power.

If you want to save power, I would ditch the desktop. A surface tablet will run on 10 watts, a basic laptop pulls about 40.

Thank you Carl, that is very helpful, especially about the inverter load, I didnt think of that.

I have decided to skimp on a number of things but i really like a good desktop computer for all it can do.  I also run various software including graphics, editing, and it can be somewhat memory hungry.

The big puter is also a source of ambient light and heat too.  

Anyway its all work in progress.

Thanks again! M

I am with you Mike, a desktop is practical for me also, I am puzzled by the uptake of laptops.
I have seen them;
- left on public transport
- dropped
- dismantled by small children
- run out of power
Whereas my desktops have;
- The screen is large enough to read,
-  it does not run out of power
- they cannot be moved so they dont get damaged
- dont run out of power
- have great storage and RAM.

If I did really need a portable, today I would look at a 2 in 1 I think they are called and then back it up to the desktop later.
But I configure what I am doing to suit my equipment.

I would invest in a kill a watt meter to figure out the actual draw...

If you want to watch your desktop run out of power, plug it into an inverter powered by too small a battery, and charged by too small a solar array.

You can't assume your panels will be putting out 70% output for 8 hours.  It doesn't work that way.  At 8am, assume your 400W of panels are not putting out more than 40W, because of the acute angle of the sun to the panels.  As the sun progresses across the ski, it gradually goes from 10% at 8am to about 85% at noon.  It will almost never reach 100%.  Then the reverse as the sun moves towards sunset.  A good concept to understand is the sunhour.  You can look that up for your location in Mississippi.  Look at this attached sunhour estimation for Jackson.

In the winter, you'll get a total of 3.64sunhours.  Multiply that by 400W of panels and you get a total of ~1460Wh of power per day (unless it rains).  So, you'll replace about 1.5kWh of power with your 4 panels in December.  If you want to assume your inverter itself will draw about 400W in 24hr, that gives you about 1.0kWh of power you can consume as you choose.

Assuming your computer consumes 130W per hour (does that include the monitor?) That gives you 1000Wh/130WPH = 7.7hours.

Now, assuming your computer consumes 130W per hour, and your computer monitor consumes 70W per hour, the total becomes 200W, or 1000Wh/200Wh = 5 hours.

Lastly, you want a lamp lighting the area while you type on the computer?  Let's assume a lamp consumes 100W.  Then, the computer, and the monitor, and the lamp consume a total of 300W, so you get 1000Wh/300wh = 3.3 hours of computer time.

Hi David, regarding the Killowatt meter, isnt this function already built into the controller? Or no?

Thanks, Mike

Agreed John on all accounts, I like the "big screen look" also, my eyes could be better.  I am also more inclined to write, budget, learn other software, create art, and do research on a larger machine and yes the advantage of laptops is also their disadvantage too, they do disappear if  you are too casual with them.

An inverter will likely at best have a little LED screen that shows you how many amps it is drawing - a kill-a-watt shows instantaneous power, plus it records energy so you can see how much a device uses per day. For something like a fridge that cycles on and off, it is good to be able to get a daily usage figure.

One thing that might be worth looking at is an inverter with "Sleep mode." Our main house inverter has that, and when you turn it on the idle draw goes from 20w down to 1.5w. The only issue is that it wont "wake up" unless you apply a fairly large load, like about 20 watts. So when you try and switch on a light at night, you have to turn on another one to wake up the inverter. Also, if you leave a wall-wart plugged in anywhere in the house, the inverter will constantly switch on and off all night, and not save you any power.

Anyway, what sort of battery bank are you going to get? Unless you have a big initial budget, Lead Acid is the logical place to start. They will wear out pretty fast, but will be a good way to get some hands-on experience.

As mentioned above a kill a watt is the best 30 dollars you will spend when it comes to figuring out power usage. Some inverters have amp draw but it's instantaneous. Some solar controllers do have a load input but again instantaneous. Just plug in your device to the kill a watt and forget about it for a full day, just live your life as you normally would. At the end of the day you will know how much total power you used as well as the instantaneous high wattage; really good for sizing a small inverter.
Also Try to stay away from a sleep mode inverter for a small system if you are running a desktop. The power supply will power up, pause, not draw, send the inverter to sleep power down due to lack of power... It creates a bad feedback loop. Basically anything with a small transformer in it like most ac fridges send the inverter into convultions when you use sleep mode. As to your production with a 400 watt system invest in an mppt charger to get the most out of your panels.

I'm not clear on what you are planning for storage. Yes, you can generate electricity, but storage stashes your surplus and regulates your output voltage.

Im not entirely clear either, Im tempted to go with lead-acid because they're cheap local and replacable.  All these things are evaluated in the context of budget.

At the size you are talking you are better off going with lead acid. A pair of 100 amp hour qw volt units would be a good start, cheap, forgiving, a good fit to 400 watts of solar.

Thanks David and would that also be a general ratio of batteries to solar panels as I expand? 100 amp hours batteries/400 amp hours panels?

Units will get you every time... For lead acid a good ratio is 1kw of panels to 400 amp hours of battery at 12 volts. So that is why I said 200 amp hours at qw volts for 400 watts of solar.
Cheers, David

Much appreciated!

There are DC-DC ATX power supplies. This would allow you to run your computer without an inverter and the corresponding energy losses. It makes sense to me because computers use DC internally. Changing DC to AC and back to DC doesn't make sense to me.

This site has power supplies that can take 12v, 24v, 48v and 120v DC input.

https://www.powerstream.com/DC-PC-12V.htm

EDIT: Looking at the ATX specifications, 12 volts is the highest output voltage of a standard computer power supply.

These electrical engineers compare some of the features of normal AC-DC power supplies and DC-DC ones. https://electronics.stackexchange.com/questions/530229/why-are-dc-dc-atx-converters-so-big

Take a look at Costco's 6V golf-cart battery.  You'll need two in series to create a 12V system.  The size of your battery should be scaled to the rest of the system.  A good rule of thumb is that charging at 1/20th of C is rock bottom acceptable, 1/10th C is good, and 1/8th of C is best.  Both Trojan and Rolls recommend 1/8C, or in other words, 0.125C

With your 400W panels outputting a max around 85% what you will likely get for a maximal amperage output is (400W X 0.85)/13V charging = 26.1Amps.  Eight times 26.1 =208.8A.

It just so happens that those Costco batteries mentioned above are 210Ah each (C=210Ah), which is close to an absolute perfect fit charging wise for what you already have.  The most power you would be able to store and use would be... 210Ah X 12V X 0.5 (50%) = 1260Wh of power.

I just noticed what I've been doing. My phone does numbers if I hold the key long enough but I was not so some brilliant person noticed qw=12..  I'll try to do better ...
Cheers,. David

Nice, another piece of the puzzle and thanks for equations which are very useful.

As the others have posted, a Kilowatt meter will be your new best friend.

I tested everything I planned to take off grid and built a spreadsheet of my estimated daily/weekly energy consumption, as compared to my daily average generation, battery array capacity, etc. Without data, you do not know where you are, it's just a guess. I wanted to know how deep was the reserve, at what depth does what draw power, what should I turn off, or not take, etc.

And all laptops are NOT the same. Gaming laptops are hungry (obviously), but you cannot go by the rating on the power supply - keep your battery mostly charged in the laptop, and you'll only see power spike up when charging or heavily processing/kicking on the fan, etc. For example, my new desktop replacement laptop rarely peaks above 35% of what the power supply can produce.

Finally, some things 'leak' power (they consume power even when you've switched them off), and a watt meter is a great way to see what leaks as such. Buy a few of something like this in that case: Power Switches

This way, I know when something is not in use, the drain has been closed.