I have a computer that uses a 500-Watt power supply. Note that this is a 500-watt power supply. How much power the computer actually uses, I don't know. 500-watt supplies are pretty typical. I think the computer actually probably uses around 150 Watts on average.

I am trying to figure out how much solar panel and battery I would need to run the computer 8 hours a day, but I find the calculations confusing.

In my case I am not going to use an inverter. My plan is to connect the 12V battery directly to the DC lines powering the computer and its fans, so there is no AC conversion loss. (although this does leave the problem of how to turn 12V battery power into 5V computer power)

Can anybody walk me through the general process to do the calculation to size the panels and battery?

I live in Massachusetts (42-degrees latitude) and would need to run the computer through the winter months. I would probably need to have 3-5 days of buffer in the battery to deal with clouds and stuff, but that is just a guess.

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This question is about calculating the design envelope of a solar energy system, which is a problem in electrical engineering. This is an on-topic question.

  • \$\begingroup\$ Don't forget about the -12V, -5V, +3.3V supplies too ... \$\endgroup\$
    – brhans
    Jun 28, 2016 at 16:21
  • \$\begingroup\$ Well, your computer will need 4kWh per day. That... seems like a lot. Could you do the job with less computer? \$\endgroup\$
    – Daniel
    Jun 28, 2016 at 16:26
  • \$\begingroup\$ Since there are very few solar systems available to choose from, I'd suggest you just stay here on earth. \$\endgroup\$ Jun 28, 2016 at 17:43
  • \$\begingroup\$ If you have a UPC (Uninterruptible power supply) some of them will provide current power usage estimates. Don't know how accurate they are, but you could have the PC run whatever software you plan to run then take the reading from that. Alternatively something like a Kill-A-Watt device could be used. \$\endgroup\$
    – mjh2007
    Jun 28, 2016 at 17:58
  • 1
    \$\begingroup\$ Note: most "12V" batteries are more like 14V, especially while charging. This may be bad for the PC. You'll need a set of DC-DC converters or (simpler) a mini-ATX power supply designed for use in cars. I think step 1 is to get a plug-in power meter and find out your actual average and peak consumption. \$\endgroup\$
    – pjc50
    Jun 28, 2016 at 18:08

1 Answer 1


As Daniel said, this is an enormous amount of power for just a computer. Not your standard computer, no? Most laptops use less than 50 watts, in comparison.

Background: I built a system that runs off of solar 24/7. The load is 150 watts. I'm using 800 watts of panels, and 6 deep-cycle lead acid batteries (Capacity about 100 Ah each). With consecutive days of no sunlight, assuming full charge on batteries, it can run 60+ hours before failing.

Your system needs to be able to charge batteries AND support your load. The advantage is you are only running 8 hours a day, and I will assume those 8 hours will occur during the solar day. With your load, I would start with 2000 watts of panels and a battery bank of 1000Ah.

With a 500W load, at 12V: 500W / 12V = 42 Amps. 1000Ah batteries / 42 Amps = 24 hours runtime (fully charged batteries, no sunlight)

2000 watts of panels: Considering your location and weather, expect no greater than 75% efficiency for 5 hours of the day. 2000*5*.75 = 7500 Watt-hours generated per day.

7500 watt-hours generated - 4000 watt-hours used =3500 watt-hours left for charging your 1000Ah * 12V = 12,000 watt-hour battery bank.

I am not including other efficiencies you may run across, so take this as a best-case scenario. I STRONGLY SUGGEST trying to reduce your load. 500W is HUGE! Building the system I described above will cost you a pretty penny too- between $5000-10000.

  • \$\begingroup\$ Just so that it's said out loud and not assumed, a computer with a 500W power supply doesn't usually USE 500W. You can factor the actual consumption in and save a lot on batteries. \$\endgroup\$
    – Daniel
    Jun 28, 2016 at 17:48
  • \$\begingroup\$ I think a desktop computer uses about 150 watts on average. \$\endgroup\$ Jun 28, 2016 at 17:49
  • \$\begingroup\$ I never said that the load was 500W, I only said the power supply in the computer is a 500W supply. \$\endgroup\$ Jul 20, 2016 at 5:50
  • \$\begingroup\$ Does your system use an inverter supplying AC to a standard switching power supply? If so, there could be large energy losses because you are converting to AC then back to DC again. Also, computer switching power supplies are wasteful, often using resistors to drop voltages. Those resistors suck energy. I am hopefully planning to supply DC directly to the computer components without involving AC at all. \$\endgroup\$ Jul 20, 2016 at 5:53

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