I use my computer to mine Bitcoins with a HD 7970 GHz Edition GPU. It currently draws a steady 465W.

I would like to offset the draw, if possible, with a reasonably priced solar panel (~240-500W). This will likely not ever power the entire computer (I live in Minneapolis, MN).

What are my options for connecting this? I'd rather not buy an inverter to make it AC and then reconvert back to DC and suffer the associated losses.

Can I regulate the output to a clean 12V1 line and connect it right to the output of the power supply, with perhaps some diodes to not have any backfeeding? Will the PSU just 'see' less draw? I'm not really concerned with the 5V or 3V3 lines, as the majority of draw is the GPU which is pulling from four 12V1 lines.

I also have this connected to a UPS on the floor, with a battery and likely it's own inverter. Can I just hook the solar panel (again with some regulation) to the battery of the UPS? What are my options here?

(also, recommendation on solar panel specifications or models would be helpful. I.E. ideal voltage output, wattage, or brand)

  • \$\begingroup\$ I should add my current power rate is $0.07 / kWh. What is the 'protected' status about? \$\endgroup\$ – Ehryk Apr 11 '13 at 7:53
  • \$\begingroup\$ Check out the 'meta' section (over there on the right), there is a thread on that 'protected' status. On another note, you could list the exact info on your PSU (model number or whatever) to see if anyone has more info on messing with its 12V1 lines. \$\endgroup\$ – Bobbi Bennett Apr 11 '13 at 13:50
  • \$\begingroup\$ I see something about someone creating accounts to troll, is that what you meant? What I'm really after is: what about my question needs protection, or is it not about my question at all? \$\endgroup\$ – Ehryk Apr 12 '13 at 2:26
  • \$\begingroup\$ I expect it has nothing (nothing bad) to do with your question. Anyway, I hope you can go ahead with your solar installation. \$\endgroup\$ – Bobbi Bennett Apr 12 '13 at 3:50
  • \$\begingroup\$ @Ehryk The protected status has absolutely nothing to do with your specific question. It's just a new thing mods do around here, it's their atonement for being mods. :-) \$\endgroup\$ – Anindo Ghosh Apr 13 '13 at 17:18
  • It can be done

  • It will probably NOT save you money.
    PV power is still not really competitive with grid power on a level energy field in many cases. Subsidies and similar may sway this somewhat, but maybe not enough.

What Bobbi says. Plus:

You could feed the panel into the 12V directly via a simple regulator that only supplied energy when the PSU power-good signal was asserted, showing that the supply was active, and the regulator could be arranged to never cause voltage to rise above 12.1V or some other limit.

You'd need to be sure that when PV input was applied that the PSU did not backfeed in unexpected manners. A diode in the PV feed stops backfeed to the PV but stopping psu backfeed may or may not be as simple as adding a diode in the PSU's 12V output to the GPU.

If you are doing this "from scratch" you want to determine if it is economic compared to grid power. If you receive local subsidies and feed in tariffs (FITS) it may be. Otherwise maybe not. And it may depend what you cost your time at.

At an average of 3 full sunshine hours per day over a year at a typical location you get about 3 units x 365 = 1095 hrs/year or say 1000 units of electricity per year from 1 kW of panels. So if electricity costs N cents per unit effective cost including any FITs subsidies etc then this will save you N x 1000 cents a year or 10 x N dollars. So if your electricity costs 20 cents /unit you save 20 x 10 = $200/year in power costs. Say $300 outside.

A very optimistic cost for installed panels* would be $2/Watt so 1 kW of panels would cost $2000 installed so would cost $2000/$300 =~7 years to break even if your money is worth 0% if invested. That's at 30c/unit and $2/Watt installed and 0% on your money. Adjust as required. * I can land panels here ex China for about $1.30/Watt at my door. Then add installation costs, mounting hardware, cabling, any control equipment, any regulatory costs, and my time at $1/hour. Add ongoing cleaning and possibly costs for hail or other damage long term.

If you feed direct DC then your panels will provide 12v/18V ~= 66% of rated output at full power unless you use an MPPT or other downconverter at extra cost. A decent grid tie inverter should beat that even allowing for the 18VDC - 240 VAC and 240VAC-12V1DC conversion BUT adds extra cost.

All up, unless you are subsidised hand over fist or are off-grid, chances are that using your own PV is not worthwhile compared to using grid power.

If you can use night-rate power for part of your grid energy this would be even more true. If night rate power is much cheaper than peak load power then bitcoin mining at night may prove more economic while taking longer.

  • \$\begingroup\$ Don't forget just how -big- 500W of solar panel is going to be! Would have to rent the neighbor's gazebo roof for it. \$\endgroup\$ – Bobbi Bennett Apr 11 '13 at 0:53
  • \$\begingroup\$ @BobbiBennett - Yes. Approaching 150 Watt per square metre for the better ones. Even slightly less if you use 23% rear contact cells. So say 3 m^2 gives 450 Wmp. With DC direct connect and panel loaded to 12V you get 2/3 of Wmp so you'd need 675 Wmp or so. On average that gives about 2 kHw/day. $US0.50/day at $US0.25/unit. More if you can get more per unit for your power. If all adds up, but, 1st, you'd have to pay for it. \$\endgroup\$ – Russell McMahon Apr 11 '13 at 2:45

Most solar panels are set up to charge 12V batteries. So their optimum output voltage is somewhat higher than 12V, and their open circuit voltage is up around 20V. Below the optimum voltage, they are essentially a constant current source.

So if you hook the panel (with a blocking diode) to your 12V1, four bad things can happen:

One, you will be getting something like 20% less power than optimum.

Two, you will be running 12V power (40A for 500W) down a long line from the panel to the power supply, heating up a lot of wire (the power loss isn't relevant, just part of that 20% above), or spending a lot on copper.

Three, if the load disconnects, you will (might. Maybe the PS has over-voltage clamp, but don't count on it) get 20Volts on your 12V1 line.

Four, if the mains power fails (and the UPS gives out), the panels will be at best useless, at worst feeding under voltage power into the MPU with no 3p3 or 5V.

Trade these against a nice simple hookup, the panel feeding a constant current into the 12V line. Your PSU will just see less draw.

But if you run the panel the way its designed, charging a small battery setup with a 'grid tie inverter' near where the panel is installed, most of these issues go away.

I (being a careless maveric with lots of cells but no money for inverters) would choose the direct connection, with a simple over-voltage shutoff, and solar panels chosen to deliver less than 400W. Probably custom built with just the right number of cells for max power at 12V.

You, being a bit-coin rich, responsible fellow with a view towards what to do with 500W when not running a GPU (maybe run an emergency wireless mesh node for when the power goes out gets my vote), would choose the grid-tie inverter approach.

As you are in Minnesota, consider the outfit up on the Iron Range that has got some press lately for their rugged solar panels.

  • 1
    \$\begingroup\$ Bobbi - Mainly good. A couple of minor points: (1) Typical 12V PV panel is about 18V loaded - even higher when OC. Doesn't affect your answer (except smoke may be denser). (2) As 18V >> 12V and system is essentially a CC (as you note) then the cable drop is not relevant energy-loss wise as long as Vdrop < ~ 6V. \$\endgroup\$ – Russell McMahon Apr 10 '13 at 23:31
  • \$\begingroup\$ Bobbi - see my answer - essentially an addition to yours. \$\endgroup\$ – Russell McMahon Apr 10 '13 at 23:57

protected by W5VO Apr 10 '13 at 21:17

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