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To maximize efficiency and reduce cost I am contemplating using the power from solar cells directly as dc without conversion to ac. I plan to wire and connect the solar panels in series and parallel to charge batteries and super capacitors. And then arrange the batteries and super-capacitors to yield 115 volts dc. This arrangement will be completely off-grid. By skipping the inverters, I hope to gain much efficiency and lower cost.

I know direct dc wiring is used in rvs but the voltage is lower. The 12 gauge and 14 gauge wiring in the house is already designed for 115 volts albeit rms ac. The dc should be ok for all loads except ac motors and compressors for which I will either keep a dedicated line from the grid or use inverters just for these.

Besides possible code violations, is there any other reason to not do this?

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    $\begingroup$ "The dc should be ok for all loads except ac motors and compressors" That is almost certainly not correct. Many electronic devices (e.g. TVs etc) have power supplies designed for AC only. You will probably find that the household appliances that will work on DC (as stated in the device's documentation- trial and error gets expensive if your errors break things, and even more expensive if they burn your house down) are only a small proportion of your total power consumption. $\endgroup$ – alephzero May 7 '17 at 18:00
  • $\begingroup$ Why would you even consider this? Nothing that I can think of is designed for 115VDC and the few things that I can think of that might work (a toaster, incandescent lights) are so horribly inefficient that any savings you might gain will be lost. If you want to skip the inverter, use things that are designed for DC where you can and use an inverter for those things that aren't. $\endgroup$ – DLS3141 May 8 '17 at 18:07
  • $\begingroup$ OK, I see. bad idea. the way the solar panels are installed now. They hook up inverters right underneath the solar panels as a modular unit. Would it be more efficient to wire as 115 v dc from the roof as proposed then feed from battery array into a resonant LC tank circuit that would resonate at 60 Hz for perfect sine wave? $\endgroup$ – Ed Kideys May 8 '17 at 21:16
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Does your "reduce cost" objective include the cost of installing a duplicated house wiring system (including safety devices like breakers - not just stringing a few wires around!), and then ensuring that you never destroy an appliance by connecting it to the wrong supply? To do that reliably, you would need a different (and probably "non-standard") type of plugs and sockets on the DC supply, for example.

If your inverter is "only" 90% efficient, just adding 10% more battery and/or solar panel capacity might be both cheaper and simpler.

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The solar panel output voltage varies with the solar energy hitting it, ie varies with time of day : angle and cloud cover, part of the function of a solar controller can be to control the voltage output to the batteries.

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  • $\begingroup$ Are you saying that a solar controller can only work with AC output? The variance in sun power would not be an issue in my setup since the sun power would just be recharging the batteries. The output of the batteries would be maintained at 115 volts but just not converted to AC. $\endgroup$ – Ed Kideys May 7 '17 at 17:36
  • $\begingroup$ I did not mention AC... Perhaps you should check out what a solar controller does... $\endgroup$ – Solar Mike May 7 '17 at 19:05

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