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I'm about to commission a total refit and rewire of a one bedroom apartment in Spain. I'm considering completely skipping a conventional 220v lighting circuit, and replacing it with a solar-battery-12v LED based installation. My fag packet calculation (taking into account, losses and winter sun-hours) reckons a 100W panel and 30Ah battery will be enough.

For me the pros are cost (as I need some areas lit throughout the day), and the ability to easily wire in Arduinos to provide smart switching.

Are there any pitfalls or downsides?

  • Keep in mind that the power draw of the Arduinos won't be zero. Your system is big enough to make it negligible if you're recharging every day, but they'll be drawing power 24/7. – Joseph Sheedy Apr 1 '18 at 19:33
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It seems reasonable to me.

For a lead-acid battery, size it so you are only dipping it about 25 percent of its capacity. That is leaving 75% remaining. If you dip it much deeper than that on a regular basis, you will destroy its life span. There is plenty about this on Google if you need more info.

You can also keep the DC system topped up using AC power. Simply have a mains battery charger of decent quality. This can be paralleled with the solar charge controller. An MPPT charge controller may not be worth it for only one 100W panel.

If power failures are a thing in your area, you may want to move more features of your home to the DC system. For instance USB charging is easy. Some modern TVs can handle 12V, as well as internet routers, cable modems, and streaming devices like Roku, Apple TV etc. Complemented with a non-electricity (millivolt) furnace, you can be watching streaming video off the Internet and have a pretty cozy night with no power.

  • The depth of discharge for a lead-acid battery is directly related to the amount of current used and is not easily summarized at 75 percent. A typical deep-discharge battery such as those in "old-timey" electric vehicles will pull currents at a level permitting no more than 50% DoD, but low currents will allow much greater discharge without harm. The question did not reference battery chemistry and could be lithium based, good for 80% DoD at high current levels. – fred_dot_u Aug 19 '17 at 11:04
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If you are considering only LED lighting and no other use, allowing for the Arduino aspect, your concept may work well enough.

The primary reason for higher voltages is to reduce current losses through wiring resistance. Transmission lines are extremely high voltage for that reason.

In a configuration such as you suggest, the low voltage will have greater power loss due to wiring resistance, but it will be greater only by comparison to more extreme situations. Your wiring runs are likely to be short enough as to permit heavier gauge wiring to ameliorate resistance losses, without an excessive expense.

You will want to incorporate some fusing or breaker device for overload protection, as a 30 ampere-hour battery can provide enough energy to melt the insulation and/or cause a fire if inadvertently shorted.

You didn't mention a charge controller, nor the battery chemistry. You will want to ensure that you do not overcharge the battery and also to follow the correct charge profile based on the battery chemistry.

Lead-acid batteries, especially valve-regulated sealed lead-acid batteries do not do well with overcharging. Flooded lead-acid batteries will release acid vapor and damage surroundings when charged at high levels.

Lithium based batteries also have special considerations, outside the scope of this forum.

On the upside, free electricity is great, if you are able to balance your expense with the local utility costs.

  • thx for the answer - upvoted – pinoyyid Aug 19 '17 at 9:14

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