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We live in a rural 250 year old colonial that was wired for ceiling and wall sconce electric lights sometime in the mid-1940s. In the early 70's outlets were added, then updated to Romex when a central heating system was installed.

We are updating to solar thermal for heating and want to add some solar PV just to put the lights off-grid, in case of another hurricane or blizzard.

The original lights are on two isolated circuits on for second floor, one for third, and I think they are just in series.

I remember doing the potato battery experiment with an old fashioned ceramic ceiling fixture years ago, and I wondered ... is there a way to use the existing wiring to go directly to DC and simply switch to DC LED edison screw lightbulb? Instead of a whole inverter, just for lights?

The DC-AC inverter is the expensive part. If I can figure this out, we'll run another USB-PD circuit for all our other power brick devices.

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  • What DC voltage you intend to use? Will you use 5 V throughout the house, or distribute a higher DC voltage, and then have a regulator in each room, or something like that? What is the "old" wire? Knob and tube, or rubber and fabric? What gauge? Could you photograph some of it? – Pigrew Aug 27 '14 at 20:57
  • bulbtown.com/… – Kate Aug 28 '14 at 22:50
  • 12VDC with 3Amp LED lights, six per circuit. – Kate Aug 28 '14 at 22:50
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In principle, you can do this. In practice, you'll need to talk to someone about local electrical codes. Running parallel low voltage DC and 110 AC circuits can be done, but like I say, you'll need to talk to a pro.

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This is problematic. The most obvious: Your PV panels will be generating power all day, while you're likely to want to operate lights all night but not so much during the day. That means your PVs will need to feed a battery bank through a PV controller/charger (which is usually contained inside the inverter anyway), and said battery bank will need to be in a well-ventilated location, which means that it'll probably be fairly distant from the area you want to light... so you're going to suffer a lot of line loss between the panels, the controller, the batteries, then up to the lighted area.

Since you're going to need a battery bank, you'll be running either 12V or 24V. 12V lights are much more common than 24V lights (at least in the USA), so we'll assume you're using 12V. Now, lumen-for-lumen, a 12V light will require ten times as much current as a 120V light will... meaning a single 100W (incandescent) would require roughly 8A. Multiply that by two floors, say three rooms per floor, and you have 48A. You cannot safely draw 48A through your 1940s-era house lighting circuit.

Even if you could, the voltage drop would kill you to death. In the early 1960s, Volkswagen was still holding onto its 6V battery instead of migrating up to a 12V system, and the result was chronic low lighting efficiency and low starting currents, just because at 6V, the voltage drop in the car's wiring was so large. Eventually 8V upgrade kits were sold, including nothing more than an 8V battery and an 8V voltage regulator... but retaining the same old 6V lights and starter, because a 2V (25%) voltage drop (inside a 20'long car!) was pretty much to be expected. In your house, a 10' run would reach from one floor to the next, but you're more likely to be looking at a total 100' or 150' run from PV panel to light bulb. Maybe 250'. That's going to buy you a huge voltage drop at 12VDC.

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