I'm trying to create an off-grid installation.

I have 200W PV panels, 20A charge regulator and two 150Ah, 12V batteries in parallel.

How can I keep the voltage constant through all the grid in the building?

Can I implement some microgrid tricks?

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    \$\begingroup\$ Use very thick wires. \$\endgroup\$ – WhatRoughBeast Jun 29 '18 at 1:21
  • \$\begingroup\$ @jsotola Over long distances appliances can't be powered. Do I create separate grids in my building or is there a way to avoid big voltage drops around big loads, all in the same circuit? \$\endgroup\$ – Mariano M. Jun 29 '18 at 1:34
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    \$\begingroup\$ "Over long distances appliances can't be powered" - define 'long distances'. \$\endgroup\$ – Bruce Abbott Jun 29 '18 at 2:21
  • \$\begingroup\$ @BruceAbbott in my case, even 100m. My inverter works on the batteries, but fails if I connect it to cables 100m far from the source. \$\endgroup\$ – Mariano M. Jun 29 '18 at 2:25
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    \$\begingroup\$ You should connect the inverter as close to the battery as possible. Inverter output voltage is higher and current lower, so voltage drop is less. If you need to run a cable 100m then it should be after the inverter (ie. AC mains cable). What is the maximum power draw, and what voltages are your solar panels and battery? (by 'battery' I mean all the 12V batteries in series/parallel). \$\endgroup\$ – Bruce Abbott Jun 29 '18 at 3:14

How can I keep the voltage constant through all the grid in the building?

Use thick enough wires for the current you're carrying.

This is made easier by using the highest distribution voltage you can.

There's a reason that country-wide power distribution grids use voltages that are now in the mega-volt ballpark. High voltage distribution allows you to economise on copper wire in two ways.

(1) High voltage means lower current for the same power. Lower current means lower voltage drop on the wires

(2) High voltage means that any voltage drop you do have is a smaller fraction of the voltage you're actually delivering, so your efficiency loss is less.

These two taken together mean that for any target distribution efficiency, your wire cross section has to vary as the inverse square of the voltage.

48v is a very common voltage used in industry for DC power distribution within telephone racks and recording studios. It's about the highest voltage that's deemed to be 'touch safe', so doesn't need the serious safety practice of mains voltages.

By distributing at (for instance) 48v instead of 12v, you could use wires of 1/16th the cross section of the equivalent 12v distribution. Whether it's worth trading cost in copper for cost in point-of-use 48v to 12v converters is a question of distance. Within one room, maybe, maybe not. Within one building, almost certainly.

To minimise on power conversion, the trick is to use a series string of batteries equal to your distribution voltage. Have a charge controller to match the voltage of your panels to the distribution voltage. Some point of use converters are efficient at low or no load, and some aren't. Find out which before deciding whether to switch particular loads at the high voltage or at 12v.


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