Off-Grid Solar Design, Education and Advice

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Michael Blankenship

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Michael Blankenship

November 15, 2024 6:14 pm

Never 12V. Guys, I know this is going to step on some toes but never design an off-grid system which relies on 12V. Period. Okay, I understand that most of you have a 12V solar system. Somebody conv... View MoreNever 12V. Guys, I know this is going to step on some toes but never design an off-grid system which relies on 12V. Period. Okay, I understand that most of you have a 12V solar system. Somebody convinced you to purchase 12V batteries and to put them parallel. Let's do some math. 1200W toaster (or coffee pot or tiny microwave oven) ------------------------------------------------------- ------------------------------- Turn it on. How much current is suddenly moving from your batteries into the inverter (the thing that turns direct current into alternating current in a typical home outlet)? The math is super easy... Battery bank Math Current Required Wire Gauge Between Batteries & From Inverter -------------------- ---------------- ------------ ------------------------------------------------------- ---------------------------------- 12V 1200 ÷ 12 100A 1AWG copper or 2/0 aluminum 24V 1200 ÷ 24 50A 6AWG copper or 4AWG aluminum 48V 1200 ÷ 48 25A 10AWG copper or 8AWG aluminum Now consider the time when you're trying to brew coffee and your refrigerator is currently cycled on. Okay, so you have to factor in additional current in those wires. Let's back up a step and look at the often-seen 3000W inverter on most of the installs I see. How much current is that for those of you with a 12V system? Math: 3000W ÷ 12V = 250A (Yipes!) In order to use that rating on your big inverter—and assuming that you've setup your battery set for 12V—you'd need copper wire gauge "250 MCM" or "400 MCM" for aluminum. It costs $9.26 per foot, the minimum purchase is ten feet and you're likely going to pay an additional $50 for shipping due to the weight. Next, you're going to find that the wire probably won't fit into the fittings or lugs on your batteries or your inverter. Good luck finding 3/8" lugs for this diameter or a pneumatic crimper to secure them to the thick cable. (A 48V system would require 4AWG copper or 3AWG aluminum wiring, for comparison.) Start things off right. If you're trying to power an off-grid home begin with a 24V or 48V battery setup with a preference for the latter.

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