Off-grid from zero · lesson 5 of 12

The battery, the inverter, and the voltage

Choose the bank chemistry and voltage, size it for autonomy, and match the inverter to the loads and the surge.

Once the load is known, the bank is the next decision and the most expensive one. Lithium iron phosphate has won this argument for full-time living: roughly 80 to 90 percent usable in practice, thousands of cycles, no maintenance. Lead-acid still makes sense for a weekend place or a very tight budget. Either way, size for days of autonomy, not for the brochure.

Voltage is the other choice people get wrong. 12 V is for small systems; above roughly 2,000 W of inverter, 48 V cuts wire size and cost and is what the serious equipment is built for. The inverter is sized from running watts and the biggest surge, and 240 V loads need a split-phase inverter, two stacked units, or an autotransformer.

Numbers to carry
  • Lithium: 80 percent usable; lead-acid: 50 percent. The same usable energy needs a lead bank nearly twice the size.
  • At 48 V a 5,000 W inverter draws roughly 120 to 140 A once efficiency and low battery voltage are counted; at 12 V it would draw 500 A or more.
  • Lithium must not be charged at cell temperatures at or below 32 F, so heated models or a warm enclosure are not optional in the north.
Read
Use
Do before the next lesson
  • Size the bank for two days of autonomy at your real daily load and compare lithium and lead-acid side by side.
  • Open the battery spec table and sort by $/kWh and by minimum charge temperature; note which ones are heated.
  • Decide the bank voltage and write it on the plan; it drives the wire and fuse lesson.
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