AGM batteries for solar

Absorbed glass mat lead-acid.

AGM (absorbed glass mat) is a sealed, maintenance-free lead-acid battery. It is cheaper upfront than lithium and tolerates cold better, but only about 50% of its capacity is usable for good life, and cycle life is far shorter. A common budget or backup choice.

Sizing a AGM bank

Because AGM allows about 50% usable depth of discharge, a bank for the same daily load will differ in size from other chemistries. To deliver the same usable energy, a chemistry with a lower usable depth needs more nominal capacity — a lead-acid bank at 50% usable must be roughly 60% larger than a LiFePO4 bank at 80% to give the same delivered energy, before efficiency and temperature are even considered. Use the battery bank size calculator with AGM selected, or the full system calculator for a complete design.

How AGM behaves in real use

Round-trip efficiency of about ~85% means some of the energy you put in is lost on the way back out, so your solar array has to be sized to cover both the load and that loss. Temperature sensitivity is rated medium: this determines how much usable capacity changes with the seasons, which matters most for a bank kept in an unheated space, a garage or outdoors. With a rated cycle life of 600–1,000 cycles, AGM also sets how many full charge-discharge cycles you can expect before capacity falls off, which in turn drives the long-run cost per kWh delivered.

To see what a AGM battery really delivers after depth of discharge, temperature and round-trip losses, try the real vs nameplate capacity tool, and check that your loads stay within a safe discharge rate with the C-rate calculator.

How it compares

The right chemistry is a balance of upfront cost, usable depth, efficiency, cycle life and cold tolerance. Compare AGM against the alternatives on the battery chemistry comparison page to decide which fits your budget and climate.

Values are typical datasheet ranges; see the methodology & sources. Educational guidance, not an electrical design.