The inverter is a load, not a battery
A 1,000-watt inverter at the coffee maker is tens of amps on a 12-volt bank after inefficiency. Reserve capacity, defined in Battery University BU-105 as minutes at 25 amperes, is one way to compare lead-acid house batteries. Amp-hours at the 20-hour rate is the other. Neither number is CCA. If the bank cannot support that current without collapsing voltage, the inverter shuts down and the owner calls it a dead battery.
Depth of discharge is the life knob
Wikipedia’s depth-of-discharge article is the frame: using more of the pack each night shortens cycle life. Flooded house batteries are often treated gently around 50 percent. AGM can go deeper; BU-201a cites about 80 percent depth of discharge versus about 50 percent for flooded. Lithium can go deeper still, with a BMS that will disconnect when it is empty. Size the bank for the night you actually live, not the brochure weekend.
Cables and the inverter’s own shutdown
Voltage drop in undersize cable makes a healthy bank look empty at the inverter terminals. We inspect the run. A house bank under a cabin step with 8-gauge leftovers from a stereo is a cable job. The inverter’s low-voltage cutoff is not a state-of-charge meter.
Charging has to refill what you used
Shore power, the engine alternator, or solar has to put those amp-hours back before the next night. A charger on the wrong chemistry never finishes. Lithium house with a lead float is the charger article. Inverter sizing without a refill plan is a one-night design.
On a cruiser visit
Tell us what you run after dark — fridge, inverter, electronics — and how you refill. Sarasota and Cape Coral overnight boats are this conversation. We size from loads and RC/Ah on the cases, not from a generic “big house battery” label.
A short list we actually want
Fridge amps if you know them, inverter size, and what you run at the same time. We can walk that list at the boat. What we cannot do is turn a single group 27 into an overnight inverter bank with optimism. If the numbers do not work, we say add batteries or drop the load. That is sizing, not a scolding.
Sources
- Battery University BU-105 — Battery Definitions (RC at 25 A)
- Wikipedia — Depth of discharge
- Battery University BU-201a — Absorbent Glass Mat (AGM)
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.