Power Budgeting: Can Your Battery Keep Up?

Living aboard means running a tiny electrical grid off your house battery bank. Three sources refill it. The alternator on your engine pushes a lot of current — often 60–120 amps — but only while the engine is running. Shore power feeds a battery charger when you're plugged into a dock, effectively unlimited. Solar trickles in during daylight: a 200-watt panel in strong sun makes roughly 16 amps, but cloud, angle, and short winter days mean you might net only 60–80 amp-hours (Ah) a day, not the theoretical 130.

The key skill is a power budget: tally what you consume per day versus what you can generate. Everything is measured in amp-hours (amps × hours). A fridge drawing 5A but cycling on ~40% of the time uses about 5 × 24 × 0.4 ≈ 48 Ah/day — usually your biggest load. Add lights, instruments, autopilot, and water pump, and a modest cruising boat lands near 80–120 Ah/day.

Now compare. At anchor with no engine and no dock, only solar generates: if you draw 100 Ah but solar makes 70, you're 30 Ah in deficit every day — the bank slowly drains. You close the gap by running the engine (the alternator can replace 100 Ah in about an hour), adding solar, or cutting consumption. Remember from earlier: you can only safely use about half a lead-acid bank's rated capacity, so a 100 Ah deficit against a 200 Ah bank is far more urgent than the raw number suggests.

The habit that keeps liveaboards off a dead battery is simple: know your daily number, and never let consumption quietly outrun generation.