Boat Battery Load Budget Calculator (Daily Amp-Hours, Days at Anchor, Solar & Engine Charging)
Add up a boat's daily 12 V loads — fridge, lights, electronics, autopilot, pumps, inverter and charging — against the usable capacity of a lead-acid or lithium house bank to see how many days you can swing at anchor, what solar covers, and how many engine hours a day it takes to break even.
How it works
daily Ah = Σ DC loads + inverter Wh ÷ 12 V ÷ 0.85 · usable Ah = bank × (0.5 flooded, 0.55 AGM, 0.85 LiFePO₄) · solar Ah/day ≈ W × sun-hours × 0.7 ÷ 12.5 V · days = usable ÷ (daily − solar × charge efficiency) · engine h/day = net deficit ÷ (alternator A × efficiency)
A boat at anchor lives off its house bank, and the arithmetic is simply consumption versus what you can put back. Refrigeration is usually the biggest single load, inverters add their own losses, and only part of a battery's nameplate capacity is usable — half for lead-acid, most of it for lithium. Solar output is far below the panel's watt rating once you count angle, shading and controller losses, and an alternator only delivers its rating into a deeply discharged bank. This calculator produces the number cruisers actually care about: how many days before you have to run the engine or move on, and how much solar would make that question go away.
Worked example
A 400 Ah AGM bank has about 220 Ah usable. Fridge 45, lights 12, electronics 15, pumps 8, devices 6 and 300 Wh of inverter use (about 29 Ah) come to about 115 Ah a day. A 300 W array at 4.5 sun-hours returns about 76 Ah, leaving a 47 Ah daily deficit — about 4.6 days at anchor, or 1.9 days with no sun — and about 0.9 hours of engine a day at 60 A to break even; roughly 510 W of solar would cover the load.
Frequently asked questions
Why does my 100 A alternator only put 40 A into the batteries?
Lead-acid batteries accept high current only when well discharged; acceptance falls off sharply above 80% state of charge, and standard internal regulators drop to float early. An external smart regulator or a lithium bank draws much closer to the rating.
How much solar do I really get from a 100 W panel?
Typically 25–35 Ah a day in summer — 100 W × 5 sun-hours × 0.7 losses ÷ 12.5 V ≈ 28 Ah. Partial shade from the boom or a shroud can halve that on a panel without bypass diodes, so mount panels where they are clear.
Is it worth switching to lithium?
Lithium gives roughly twice the usable capacity for the same nameplate Ah, charges faster, weighs half as much and lasts several times longer; the costs are the battery price, a BMS and charging-system changes (alternator protection, chargers with lithium profiles). For boats that anchor a lot it usually pays back in fewer engine hours.
Related calculators
Boating figures are planning estimates based on published rules of thumb and standards (USCG, ABYC, manufacturer tables). Your boat's capacity plate, builder documentation and local regulations govern; conditions at sea vary.