> ## Content Index > Fetch the complete content index at: https://www.goodkit.io/llms.txt > Use this file to discover other available public pages before exploring further. # Marine Battery Bank Sizing Calculator - URL: https://www.goodkit.io/marine-battery-bank-sizing-calculator/ - Published: 2026-05-10T12:43:05.000Z - Updated: 2026-09-06T01:59:04.000Z - Description: Free marine battery-bank sizing calculator. Enter daily consumption, autonomy, voltage and chemistry to estimate the required capacity in amp-hours. - Author: goodkit crew - Tags: Marine, Electrical, Calculator Daily Energy Consumption (Wh) Add up everything you use in 24 hours. Fridge \~600 Wh, lights/fans \~300 Wh, laptop \~150 Wh, watermaker varies. System Voltage 12V 24V 48V Days of Autonomy Days you want to run without charging. Cruisers typically 1–3, off-grid liveaboards 3–5, conservative 5–7. Battery Chemistry LiFePO4 (Lithium) — 80% usable AGM — 50% usable Gel — 50% usable Flooded Lead-Acid — 50% usable Charging Reserve No reserve (size to exactly meet need) 20% reserve (recommended — handles cloudy days, surprises) 50% reserve (very conservative) Calculate ## How to Use This **Start with daily Wh consumption.** That's the input that matters most. Add up your fridge, lights, electronics, watermaker, and anything else that runs from the bank in a typical 24-hour day. If you don't know — start measuring with a battery monitor (the [Victron SmartShunt](https://www.goodkit.io/best-marine-battery-monitor-victron-smartshunt-500a-review/) is what we use on our boat). **Days of autonomy is judgment.** Most cruisers size for 1–3 days. Off-grid liveaboards 3–5\. Belt-and-suspenders 5–7\. The bigger the bank, the more it costs and the longer it takes to recharge — but the more cushion you have for cloudy weeks. **Pick the right chemistry.** LiFePO4 is the right answer for almost any new install — 80% usable vs 50% for lead-acid means a 100Ah lithium bank does the same job as a 200Ah lead-acid bank, weighs half as much, and lasts 3–5× longer. The price gap has narrowed significantly. Lead-acid still wins on cold-weather use (LiFePO4 won't charge below \~32°F without internal heating) and absolute lowest upfront cost. **The reserve multiplier matters more than people think.** Sizing exactly to your daily need leaves zero margin for cloudy weeks, surprise loads, or aging cells. 20% reserve is the practical default. 50% is paranoid but cheap insurance on a system you live with for years. ## What This Doesn't Account For A few things this calculator simplifies that you should think about separately: **Temperature derating.** All batteries lose capacity in cold. LiFePO4 drops about 10–20% at freezing. Lead-acid drops more. If you're cold-weather cruising or storing the boat in winter, size up. **Aging.** Both lithium and lead-acid lose capacity over the bank's life. Plan to replace lead-acid every 3–7 years, lithium every 8–15 years. The numbers above are for a new bank. **Peak loads.** A windlass or thruster pulls 100–300A momentarily. The bank has to deliver that without sagging out of spec. Most modern lithium and AGM banks handle this, but flooded lead-acid struggles with high-current pulses. **Inverter efficiency.** If you're powering AC loads through an inverter, the inverter itself is 85–95% efficient. The Wh you measure on the AC side gets a 5–15% bump on the DC side. The 95% round-trip efficiency for lithium in this calculator approximates this for typical use. ## Common Mistakes **Sizing for "100Ah lithium = 100Ah lead-acid."** It doesn't. 100Ah lithium gives you 80Ah usable. 100Ah lead-acid gives you 50Ah usable. To replace a 200Ah lead-acid bank with lithium, you only need about 125Ah of lithium for the same usable capacity. **Forgetting the inverter and watermaker.** The big draws are easy to forget. A watermaker can pull 30A while running, and even an hour a day adds up to 360 Wh. Inverter standby pulls 10–30W constantly even with no AC load. **Not accounting for charge time.** A bigger bank takes longer to recharge. If you have 2,000 Wh of solar but a 10,000 Wh bank, you'll never fully recharge on a sunny day. **Going lead-acid because lithium "is too expensive."** Run the math over 10 years. Lithium has been cheaper in total cost of ownership for at least 5 years now. ## Where to Buy Cells, BMS, and complete LiFePO4 systems for marine and RV builds: → [Battery Chargers and Monitoring on Dupree Products](https://dupreeproducts.com/collections/electrical?utm%5Fsource=goodkit&utm%5Fmedium=referral&utm%5Fcampaign=marine-battery-bank-sizing-calculator&utm%5Fcontent=article-body) ## Cross-References - [Best Marine Battery Monitor: Victron SmartShunt 500A Review](https://www.goodkit.io/best-marine-battery-monitor-victron-smartshunt-500a-review/) — measure your actual consumption before sizing - [Marine Wire Gauge Calculator](https://www.goodkit.io/marine-wire-gauge-calculator-abyc-e-11/) — size the cables to your bank - [Marine Fuse Sizing Calculator](https://www.goodkit.io/marine-fuse-sizing-calculator-abyc-e-11/) — required within 7 inches of the battery - [RV Battery Monitoring: What Actually Works](https://www.goodkit.io/rv-battery-monitoring-what-actually-works/) --- Spotted something off? Email braden@goodkit.io. We use this on our boat. Once you have a bank-size estimate - [Compare battery monitoring](https://www.goodkit.io/rv-battery-monitoring-what-actually-works/) — check what a monitor measures and what it estimates - [SmartShunt 500A](https://dupreeproducts.com/products/victron-smartshunt-500amp-50mv-bluetooth-smart-battery-shunt-shu050150050?utm%5Fsource=goodkit&utm%5Fmedium=referral&utm%5Fcampaign=marine-battery-bank-sizing-calculator&utm%5Fcontent=next-step) — a monitoring option, not a battery-size recommendation - [Plan the monitor installation](https://www.goodkit.io/how-to-wire-a-victron-smartshunt-step-by-step-install-guide/) — connections, cable and optional GX integration