Boat Battery Runtime Guide
Runtime on a boat is a critical engineering constraint. Unlike grid-tied systems, a marine battery bank must carry all loads unaided for hours — from overnight anchoring to extended trolling sessions. This guide covers the method for estimating runtime across common marine load categories and explains the factors that reduce real-world performance below rated specifications.
Understanding Marine Runtime
Marine runtime depends on three variables: total available energy (Wh), average load (W), and system efficiency. The fundamental calculation is straightforward — divide usable energy by load power — but marine applications introduce complications. Vibration, temperature extremes, salt air corrosion, and intermittent high-current loads (pumps, windlass) all degrade usable capacity below the manufacturer's rated specification.
The most common mistake is using nameplate Ah rating directly. Lead-acid batteries lose 20–40% of usable capacity at higher discharge rates due to the Peukert effect. Lithium batteries perform better but still lose capacity at low temperatures and when aged. Always derate your calculations by 15–25% for marine conditions.
Runtime Formula
Usable Energy = Battery Capacity (Ah) × System Voltage (V) × DoD × Efficiency Factor
This accounts for Peukert losses (lead-acid), temperature derating, and parasitic draws not captured in your load inventory.
Common Marine Loads and Runtime Estimates
The following table provides typical power consumption for common marine electrical loads. Values assume LED lighting and modern electronics. Older incandescent fixtures or analog instruments will draw significantly more power.
| Load | Typical Power (W) | Runtime on 100Ah 12V |
|---|---|---|
| Trolling motor (medium speed) | 150–300 W | 3–6 h |
| Fish finder / chartplotter | 15–30 W | 25–50 h |
| Navigation lights (LED) | 5–15 W | 50–150 h |
| Cabin lights (LED) | 20–40 W | 20–40 h |
| Refrigerator | 40–80 W | 10–20 h |
| VHF radio | 5–25 W | 30–150 h |
| Water pump | 5–10 W | 70–140 h |
| Anchor light (LED) | 5–10 W | 70–140 h |
| Bilge pump | 20–40 W | 20–40 h |
Runtime estimates assume a 100Ah 12V battery at 80% DoD with 85% system efficiency. Actual runtime will vary based on load duty cycles and battery age.
Load Profile Method
A single load does not tell the full story. Boats run multiple loads simultaneously — a fish finder and trolling motor during the day, navigation lights and cabin lighting at night. To estimate real-world runtime, build a load profile for each operating mode and sum the watt-hours consumed during each period.
| Mode | Active Loads | Combined Draw |
|---|---|---|
| Day fishing | Trolling motor + fish finder + nav lights | 180–350 W |
| Overnight at anchor | Anchor light + cabin lights + fridge + VHF | 80–160 W |
| Cruising | Electronics + instruments + pumps | 100–250 W |
Worked Example: Overnight Anchor Runtime
Scenario: 12V 200Ah AGM battery bank, anchored overnight with the following loads:
| Load | Power | Hours | Wh |
|---|---|---|---|
| Anchor light | 8 W | 12 h | 96 Wh |
| Cabin lights | 30 W | 4 h | 120 Wh |
| Refrigerator | 50 W | 12 h | 600 Wh |
| VHF radio | 8 W | 2 h | 16 Wh |
Step 1: Total overnight energy:
Step 2: Apply marine derating (0.85):
Step 3: Convert to Ah at 12V with 50% DoD (AGM):
Step 4: A 200Ah AGM bank provides 100 Ah usable at 50% DoD — sufficient with margin. A 300Ah bank would be recommended if daytime trolling loads are also expected.
Factors That Reduce Runtime
Temperature
Battery capacity drops 10–20% in cold conditions (below 10°C / 50°F). Cold anchors in early morning can significantly reduce overnight runtime compared to warm-weather calculations.
Battery Age
Lead-acid batteries lose 20–30% capacity after 300–500 cycles. A 2-year-old AGM bank may deliver only 70–80% of its rated capacity. Factor in expected aging when sizing for multi-season use.
Peukert Effect
Lead-acid batteries lose usable capacity at higher discharge rates. A 100Ah battery discharging at 10A (1C) may only deliver 60–70Ah. Discharging at 2A (0.02C) yields closer to the full 100Ah rating.
Parasitic Loads
Battery monitors, bilge pump controllers, clock circuits, and alarm systems draw small but continuous loads (5–20W total). Over 12–24 hours, these parasitic draws add 60–480 Wh to your consumption.
Try It
Use the Marine Battery Sizing Calculator to input your specific loads and get a bank recommendation.
Open Marine Battery Sizing CalculatorNext Step
Calculate how long your sized battery bank will run your specific load profile with the Runtime Calculator.
Open Runtime CalculatorRelated Articles
The complete method for sizing a marine house battery bank, including hotel loads, cruising vs anchored modes, and battery chemistry selection.
How to inventory and calculate marine hotel loads — the foundation of accurate runtime and bank sizing calculations.
Frequently Asked Questions
How long will a 100Ah marine battery last?
At a moderate 50W hotel load, a 100Ah 12V battery provides approximately 12 hours of runtime (1,200Wh ÷ 50W). Real-world runtime depends on load profile, battery chemistry, temperature, and age. Always derate by 15–20% for marine conditions.
What is the biggest marine battery drain?
Refrigeration is typically the largest single load, drawing 40–80W continuously. Trolling motors can spike to 1,000W+ under load but run intermittently. Navigation electronics, cabin lighting, and water pumps also contribute significantly to daily consumption.
Should I size my battery bank for 2 days or 3 days?
For weekend fishing trips, 1–2 days of autonomy is typical. For cruising yachts or liveaboards, 2–3 days provides a safety margin for weather delays or extended anchoring. A 2-day autonomy at 50% DoD gives a practical buffer without oversized banks.
Can I use car batteries on my boat?
No. Car (starter) batteries are designed for short high-current bursts, not sustained deep cycling. Using them for marine house loads will cause rapid capacity loss and premature failure. Always use deep-cycle marine batteries rated for the discharge profile.