How to Size Marine Solar and LiFePO4 for Reliable Boat Power

Marine solar and LiFePO4 can run lights, electronics, refrigeration, and chargers quietly. The trick is sizing both the solar array and the battery bank to match your daily loads, shading, and cruising style. This piece gives a step-by-step method, practical formulas, and tables you can reuse on any boat.

Why this sizing method works

Solar on boats is modular and scalable. You can add panels and storage as space and budget allow. That flexibility is a core PV strength noted by Energy.gov. Global market data still shows PV delivering competitive electricity costs despite recent input price swings, according to World Energy Investment 2023. For batteries, using days of autonomy, allowable depth of discharge (DoD), and round-trip efficiency is a standard approach echoed in IRENA’s design guidance. Finally, matching variable solar with storage and smart controls reflects integration practices discussed in IEA’s grid integration work.

Step 1 — Build a realistic energy budget

List each load, its average current or power, and hours per day. Convert to daily Wh. Add 10–20% overhead for inverter and wiring losses if you use AC loads.

• Daily Wh = Power (W) × Hours
• If you only have current (A): Power ≈ A × 12 V for a 12 V DC load

Typical daily loads on a small cruiser

Two common scenarios:

• At anchor day: 550–650 Wh
• Coastal sailing day (autopilot + instruments): 1,000–1,300 Wh

PV orientation flexibility and DC/AC ratios matter for rooftops and land systems, and the core idea carries to boats that change heading all day. As the IEA’s Next-Generation Wind and Solar notes, non-ideal orientation can still be productive with modern controls. On boats, expect a derate for heading, shadows, and temperature.

Step 2 — Size the LiFePO4 battery bank

LiFePO4 offers high usable capacity, flat discharge voltage, and long cycle life. Use 80% usable DoD for conservative sizing unless your BMS and charge settings are dialed in. Round-trip efficiency is typically 95%.

• Battery Ah (12 V) = Daily Wh × Autonomy days ÷ (12 V × DoD × 0.95)

Example for a 600 Wh/day anchor profile with two days of autonomy and 80% DoD: 600 × 2 ÷ (12 × 0.8 × 0.95) ≈ 132 Ah. Choose a 12 V 150 Ah LiFePO4 bank.

This approach aligns with battery sizing practice using DoD, efficiency, and autonomy as cited by IRENA.

Charge profile targets for 12 V LiFePO4

• Bulk/absorption voltage: 14.0–14.4 V (typical target 14.2 V)
• Absorption time: short; end on current drop (C/20–C/50) or 10–30 min per 100 Ah if near full
• Float: 13.4–13.6 V or disabled on solar
• Low-temp charge inhibit: at or below 0°C; rely on BMS or external sensor

Use an MPPT with a lithium profile and a temperature sensor. A BMS is non-negotiable. Energy system best practice is to pair variable renewables with storage and controls, consistent with IEA integration guidance.

Step 3 — Size the marine solar array

Array watts depend on your daily Wh, sun hours, and real derates (angle, shadows, temperature, controller, wiring). Boats rarely hit nameplate output for long.

• Array W ≈ Daily Wh ÷ (Peak Sun Hours × System derate)

Practical derate on boats: 0.55–0.70. Use 0.65 unless you mount on a rigid arch with minimal shading. Typical peak sun hours on a clear day: mid-latitudes 4.0–5.0, subtropics 5.0–5.5, tropics 5.5–6.0. These ranges sit within PV yield expectations discussed by public datasets and are consistent with the modular, scalable nature of PV noted by Energy.gov.

PV pricing remains attractive in many regions, which makes adding margin on array watts sensible for cloudy strings of days, as shown in IEA’s investment review.

Controllers, wiring, and layout that actually hold voltage

MPPT vs PWM

MPPT harvests more energy, especially on cool, bright days and with partial shading. Use independent MPPTs for separate panels if shadows differ across the boom, backstay, or radar. That reduces mismatch losses.

Series vs parallel

• Parallel (with fuses per panel) limits shading loss to the shaded panel and keeps voltage safe at 12 V systems.
• Series needs fewer amps in wiring but suffers hard from any shadow and pushes higher voltages through deck glands.

Voltage drop and wire gauge

• Target ≤3% drop from panels to controller and ≤2% from controller to battery.
• Use tinned marine-grade cable. Example: two 200 W panels, Imp ≈ 10–11 A each, parallel run 20–22 A over a 10 m loop. AWG 8 keeps drop near 2–3%.
• Fuse each panel lead near the combiner. Add a breaker between controller and battery.

Smart controls and right-sized balance-of-system gear raise useful output and reduce stress on batteries, a theme echoed in IEA’s system-friendly PV discussion.

Worked example: 28-foot coastal cruiser

Energy budget

• At anchor: fridge 450 Wh, lights 60 Wh, pumps/USB 40 Wh → ≈ 550 Wh/day
• Sailing day: add instruments 144 Wh and autopilot 220 Wh → ≈ 950–1,100 Wh/day

Battery bank

Target two days at anchor: 550 × 2 ÷ (12 × 0.8 × 0.95) ≈ 120 Ah. Choose 12 V 150 Ah LiFePO4. If frequent long sails, a 200 Ah bank gives more cushion.

Solar array

• For 550 Wh/day with 5 PSH and 0.65 derate: ≈ 170 W → fit 200–250 W
• For 1,000 Wh/day sailing with 5 PSH: ≈ 308 W → fit 360–420 W

Layout: two 180–220 W panels on a stern arch, each with its own MPPT to isolate shading. Keep the controller near the battery. Route short, fat cables to limit drop.

Charge settings, health, and safety

• Program lithium profile: 14.0–14.4 V bulk/absorb, short absorb, 13.4–13.6 V float or disabled.
• Watch temperature. Block charge at 0°C and below unless the pack has a heater.
• Use a shunt monitor for accurate state of charge. Voltage-only indicators mislead on LiFePO4.
• Vent the compartment. LiFePO4 is stable, yet any battery bank and wiring can heat under fault.

Pairing PV with storage and flexible operation raises uptime, consistent with benchmarks on storage roles in future power systems presented by the IEA and the modular PV benefits noted by Energy.gov.

Quick sizing lookup

Add 20–30% margin if you cruise in cloudy seasons or plan to anchor in shade. If space is tight, consider adding alternator or shore charging as a hybrid plan. Policy tailwinds and maturing storage tech, as tracked by the IEA, support strong supply chains for these upgrades.

Frequently cited insights you can trust

• PV is modular and works across scales, from rooftops to small craft (Energy.gov).
• PV costs remain competitive across regions, aiding off-grid uses (IEA investment review).
• Battery sizing based on DoD, efficiency, and autonomy is industry-standard (IRENA technical note).
• System-friendly PV and right DC/AC ratios improve real output (IEA Next-Generation PV).
• Smart operation aligns variable generation with demand using storage (IEA integration report).

Pulling it together

Start with a clear daily Wh target. Size LiFePO4 Ah with honest autonomy and DoD. Size marine solar with realistic derates and site shading. Use MPPT, short cable runs, and proper fusing. Aim for a system that holds steady voltage across anchor days and sailing days without babysitting. This approach keeps your boat power system reliable and scalable.

Disclaimer: Technical information only. Not legal or investment advice.

FAQ

Can marine solar run a 12 V fridge all day?

Yes, with the right array watts. A typical small compressor fridge uses 360–600 Wh/day in warm weather. Plan 250–350 W of panels in summer mid-latitudes, more in shoulder seasons.

How big should a LiFePO4 bank be for weekend cruising?

For 500–700 Wh/day and two days of autonomy, a 12 V 120–160 Ah LiFePO4 bank works well. Use 80% DoD and 95% efficiency in your math.

Is series wiring better for boat panels?

Parallel often performs better with partial shading. Each panel keeps contributing. Use separate MPPTs or a dual-input controller if shadows differ.

Do I need to float charge LiFePO4?

Not necessarily. Many sailors set a modest float (13.4–13.6 V) or disable float. End absorption on current taper to avoid holding at high voltage.

Should I choose 12 V or 24 V for a small boat?

Under 600 W of PV and 200 Ah of battery, 12 V is common. At higher loads or longer cable runs, 24 V reduces current and voltage drop. Match system voltage to your loads and inverter.