Solar Boat Lift Chargers: How to Size the Panel, Battery, and Controller

A solar charger on a boat lift is a small, off-grid power system: a panel collects energy during the day, a charge controller feeds it safely into a battery, and the lift draws from the battery whenever you press the button. Most failures are not panel failures. They are budget failures: the system was sized for a sunny July afternoon, then asked to work through a cloudy week in October with a canopy shading the panel.

The sizing method in four steps

Solar sizing is a chain of simple conversions. You can do it on paper, or use the solar charger sizing tool, which follows the same steps.

1. Daily load (Ah): amp-hours per round trip times cycles per day, plus standby draw from a wireless receiver or controller.
2. Daily energy (Wh): daily Ah times nominal battery voltage (use about 12.5V for a 12V lead-acid system).
3. Panel watts: daily Wh divided by (peak sun hours x system derate). A derate of 0.6 to 0.7 covers controller losses, battery charging inefficiency, heat, dust, and less-than-ideal panel angle.
4. Battery Ah: enough to cover the daily load for several sunless days without going below about 50% state of charge (lead-acid) or about 20% (lithium iron phosphate).

Step 1: amp-hours per lift cycle

Lift energy follows from power = force x velocity. A lift moving a 4,500 lb load at 3 ft/min needs about 0.41 hp (305 W) of lifting power. With a drivetrain efficiency near 0.5 and a DC motor efficiency near 0.7, the battery supplies roughly 870 W, or about 73 A at 12V. The full derivation is in AC vs DC boat lift motors. Typical round-trip figures, including lowering and a margin for cold and battery aging:

Approximate DC energy per round trip (12V system, typical gearing and travel)

Lift and load	Typical current while lifting	Approx. Ah per round trip
Single PWC lift, about 1,000 to 1,300 lb moving load	25 to 40 A	0.5 to 1
Small boat or light pontoon, about 2,500 lb	40 to 60 A	1.5 to 2.5
Pontoon or tritoon lift, about 4,500 lb	60 to 80 A	3 to 4
Same 4,500 lb lift at 24V	30 to 40 A	1.5 to 2 (at 24V)

These are estimates under stated assumptions. A dry gearbox, a binding cradle, or a corroded terminal can raise them substantially. If you can, measure: a DC clamp meter on the motor cable during a lift, multiplied by run time in hours, gives a real number.

Step 2 and 3: sun hours and panel watts

"Peak sun hours" is the day's solar energy expressed as hours of full 1,000 W per square meter sunlight. A panel rated at 50 W produces close to 50 W only under those conditions. Typical ranges for a reasonably tilted panel in the continental US:

• Summer: roughly 5 to 6.5 peak sun hours in much of the country, including the northern lake states.
• Spring and fall: roughly 3 to 5, with the north dropping faster in October.
• Winter: roughly 1.5 to 3 in northern states, 3.5 to 4.5 in Florida and the Gulf Coast.

For site-specific values, NREL's solar resource data and PVWatts calculator give monthly figures by location. Always size for the worst month in which you will actually use the lift.

Step 4: choosing the battery

Battery chemistries for solar boat lifts

Type	Usable capacity	Strengths	Cautions
Flooded deep-cycle lead-acid	About 50% of rating	Lowest cost, tolerant of crude charging	Needs water checks, vents hydrogen, can freeze when discharged
AGM sealed lead-acid	About 50%	No watering, spill-resistant, handles vibration	Sensitive to overcharge; set controller to AGM profile
Lithium iron phosphate (LiFePO4)	About 80 to 90%	Light, long cycle life, holds voltage under load	Higher cost; most must not be charged below about 32°F unless they have built-in heating; needs a compatible charge profile

Do not use a car starting battery. It is designed for short bursts and degrades quickly when deep-cycled.

Charge controllers: PWM vs MPPT

Never connect a panel directly to a battery except for very small maintenance panels designed for it. A controller prevents overcharging, sets the right voltages for the chemistry, and stops the battery from discharging back through the panel at night.

• PWM controllers connect the panel to the battery in rapid pulses. They are inexpensive and fine for small 12V panels (under roughly 100 W) whose voltage closely matches the battery.
• MPPT controllers convert excess panel voltage into extra current. They typically harvest 10 to 30% more energy, with the biggest gains in cold weather and with higher-voltage panels. Worth it on larger or 24V systems.
• Settings matter: choose the profile for your battery type. Lead-acid absorption is typically around 14.4 to 14.8 V with float around 13.5 to 13.8 V at room temperature; follow the battery maker's numbers. Temperature compensation helps lead-acid batteries that see wide seasonal swings.

Mounting the panel

• Orientation: face the panel toward true south (in the US) where possible. A tilt near your latitude is a good year-round compromise; flatter favors summer, steeper favors spring and fall. A flat panel can lose a meaningful share of output and collects dirt and water.
• Shade: a canopy edge, a tree, or a piling shadow across even part of a panel can cut output dramatically, because shaded cells restrict current through the whole string. Check shade in the morning, midday, and late afternoon. Lift canopies are the most common culprit.
• Birds: droppings on the panel act like shade. A clean panel is a working panel; see boat lift cleaning.
• Wiring: use tinned marine wire, sealed connectors, and a fuse or breaker close to the battery positive terminal. ABYC E-11 calls for overcurrent protection near the battery, and it is cheap insurance against a dock fire.
• Theft and storms: use tamper-resistant fasteners and remove the panel ahead of hurricanes; see hurricane preparation.

Failure modes and their causes

Common solar lift problems

Symptom	Likely cause	Fix
Lift slows, then stops partway up	Battery deeply discharged or sulfated; voltage sags under load	Full charge with a shore charger, load test, replace if it fails
Fine in summer, weak in fall	Panel sized for summer sun; recovery too slow	Larger panel, better tilt, or occasional shore charging
Battery always low despite sun	Shading, dirty panel, failed controller, blown fuse, corroded connector	Check panel open-circuit voltage and controller charging indicator in sun
Battery boiling dry or swollen	Wrong controller setting or failed controller overcharging	Correct profile, replace controller and damaged battery
Dead battery every spring	Left discharged over winter or frozen	Charge fully and store above freezing or on a maintainer

For step-by-step diagnosis with a multimeter, see solar and battery problems.

Regional notes

Northern lakes and ice country: the season ends as sun hours fall. Take the battery off the lift, charge it fully, and store it cool but above freezing. A fully charged lead-acid battery resists freezing far better than a discharged one. See winterizing a boat lift.

Florida, Gulf, and Southwest: plenty of sun, but heat accelerates battery aging. Shade the battery box (not the panel), keep it ventilated, and expect shorter battery life than in cool climates.

Saltwater sites: salt fog corrodes terminals and controller connections quickly. Use sealed enclosures with drainage, dielectric grease, and stainless fasteners, and inspect connections every few months.

Cost

Typical 2026 ranges (USD, vary by region and supplier, not quotes): a 20 to 100 W panel kit with controller and mount about 100 to 450; a deep-cycle lead-acid battery about 120 to 300; a 100 Ah LiFePO4 battery about 250 to 800. Manufacturer-specific solar kits for lifts often cost more than generic parts but include mounts and wiring designed for the lift.

Frequently asked questions

What size solar panel do I need for a boat lift?

Divide your daily energy use in watt-hours by peak sun hours for your worst month, then by a derate of about 0.65. A PWC lift used once or twice a day may need only 10 to 30 W. A 12V pontoon lift cycled twice a day typically needs 40 to 100 W, with the larger end giving faster recovery after cloudy weather.

How long does a boat lift battery last?

A deep-cycle lead-acid battery on a solar lift typically lasts about 3 to 5 years, less if it sits partially discharged, overheats, or freezes. Lithium iron phosphate batteries can last considerably longer in cycle terms but cost more and need cold-weather charging protection. Keeping the battery near full charge is the biggest factor.

Do I need a charge controller for a solar boat lift?

Yes, for anything other than a tiny maintenance panel designed to connect directly. The controller prevents overcharging, applies the correct voltages for your battery chemistry, and stops reverse current at night. PWM controllers suit small 12V panels; MPPT controllers recover more energy on larger or 24V systems and in cold weather.

Will a solar boat lift work in winter?

In northern states, winter sun hours often fall to about 1.5 to 3 per day, and cold reduces battery capacity, so most systems cannot keep up with regular use. If the lift stays in service, add panel capacity or charge periodically from shore power. If not, remove the battery, charge it fully, and store it above freezing.

Why does my solar boat lift battery keep dying?

The usual causes are a panel too small for your use, shade from a canopy or tree, a dirty panel, a failed controller or blown fuse, corroded connections, or an old sulfated battery. Check panel voltage in full sun, confirm the controller shows charging, and load test the battery after a full charge.

Sources and further reading

• National Renewable Energy Laboratory (NREL), solar resource data and the PVWatts calculator, for monthly peak sun hours by location.
• ABYC E-11, AC and DC Electrical Systems on Boats (DC overcurrent protection and conductor practice). https://www.abycinc.org/
• Battery manufacturer technical manuals for flooded, AGM, and lithium iron phosphate deep-cycle batteries (charge voltages, depth of discharge, temperature limits).
• Charge controller manufacturer manuals (PWM and MPPT setup and battery profiles).
• Boat lift manufacturer owner's manuals for DC drive and solar kits.