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Tools & Calculators

Boat Lift Solar Charger and Battery Sizing

By Boat Lift Lab Updated 4 min read

Quick answer

Multiply motor current by lift time to get amp-hours per trip, average it over a week, then size the battery for several sunless days at a safe depth of discharge and the panel to replace a day's use in your local peak sun hours. A typical 12 V lake lift used a few times a week runs well on a 20 to 50 W panel and a 75 to 100 Ah battery.

On this page
  1. The arithmetic behind the result
  2. Why lifts slow down before the battery is empty
  3. Panel placement matters as much as panel size
  4. Frequently asked questions
  5. Sources and further reading

Solar charger and battery sizing

Lift motor
A
From the motor label or manual. Measure with a clamp meter if unsure.
min
min
Lowering usually draws much less current.
Use and site

Results appear here as you type.

Sizing assumes a charge controller and about 25% loss between panel nameplate and stored energy (heat, angle, dirt, charging losses), plus a small standby draw for a remote receiver.

The arithmetic behind the result

A DC lift's energy use is simple to estimate because it only draws real power while it moves. The calculator works in amp-hours (Ah) at the system voltage:

  1. Per trip: current while lifting x minutes up / 60, plus about 40% of that current x minutes down / 60 (lowering draws less because gravity does much of the work).
  2. Per day: trips per week / 7, plus a standby draw of about 0.03 A around the clock (roughly 0.7 Ah a day) for a remote receiver or controller.
  3. Battery: the larger of two figures. Energy: the larger of (daily use x days of reserve) and three full trips, divided by the usable fraction of the battery (50% for lead-acid, 80% for lithium iron phosphate). Current: 2 Ah of lead-acid capacity, or 1 Ah of lithium capacity, per amp of motor current, which keeps voltage up under load.
  4. Panel: daily use x system voltage / (peak sun hours x 0.75). The 0.75 covers panel heat, angle, dirt, controller and battery charging losses.

Worked example: 12 V lake lift, weekend use

Motor draws 35 A lifting. Up takes 3 minutes, down 2.5 minutes. Six round trips a week, 4 peak sun hours, 3 days of reserve, AGM battery.

  • Per trip: 35 x 3/60 = 1.75 Ah up, plus 35 x 0.4 x 2.5/60 = 0.58 Ah down, total about 2.33 Ah.
  • Per day: 2.33 x 6/7 = 2.0 Ah, plus 0.72 Ah standby = 2.72 Ah.
  • Battery for energy: max(2.72 x 3, 2.33 x 3) = 8.2 Ah usable, / 0.5 = 16.3 Ah rated.
  • Battery for current: 35 A x 2 Ah per amp = 70 Ah rated, so the lead-acid battery is not drained at a punishing rate.
  • Battery chosen: the larger figure, 70 Ah, rounded up to a common 75 Ah deep-cycle size.
  • Panel: 2.72 x 12 / (4 x 0.75) = 10.9 W, so a 20 W panel.

Notice that current, not energy, sized the battery. Energy alone suggested a tiny battery, but a small lead-acid battery under a 35 A load sags in voltage and the lift slows near the top. Lithium iron phosphate batteries hold voltage far better under load, so the calculator applies 1 Ah per amp for them instead of 2.

Why lifts slow down before the battery is empty

A lead-acid battery's voltage drops under heavy current, and the drop is larger in a small, old, cold or partly discharged battery. Lift motors slow as voltage falls, and controllers may cut out at a low voltage threshold. So a battery that is "big enough" on amp-hours can still disappoint. Check a struggling system with a meter: a healthy 12 V lead-acid battery at rest reads about 12.6 to 12.8 V, and voltage under the lift's load should stay well above 11 V. The solar and battery troubleshooting guide walks through the tests.

Panel placement matters as much as panel size

  • Shade kills output. Partial shade on one cell of a small panel can cut output by much more than the shaded area suggests. Mount clear of canopy shadows and trees, especially in the morning and afternoon.
  • Tilt toward the sun. In the continental US, a panel facing south and tilted at roughly your latitude collects more over a season than one lying flat on a canopy.
  • Keep it clean. Bird droppings on a small panel are a common cause of "the solar stopped working". See boat lift cleaning.

Frequently asked questions

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

For a typical 12 V lift used a few times a week, a 20 to 50 W panel is usually enough when it gets 4 or more peak sun hours and no shade. Heavy daily use, 24 V systems, cloudy climates or shaded sites need more. Use the calculator with your motor's current and lift time.

How long will a boat lift battery last on one charge?

A charged 100 Ah lead-acid battery has about 50 Ah of usable energy. At around 2 to 4 Ah per round trip for a typical 12 V lift, that is roughly 12 to 25 trips, less in cold weather or with an aging battery. Lithium iron phosphate batteries deliver more of their rating.

Can I use a car battery for a boat lift?

A car starting battery is built for short bursts and wears out quickly when deeply discharged. Use a deep-cycle battery (flooded, AGM or lithium iron phosphate) and a charge controller that matches the battery chemistry.

Do I need a charge controller with a solar panel?

Yes, except for very small maintainer panels designed for direct connection. A controller prevents overcharging and, with lithium batteries, applies the correct charging profile. Many lift solar kits include one, so check before adding another.

Sources and further reading

  • NREL (National Renewable Energy Laboratory), solar resource data and peak sun hour maps for the United States.
  • ABYC E-11, AC and DC Electrical Systems on Boats, and battery manufacturers' deep-cycle charging specifications: abycinc.org.
  • Boat lift motor and solar kit owner's manuals for rated motor current and charge controller settings.