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Free tool · Sailboat sizing

What size battery bank and solar does your sailboat need?

Tick the loads on your sailboat, set your battery type and how many days you want to swing on the hook, and this tool sizes your house bank, solar array, charge controller, and inverter using accepted marine math. It is a planning estimate to get you in the right ballpark, not a wiring diagram and not a substitute for a marine electrician.

Read first: this is a free planning estimate, not an engineered design. It does not account for your specific wiring, fusing, battery placement, ventilation, or charging sources, all of which must follow ABYC standards and your equipment manuals. Real loads vary by make and model. Confirm the final system with a qualified marine electrician before you buy or install. Use it as a starting point only.

Your system

Your daily loads

Tick what you run and adjust the hours per day. Amps are typical 12V figures; check your own gear's draw where you can. On a 24V system these still apply to 12V devices; convert any native-24V gear separately.

Use Load Amps Hrs/day
cycles; ~45 Ah/day suits a well-insulated box, a hot or large box can reach 60 to 80 4.5
overnight at anchor 0.2
0.6
underway after dark 0.8
standby; receiving draws closer to 0.6 A 0.35
1
depth and wind, underway and on anchor watch 0.6
passage only; 4 A suits a tiller pilot in calm water, a below-deck drive in a seaway averages 6 to 10 A or more 4
intermittent 2.5
duty cycle 4
1.5
0.25
2.5

A planning estimate using accepted marine sizing math, not an engineered design. Have the final system sized and installed to ABYC standards by a qualified marine electrician.

How the marine sizing math works

The calculator above is just this five-step method, applied to the loads you pick. You can run it by hand with the same numbers.

Step 1: Add up your daily power use

List each 12V load, multiply its amps by the hours you run it per day, and total it to amp-hours per day. Size to your heaviest realistic day, which on a cruising sailboat is usually a passage day with the autopilot and instruments running, not a quiet night at anchor. If you make passages, switch the autopilot on and give it real hours.

Step 2: Size the house battery bank

Bank capacity = daily amp-hours x days of reserve, divided by the usable share of the battery. Lead-acid (flooded, AGM, or gel) is sized to about 50% usable; LiFePO4 lithium to about 80%. Two days of reserve with no charging is a common baseline. Add 20 to 30% on top for growth, ageing, and cold weather. Lead-acid loses about a quarter of its capacity near freezing and delivers less than its rating under heavy loads (the Peukert effect), so lean high in the cold or with a big inverter. LiFePO4 needs a battery-management system under ABYC E-13, and gel cells prefer the conservative end of the 50% figure.

Step 3: Size the solar array

Array watts = daily watt-hours divided by (peak sun-hours x 0.7). The 0.7 is the real-world marine derate for panel heat, salt film, wiring, and charge losses. It is lower than the 0.85 used for home roofs because shading from rigging, the boom, and a bimini hurts a boat array far more. Four sun-hours is a rough continental-US annual average, not a universal figure: a Pacific Northwest winter is closer to 2 and a Gulf Coast summer can top 6, so look up your own area (NREL's PVWatts data is free). On a sailboat with panels under the rig or a bimini, knock another half to one hour off the figure you pick, or use 0.65 in place of 0.7.

Step 4: Size the charge controller

Controller amps = array watts divided by battery voltage, times 1.25. The 1.25 is the standard safety factor that covers bright cloud-edge irradiance spikes. Choose an MPPT controller over PWM for the extra harvest in low and partial light, and check the controller's input voltage against the array's open-circuit voltage in cold weather.

Step 5: Size an inverter, if you run AC

Continuous watts = your largest simultaneous AC load times 1.25, then confirm the surge rating covers motor startup (a fridge, pump, or power tool can pull two to five times its running watts for a moment). Choose a pure sine wave inverter for sensitive electronics. Keep enough bank to deliver the current: about 200 Ah of 12V lithium, or 300 Ah of 12V lead-acid (which sags more under load), per 1,000 W of inverter. On a 24V system the amp draw, and the Ah you need, both halve.

Typical sailboat 12V load reference

Representative draws for common cruising-sailboat equipment, the same figures the tool starts from. Your own gear's amps vary by make and model, so check the label or manual where you can. The fridge is almost always the biggest single house load; the autopilot is the one that defines a passage.

Device Amps (12V) Typical hrs/day
Marine fridge or cooler 4.5 10
LED anchor light 0.2 10
LED cabin lights 0.6 4
LED navigation lights 0.8 3
VHF radio (standby) 0.35 12
Chartplotter or MFD 1 4
Sailing instruments (wind, depth, speed) 0.6 6
Autopilot 4 0
Fresh-water pump 2.5 0.5
Bilge pump 4 0.5
Phone and tablet charging 1.5 3
Cabin fans 0.25 8
Marine stereo 2.5 2

Once you have your numbers, here is the gear

Each piece of the house-bank stack has its own tested-picks guide. Size first with the tool, then shop with these.

How we are paid: some links above go to Amazon, and if you buy through them we earn a commission at no extra cost to you. It does not change what we recommend. For the whole picture, see our complete boat gear guide.

Common questions

How do I calculate the battery bank size for my sailboat?

Total your daily power use in amp-hours (each load's amps times the hours per day you use it), multiply by the days of reserve you want without charging (two is a common baseline), then divide by the usable share of the battery. Lead-acid is sized to about 50% usable and LiFePO4 lithium to about 80%, so the same daily use needs a much larger lead-acid bank. Most cruisers add 20 to 30% on top for ageing and cold. The calculator above does this math from the loads you select.

How much solar do I need to keep my sailboat's batteries charged?

Take your daily energy use in watt-hours and divide by your peak sun-hours times 0.7. The 0.7 is a marine real-world derate that accounts for panel heat, salt film, wiring loss, and partial shading from the rig, boom, and bimini, and it is deliberately lower than the figure used for home rooftop solar. As a rough feel, a sailboat using 100 amp-hours a day at 12V (about 1,200 watt-hours) needs roughly 350 to 480 watts of panel in average to weak sun.

Lead-acid or lithium: how much capacity can I actually use?

Usable capacity is the catch. Flooded, AGM, and gel lead-acid batteries should be sized to about 50% depth of discharge to protect their life, so a 200 amp-hour lead-acid bank gives you about 100 usable amp-hours. LiFePO4 lithium is commonly designed to about 80%, so a 100 amp-hour lithium battery gives roughly 80 usable. Some lithium vendors advertise 100% usable, but reputable system designers still leave a margin, which is why this tool uses 80%.

What size charge controller do I need for my solar?

Divide your total array watts by the battery voltage to get the charging current, then multiply by 1.25 as a safety factor for bright-edge irradiance spikes. A 400 watt array on a 12V bank works out to about 33 amps times 1.25, so roughly a 40 amp controller. Pick MPPT over PWM for the extra harvest, and confirm the controller's maximum input voltage covers your panels' open-circuit voltage on a cold, sunny day.

What size inverter do I need on a sailboat?

Size the inverter to your largest AC load running at once, plus about 25% headroom, then check its surge rating covers motor startup (a fridge, pump, or power tool can briefly pull two to five times its running watts). A boat running a laptop, a blender, and phone chargers is usually fine on 1,000 to 2,000 W; a watermaker or air conditioner pushes you higher. Just as important, the battery bank has to deliver the current: keep at least 200 Ah of 12V lithium, or about 300 Ah of 12V lead-acid, per 1,000 W of inverter, and choose a pure sine wave model for sensitive electronics.

Does this calculator replace a marine electrician?

No. It is a planning estimate to get you in the right ballpark and help you shop. The actual installation, wire gauge, fusing, battery placement, and ventilation must follow ABYC standards and your equipment's manuals, and a qualified marine electrician should size and sign off the final system. Treat the numbers here as a starting point for that conversation, not the last word.

Disclaimer

This calculator and page are provided for general informational and educational purposes only and give a planning estimate based on accepted marine sizing practice as of 2026. They are not engineering advice, not a system design, and not a substitute for a qualified marine electrician or ABYC-certified technician. Real component draws, charging sources, wiring, fusing, ventilation, and battery placement vary by boat and must follow ABYC standards and the manufacturers' instructions. Every battery bank needs properly sized overcurrent protection (a fuse or breaker) at the battery per ABYC E-11, and a LiFePO4 bank needs a battery-management system per ABYC E-13 plus alternator protection if it is engine-charged; this tool sizes capacity only and does none of that. We make no warranty, express or implied, as to accuracy, completeness, or fitness for any particular boat or installation. You alone are responsible for verifying and safely installing your electrical system, and Sorted Gear accepts no liability for any loss, injury, fire, or damage arising from reliance on this information. When in doubt, consult a qualified marine electrician. Last reviewed June 2026.

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