What Size Battery Bank Do I Need for
My Boat?
What Size Battery Bank Do I Need for My Boat? How Big a Battery Bank Does a Cruising Boat Need? A cruising yacht’s house battery bank must support every domestic load between charging periods without reaching the battery system’s discharge limit. The calculation begins with daily energy use, not the capacity of an available battery compartment or the rating of the alternator. Refrigeration, communications, navigation equipment, pumps, lighting, water production and inverter loads must be converted into one daily energy figure before battery capacity can be selected. The topic covers sailboat house battery bank sizing, AGM battery bank sizing and lithium battery bank sizing, all issues to address.
Lithium iron phosphate and AGM banks cannot be sized by applying the same nominal amp-hour figure. They have different usable discharge ranges, voltage behaviour, charge acceptance, temperature limits and high-current performance. An installation must also account for poor solar conditions, battery ageing, conversion losses, engine-start isolation and the current limits of the battery-management system, fuses, cables and disconnects.
How Big a Battery Bank Does a
Cruising Boat Need? - Use Watt-Hours as the Main Unit
Amp-hours are meaningful only when the system voltage is known. A 200 Ah bank at 12 volts contains approximately half the nominal energy of a 200 Ah bank at 24 volts.
Convert each load into watt-hours:
- Power in watts × operating hours = watt-hours
- Where only current is known: Current in amps × system voltage × operating hours = watt-hours
A 12-volt refrigerator drawing 5 amps while operating for 10 hours consumes:
- 5 A × 12 V × 10 h = 600 Wh
At 24 volts, a 5-amp load operating for the same period consumes 1,200 Wh. Do not compare amp-hours across different system voltages without conversion. Use measured voltage and current where possible. Equipment labels usually state maximum or rated power rather than average daily consumption.
What Size Battery Bank Do I Need for My Boat? - Step One: List Every House Load
I have a template for listing values in The Marine Electrical and Electronics Bible. Begin with every item supplied by the domestic bank. Include continuous, intermittent and occasional loads. The inventory normally includes the following systems and equipment:
- Refrigeration and freezer
- Cabin and navigation lighting
- Freshwater and sanitation pumps
- Bilge pumps and alarms
- Navigation instruments includes MFD’s and so on
- Autopilot
- VHF, AIS and other communications
- Chartplotter and radar
- Mobile telephone and tablet charging
- Computers, routers and satellite terminals
- Fans, heating controls and diesel-heater pumps
- Watermaker
- Inverter standby load
- AC appliances supplied through the inverter
- Electric winches or furling equipment
- Entertainment equipment
Do not include an appliance at zero because it is used only occasionally. Convert irregular loads into a daily average or size the system for the day on which they operate. An 800 Wh watermaker cycle used every second day creates an average demand of 400 Wh per day. The bank and inverter must still carry the complete 800 Wh load on the production day.
What Size Battery Bank Do I Need for
My Boat? - Step Two: Measure
Operating Current and Run Time
I have a template for listing in The Marine Electrical and Electronics Bible. Begin with every item supplied by the domestic bank. Include continuous, intermittent and occasional loads Measure current while each load operates. Record its daily run time under the conditions expected during cruising.
Refrigeration cannot be calculated from compressor current alone. Compressor run time changes with ambient temperature, insulation, food loading, ventilation and thermostat setting. Measure its complete 24-hour consumption.
The autopilot also requires measurement under sail. Flat-water current draw does not represent operation in quartering seas or with excessive weather helm.
Measure communications equipment over its actual operating schedule. A device drawing 20 watts continuously consumes:
- 20 W × 24 h = 480 Wh per day
This can exceed the energy used by equipment with a higher power rating that operates briefly. Use a battery monitor with a correctly installed shunt to verify the complete daily total. All domestic loads and charging sources must pass through the shunt
What Size Battery Bank Do I Need for
My Boat? - Step Three: Build
the Daily Energy Budget
Add the watt-hours consumed by all loads during a representative 24-hour period. Create separate profiles for marina, anchorage, coastal passage and offshore passage because the loads differ. A marina profile may include computers, lighting and refrigeration but little navigation equipment. An anchorage profile adds anchor monitoring, water production, fans and tender-related charging. A passage profile adds autopilot, instruments, AIS, navigation lights, radar and communications.
Size the bank for the highest recurring profile, not the lowest average across incompatible operating conditions. Add 10–15 per cent for wiring, controller and conversion losses where the individual measurements do not already include them. Add a separate allowance for inverter conversion and standby load.
What Size Battery Bank Do I Need for
My Boat? - Step Four:
Separate Essential and Discretionary Loads
Essential loads include bilge monitoring, navigation, communications, required lighting, refrigeration where food or medication depends on it, and the pumps required to maintain the yacht. Discretionary loads include entertainment, unrestricted computer use, electric cooking, domestic water heating and non-essential appliances. The battery bank must maintain essential services after discretionary loads have been removed. A bank that supports normal comfort but reaches its protection limit immediately after solar failure has no useful contingency reserve. Define which circuits will be switched off when the bank reaches the reduced-consumption threshold. Do not include engine starting in the house-bank reserve where an isolated engine-start battery is installed.
Include every load that remains energized when no equipment appears to be operating. Battery monitors, LPG detectors, bilge-pump controls, alarm systems, stereo memory circuits, network equipment, inverter standby modes, solar-and wind controller electronics and permanently powered instrument circuits can draw current throughout the day. Measure the vessel’s baseline current at the battery monitor after switching off all normal loads, then multiply that current by 24 hours. A continuous draw of 0.5 A consumes 12 Ah per day, while 1 A consumes 24 Ah per day. Add this measured consumption to the daily load calculation before applying depth-of-discharge, reserve and autonomy factors; otherwise, the resulting battery bank will be undersized.
What Size Battery Bank Do I Need for
My Boat? - Step Five: Select
the Required Autonomy
Autonomy is the period the bank must supply loads without useful charging. It does not mean the yacht will deliberately operate without charging for that period. For marina-based use with reliable shore power, one day may be adequate. For coastal cruising with solar, use at least one complete overnight period plus allowance for reduced solar. For sustained anchoring or offshore use, two days of usable storage provides a workable planning basis. Higher-latitude, winter or cloud-dependent operation may require more.
A yacht using 2.4 kWh per day and requiring two days of autonomy needs:
- 2.4 kWh × 2 days = 4.8 kWh of usable energy
This is usable capacity before reserve and battery-specific discharge limits are applied. Do not use a larger bank as a substitute for insufficient charging. Increasing storage without increasing replacement energy lengthens the time before failure but also lengthens recovery.
What Size Battery Bank Do I Need for
My Boat? - Step Six: Add an
Operational Reserve
Retain energy for abnormal demand, forecast error, battery ageing and loss of one charging source. A reserve of 15–25 per cent of usable capacity provides a practical planning basis. This reserve is separate from the battery-management system’s emergency low-voltage disconnect.
For a 4.8 kWh usable requirement with a 20 per cent operating reserve:
- 4.8 kWh ÷ 0.80 = 6.0 kWh of required usable bank capacity
The bank then reaches its normal operating limit before the protection system disconnects it. Do not plan routine operation down to the BMS cutoff or inverter low-voltage shutdown. Those are protection limits, not daily operating targets.
What Size Battery Bank Do I Need for My Boat? - Step Seven: Convert Usable Energy into Nominal Lithium Capacity
Lithium iron phosphate batteries provide a larger usable proportion of their nominal capacity than AGM batteries. The exact operating range comes from the battery manufacturer and system design. For planning, a LiFePO₄ bank is commonly operated over approximately 80–90 per cent of nominal capacity while retaining margins above the BMS limits.
If 6.0 kWh of usable capacity is required and the planned usable fraction is 85 per cent:
- 6.0 kWh ÷ 0.85 = 7.06 kWh nominal
At a nominal 12.8 volts:
- 7,060 Wh ÷ 12.8 V = approximately 552 Ah
The practical selection becomes a bank around 560–600 Ah at 12 volts, depending on available battery modules and manufacturer requirements.
At 25.6 volts, the same energy requires approximately:
- 7,060 Wh ÷ 25.6 V = approximately 276 Ah
The 24-volt bank carries the same energy at half the amp-hour figure.
What Size Battery Bank Do I Need for
My Boat? - Step Eight:
Convert Usable Energy into Nominal AGM Capacity
AGM batteries require a lower routine depth of discharge if service life is to be preserved. Voltage sag and the Peukert effect also reduce available capacity under high current. For planning, use approximately 50 per cent of nominal capacity as the routine usable fraction unless the battery manufacturer specifies another operating basis.
If 6.0 kWh of usable capacity is required:
- 6.0 kWh ÷ 0.50 = 12.0 kWh nominal
At a nominal 12 volts:
- 12,000 Wh ÷ 12 V = approximately 1,000 Ah
This produces an AGM bank around 1,000 Ah at 12 volts for the same planned usable energy supplied by approximately 560–600 Ah of LiFePO₄ capacity. The AGM bank will also weigh more, occupy more volume and accept charge more slowly near full state of charge.
What Size Battery Bank Do I Need for
My Boat? - Step Nine: Account
for AGM’s Peukert Effect
AGM capacity is normally stated at a defined discharge rate. High-current loads reduce the amount of energy that can be recovered before voltage reaches the equipment cutoff. An inverter, windlass or electric cooker can therefore make the effective AGM capacity lower than the arithmetic value. Voltage sag may also cause an inverter to disconnect while significant chemical capacity remains in the bank.
Use the battery manufacturer’s discharge curves for the expected current. Do not size an AGM bank solely from the published 20-hour amp-hour rating where the installation includes sustained inverter loads. Large parallel AGM banks can supply high current, but every battery requires equal cable resistance and a balanced connection arrangement. Unequal current sharing causes some batteries to cycle harder and fail first.
What Size Battery Bank Do I Need for
My Boat? - Step Ten: Check
Lithium BMS Current Limits
A lithium bank can contain enough energy for an appliance while its BMS cannot supply the required current. Check continuous discharge current, peak current, peak duration and low-temperature restrictions for each battery and for the complete parallel bank. A 2,000-watt inverter on a 12-volt system can draw more than 180 amps after inverter losses and voltage variation are included. Starting surges can be higher.
The BMS, main fuse, isolator, busbar and cable must all carry the expected continuous and surge current. The rating of the battery cells alone does not establish the usable system current. Where several lithium batteries are connected in parallel, confirm that the manufacturer permits the arrangement and states how current limits combine. One battery disconnecting under load can transfer the complete current to the remaining batteries and trigger a bank-wide cascade.
What Size Battery Bank Do I Need for
My Boat? - Step Eleven: Check
the Largest Simultaneous Load
Add the loads that can operate at the same time. A watermaker, inverter appliance, refrigerator, pumps and electronics may overlap. For example:
- Inverter appliance: 1,500 W
- Watermaker: 400 W
- Refrigeration and pumps: 150 W
- Electronics and communications: 150 W
The simultaneous load is 2,200 W. At 12 volts and 90 per cent inverter efficiency where applicable, battery current can exceed 190 amps. The cable and protection system must be sized for current, voltage drop, insulation temperature, routing and applicable installation requirements. Increasing battery capacity does not correct undersized cables.
What Size Battery Bank Do I Need for
My Boat? - Step Twelve: Match
the Charging System to the Bank
A battery bank is not correctly sized until the charging system can restore its daily use. If the yacht consumes 2.4 kWh per day, charging sources must return more than 2.4 kWh because conversion and battery losses occur. The required margin depends on battery chemistry and charging method.
Lithium batteries accept high charge current through much of the cycle. This can overload an alternator designed for intermittent lead-acid charging. Alternator temperature regulation, current limitation and belt capacity require assessment.
AGM batteries accept less current as they approach full charge. Solar or engine charging may replace the bulk portion quickly but require additional time to complete absorption. Repeatedly ending the charge early leaves the bank below full charge and promotes capacity loss.
Size solar generation from daily watt-hour demand:
- Daily energy ÷ effective sun hours ÷ system efficiency = required solar rating
For 2.4 kWh per day, 4.5 effective sun hours and 70 per cent system efficiency:
- 2,400 ÷ 4.5 ÷ 0.70 = approximately 760 W
A practical installation requires approximately 800 watts of effective solar capacity before adding recovery margin.
What Size Battery Bank Do I Need for
My Boat? - Step Thirteen:
Plan Recovery After a Low-Generation Day
Daily generation equal to daily consumption maintains the current state of charge but does not recover a deficit.
If the yacht uses 2.4 kWh during one cloudy day and solar provides only 0.8 kWh, the deficit is 1.6 kWh. The following day must supply normal use plus that deficit.
If the array produces 2.8 kWh on the next day while the yacht again uses 2.4 kWh, only 0.4 kWh is available for recovery. Restoring the deficit takes four days.
The charging system requires surplus capacity or a separate charging source. Engine alternators, generators and shore chargers form contingency charging, but their fuel use and operating limitations belong in the cruising plan.
What Size Battery Bank Do I Need for
My Boat? - Step Fourteen:
Account for Battery Age and Temperature
Battery capacity falls with age, use and operating conditions. A bank sized without margin may meet demand when new and fail the same profile later.
AGM performance falls at low temperature, and chronic partial-state-of-charge operation causes capacity loss. Elevated temperature accelerates ageing.
LiFePO₄ batteries retain useful discharge performance at low temperature but normally cannot accept unrestricted charging below the manufacturer’s minimum cell temperature. The BMS must block or control charging when cells are too cold.
Install battery temperature sensing where required by the battery, charger or regulator. Compartment temperature also affects ventilation, cable loading and equipment life.
What Size Battery Bank Do I Need for
My Boat? - Step Fifteen:
Check Weight and Installation Space
A large AGM bank can impose substantial weight in one part of the yacht. Include batteries, boxes, restraint systems, cables and busbars in the installation weight. The support structure must carry the bank under vessel motion. Batteries require restraint in all directions and protection against conductive objects.
Lithium reduces weight but requires a compatible electrical system. Do not treat lower battery weight as permission to install unsecured modules or unsupported busbars. The installation must preserve access to terminals, fuses, isolation switches and monitoring equipment. A bank that cannot be inspected or isolated creates a maintenance and emergency-access defect.
What Size Battery Bank Do I Need for
My Boat? - Real-World
Profile: Basic Coastal Cruising
This profile covers a yacht using one refrigerator, LED lighting, freshwater pumps, mobile devices, limited instruments and no regular inverter appliances. Typical daily use is as follows but verify your own:
- Refrigeration: 600–900 Wh
- Lighting and pumps: 150–250 Wh
- Instruments and communications: 200–350 Wh
- Personal-device charging: 100–250 Wh
- Loss allowance: 150–250 Wh
Total daily consumption is approximately 1.2–2.0 kWh.
With two days of autonomy and a 20 per cent operational reserve, the required usable storage is approximately 3.0–5.0 kWh.
A practical LiFePO₄ bank is approximately:
- 300–450 Ah at 12.8 volts
A practical AGM bank is approximately:
- 500–850 Ah at 12 volts
Solar generation falls around:
- 400–700 watts, depending on location, season and shading
The upper part of each range applies where refrigeration demand is high or the yacht remains at anchor for sustained periods.
What Size Battery Bank Do I Need for
My Boat? - Real-World
Profile: Full-Time Liveaboard Cruising
This profile adds computers, internet equipment, fans, a watermaker and regular inverter use for small appliances. Typical daily use is:
- Refrigeration or fridge/freezer: 800–1,300 Wh
- Lighting and pumps: 200–350 Wh
- Instruments and anchor monitoring: 150–300 Wh
- Computers and communications: 500–1,200 Wh
- Watermaker averaged across production days: 300–800 Wh
- Inverter loads and conversion losses: 200–600 Wh
Total daily consumption is approximately 2.2–4.0 kWh.
With two days of autonomy and a 20 per cent reserve, the usable requirement is approximately 5.5–10.0 kWh.
A practical LiFePO₄ bank is approximately:
- 500–900 Ah at 12.8 volts
A practical AGM bank is approximately:
- 900–1,700 Ah at 12 volts
Solar generation falls around:
- 700–1,300 watts
The upper AGM capacity creates substantial weight and charging demand. At this energy level, a 24-volt house system can reduce current and cable size for inverters, watermakers and other large loads.
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What Size Battery Bank Do I Need for
My Boat? - Real-World
Profile: Offshore Passage Making
An offshore profile removes some domestic loads but adds continuous navigation, autopilot, AIS, communications and night lighting. Typical daily power usage is as follows:
- Refrigeration: 600–1,100 Wh
- Autopilot: 500–1,800 Wh
- Navigation instruments and AIS: 300–700 Wh
- Communications: 150–500 Wh
- Navigation and cabin lighting: 150–300 Wh
- Pumps and other loads: 200–400 Wh
Total daily consumption is approximately 1.9–4.8 kWh.
Autopilot demand produces the widest variation. A balanced yacht in moderate conditions uses less energy than a yacht with weather helm, stiff steering or quartering seas. Bank capacity alone cannot guarantee passage endurance. Solar output can fall through cloud, shading and sail position. The engine alternator, hydrogenerator, wind generator or other charging source must cover periods of reduced solar generation. Size the bank for the passage load and retain engine-start capacity outside it.
What Size Battery Bank Do I Need for
My Boat? - Real-World
Profile: High-Load Electric Galley
An electric galley changes both daily energy and peak-current requirements. A 1,500-watt induction unit operating for one hour uses 1.5 kWh before inverter losses. An electric kettle, coffee machine, microwave or water heater adds further demand.
A high-load liveaboard profile can exceed 5–8 kWh per day. Two days of autonomy then requires a substantial lithium bank and charging system. An equivalent AGM bank becomes large in weight, volume and recharge time.
At this level, use a 24-volt or 48-volt architecture where the complete vessel design supports it. Do not add high-load appliances to a 12-volt system without reviewing the inverter, battery BMS, cable routes, fuses, switches, alternator and solar capacity.
Include every load that remains energised when no equipment appears to be operating. Battery monitors, LPG detectors, bilge-pump controls, alarm systems, stereo memory circuits, network equipment, inverter standby modes, solar-controller electronics and permanently powered instrument circuits can draw current throughout the day. Measure the vessel’s baseline current at the battery monitor after switching off all normal loads, then multiply that current by 24 hours. A continuous draw of 0.5 A consumes 12 Ah per day, while 1 A consumes 24 Ah per day. Add this measured consumption to the daily load calculation before applying depth-of-discharge, reserve and autonomy factors; otherwise, the resulting battery bank will be undersized.
What Size Battery Bank Do I Need for
My Boat? - Lithium-Specific
Installation Requirements
LiFePO₄ installation requires more than replacing AGM batteries with lithium units. Confirm the following:
- Alternator current control and temperature protection
- Compatible shore charger and solar-controller profiles
- Low-temperature charging protection
- BMS continuous and surge-current limits
- Main fuse capacity and interrupt rating
- Cable and busbar current ratings
- Battery isolation and emergency disconnection
- Monitoring based on current measurement
- Manufacturer limits for series or parallel connection
A BMS disconnect under high alternator output can expose charging equipment to a sudden load change. The installation requires a control arrangement that does not depend on the BMS repeatedly opening the main battery circuit during normal operation.
What Size Battery Bank Do I Need for
My Boat? - AGM-Specific
Installation Requirements
AGM banks require balanced current paths, complete charging and ventilation for heat and abnormal gas release. Confirm the following and record the following:
- Correct absorption voltage and duration
- Temperature-compensated charging
- Full recharge at required intervals
- Equal cable resistance across parallel batteries
- Protection against chronic partial-state-of-charge operation
- Ventilation required by the battery manufacturer
- Access for individual battery testing
- Secure restraint and protected terminals
Do not combine batteries of different ages, capacities or models within one AGM bank unless the manufacturer permits the arrangement. A weak battery can reduce the performance of the complete parallel bank.
What Size Battery Bank Do I Need for
My Boat? - Do Not Size from
Solar Output Alone
Solar production is variable and cannot define battery capacity by itself. The bank must carry overnight use and provide time to respond to poor generation. A large solar array with a small bank may reach full charge early and waste available production while leaving little overnight reserve. A large bank with a small array stores more energy but remains undercharged after use. Size loads first, usable storage second and charging capacity third. Then test whether the three parts recover together under the expected operating conditions. Most boats I attend have done no calculations at all.
What Size Battery Bank Do I Need for
My Boat? - Do Not Combine
House and Engine Starting Without Control
The engine-start battery remains a separate reserve for propulsion and charging recovery. Routine domestic loads must not discharge it. A parallel switch or emergency combiner can provide controlled backup, but its normal position and circuit protection must prevent unnoticed domestic discharge. If the house bank BMS disconnects, the alternator arrangement must retain an acceptable electrical path or be controlled to stop charging safely. This requires system design rather than dependence on manual reaction.
What Size Battery Bank Do I Need for
My Boat? - Final Sizing
Method
Use the following calculation sequence:
- Measure every load in watt-hours per day.
- Create separate marina, anchorage and passage profiles.
- Select the highest recurring daily profile.
- Add conversion and system losses.
- Multiply by the required autonomy period.
- Add a 15–25 per cent operational reserve.
- Divide by the planned usable fraction for the battery chemistry.
- Convert nominal watt-hours into amp-hours at the system voltage.
- Check BMS, inverter and cable current against simultaneous loads.
- Size charging sources to replace daily use and recover a deficit.
- Confirm weight, restraint, ventilation and service access.
- Preserve engine-start capacity outside the house-bank calculation.
For LiFePO₄, use the manufacturer’s permitted discharge range and BMS limits. For AGM, account for routine depth of discharge, voltage sag, charge acceptance and the Peukert effect.
What Size Battery Bank Do I Need for
My Boat? - Final Assessment
House-bank sizing begins with measured daily energy use. A light coastal yacht may use 1.2–2.0 kWh per day, a full-time liveaboard cruiser 2.2–4.0 kWh, and an electrically intensive yacht more than 5 kWh. LiFePO₄ provides more usable energy from a given nominal capacity and accepts charging at a higher rate, but it requires BMS coordination, alternator protection and temperature control. AGM requires greater nominal capacity for the same usable energy and longer charging time, but it can remain suitable where the charging system, weight allowance and operating profile support it. The correct bank is not the largest bank that fits the compartment. It is the bank that carries the measured load through the selected autonomy period, retains an operating reserve and can be fully recovered by the yacht’s charging system. What Size Battery Bank Do I Need for My Boat? All you need to know.