Powering Priscilla: Planning an All-Electric Narrowboat

An all-electric narrowboat needs considerably more than a list of appliances and a large battery bank.

Priscilla’s electrical system must support two very different demands:

Domestic energy for cooking, refrigeration, lighting, hot water and everyday equipment
Propulsion energy for moving the boat

Both ultimately draw from the same finite supply, but propulsion can consume far more energy than the domestic systems. The design must therefore consider energy consumption, peak power, charging capacity, seasonal generation and operational resilience together.

Power and energy are different

Two measurements are central to a power audit:

Watts (W) measure the power being used at a particular moment.
Watt-hours (Wh) or kilowatt-hours (kWh) measure the energy consumed over time.

A 2,000W induction hob operating for 15 minutes uses 500Wh:

2,000W × 0.25h = 500Wh

This distinction matters because the inverter, cabling and protective devices must cope with peak power, while the battery bank must provide sufficient stored energy. A system could have enough battery capacity for the day but still be unable to start an appliance because the inverter or cabling cannot safely deliver its peak demand.

Begin with an equipment schedule

Every planned electrical consumer should be recorded, including equipment that runs continuously, intermittently or only in emergencies.

Domestic services

LED lighting
Fridge and freezer
Freshwater and waste pumps
Toilet system
Ventilation fans
Heating controls and circulation pumps
Induction hob
Combination oven or microwave
Kettle
Washing machine
Dishwasher
Immersion heater
Television and audio equipment
Laptops, cameras and editing equipment
Mobile phones and network equipment
Hairdryer and personal appliances

Boat systems

Navigation lights
Horn
Bilge pumps
Monitoring and alarm systems
Mooring and security equipment
Hydraulic or electric bow thruster
Communications equipment
Electric propulsion
Steering or control equipment
Battery heating or cooling
Pumps associated with propulsion and charging

Occasional or exceptional loads

Power tools
Vacuum cleaner
Dehumidifier
Guest equipment
Emergency pumps
Portable heating
Additional filming and production equipment

Small continuous loads should not be overlooked. A device drawing only 10W consumes 240Wh if it operates throughout the day.

Record the right information

For each item, the audit should record:

Supply voltage
Rated power
Typical operating power
Starting or surge power
Expected daily running time
Duty cycle
Whether it is supplied directly from DC or through an inverter
Whether its use can be deferred
Whether it is essential, desirable or optional
Expected seasonal variation

Where only current is given, power can be estimated using W = V × A. For example, a 24V pump drawing 4A uses approximately 96W.

For AC equipment, the manufacturer’s measured energy-consumption information is preferable to a simple calculation from the fuse rating. A 13A plug does not mean an appliance continuously consumes 13A.

Use realistic operating times

A 50W fridge running for 24 hours would consume 1,200Wh each day only if its compressor operated continuously. Refrigerators cycle according to their insulation, thermostat setting, ambient temperature, ventilation and frequency of use. A better estimate would come from the manufacturer’s annual energy figure or direct measurement.

The same principle applies to many devices:

A water pump runs only when water is drawn.
An induction hob varies its power electronically.
A washing machine’s heater operates for only part of its programme.
A calorifier immersion heater stops when its thermostat is satisfied.
A laptop charger does not continuously draw its maximum rating.
A bow thruster has a very high load but normally operates for only seconds.

The audit should use realistic daily energy rather than multiplying every appliance’s maximum rating by the time it remains switched on.

A worked domestic example

The figures below are illustrative rather than a proposed specification.

Equipment	Typical power	Daily use	Estimated energy
Fridge/freezer	Variable	—	700Wh
LED lighting	40W	5 hours	200Wh
Water pumps	100W	0.5 hours	50Wh
Induction cooking	1,500W average	1 hour	1,500Wh
Combination oven	1,200W	0.4 hours	480Wh
Kettle	2,000W	0.15 hours	300Wh
Washing machine	Programme total	—	700Wh
Television	60W	3 hours	180Wh
Laptops and editing	200W	5 hours	1,000Wh
Phones, cameras and accessories	—	—	300Wh
Router and monitoring	25W	24 hours	600Wh
Heating pumps and controls	—	—	350Wh
Miscellaneous and standby loads	—	—	300Wh
Estimated daily total			6,660Wh

This example produces a domestic requirement of approximately 6.7kWh per day before conversion losses. Actual consumption could be considerably lower or higher. Electric water heating, space heating and intensive video editing would have a particularly significant effect.

Account for inverter losses correctly

A 230V appliance supplied from the batteries requires an inverter. If AC appliances require 4kWh and the inverter is 90% efficient at the relevant loads, the battery must supply approximately 4kWh ÷ 0.90 = 4.44kWh.

Only the AC portion of the audit should normally receive this adjustment. Applying an inverter-loss allowance to equipment supplied directly from DC would overstate its consumption. Inverter efficiency also varies with load, and the inverter itself may consume energy whenever it remains active.

Priscilla may therefore benefit from:

Efficient direct-DC supplies where appropriate
A low-power mode for the main inverter
Timed or switchable AC circuits
Avoiding unnecessary conversion from DC to AC and back to DC
Separating essential continuous loads from discretionary appliances

Peak demand matters

Daily energy alone does not determine the required inverter capacity. An induction hob, kettle, combination oven, washing-machine heater and immersion heater could together demand 8.5kW before other equipment.

Designing for every appliance to run simultaneously could result in an unnecessarily large and expensive system. Designing without considering simultaneous use could cause overloads. Priscilla should therefore have a deliberate load-management strategy, potentially pausing the immersion heater or preventing selected appliances from operating together.

The audit should identify maximum theoretical demand, credible simultaneous demand, short-duration starting loads, continuous and surge inverter ratings, battery discharge-current limits, cable and protective-device capacity, and loads that can be shed automatically.

Keep propulsion separate

Propulsion must not be treated as another domestic appliance. Its energy consumption depends upon boat length and displacement, propeller and drivetrain efficiency, cruising speed, water depth and channel width, wind, river flow, manoeuvring, hull condition and bow-thruster use.

The relationship between speed and required power is not linear. A modest increase in cruising speed can require a disproportionately large increase in propulsion power.

A motor drawing an average of 4kW for six hours would consume 24kWh; at 8kW, the same cruising period would require 48kWh. Either substantially exceeds the illustrative domestic requirement.

The propulsion audit should use modelling and measured evidence from comparable boats. It should include low-speed and typical canal cruising, extended river passages, strong currents or adverse wind, manoeuvring and locking, emergency reserve, stationary days, charging while cruising and equipment-failure scenarios.

Battery capacity should be expressed in kWh

Battery capacity is often quoted in amp-hours, but that figure is meaningful only with voltage. For example, 200Ah at 12V is 2.4kWh, while 200Ah at 48V is 9.6kWh.

The correct conversion is Ah = Wh ÷ V. However, kilowatt-hours provide a clearer common measure for domestic loads, propulsion, batteries and charging.

Nominal capacity is not usable capacity

A battery bank’s advertised capacity is not necessarily the amount available for routine use. Allowances may be required for permitted depth of discharge, battery-management-system limits, ageing, low-temperature performance, conversion losses, cell balancing and emergency reserve.

If Priscilla requires 30kWh between charging opportunities and only 80% of nominal capacity is allocated to normal operation, the bank would need at least 30kWh ÷ 0.80 = 37.5kWh, before any further design allowance.

Two or three days of autonomy may be unrealistic

Providing several days without charging sounds reassuring, but it can create an exceptionally large, heavy and expensive battery bank. If domestic consumption is 7kWh per day and propulsion averages 25kWh per cruising day, two days of autonomy would require 64kWh before reserve and system losses; three days would require 96kWh.

The better question is: What operating pattern must Priscilla sustain, and what reliable charging opportunities will be available?

Different scenarios include stationary days with domestic loads only, canal cruising followed by overnight charging, consecutive cruising days with solar assistance, demanding river passages with a protected reserve, winter occupation with little solar generation and extended periods away from shore power.

Battery chemistry and safety

Lithium batteries can provide high usable capacity, good charging efficiency and substantial power from comparatively limited space and weight. They also require a properly engineered installation.

The Boat Safety Scheme’s lithium-battery guidance advises the use of marine-suitable components and competent installation to appropriate industry standards.

The design should cover battery chemistry, battery management, contactors and emergency isolation, overcurrent protection, short-circuit capability, temperature monitoring, low-temperature charging, compartment design, fire detection, ventilation where required, mechanical restraint, water protection, system communication and safe loss-of-power behaviour.

Solar generation

Solar can make an important contribution to domestic energy and may extend cruising range, but roof-mounted panels are unlikely to provide guaranteed propulsion energy in every season.

A simplified estimate is: daily generation = array rating × equivalent sun hours × system efficiency. A 2kW array receiving four equivalent full-sun hours at 80% overall efficiency might produce 6.4kWh, but this is not a reliable year-round daily output.

Production is affected by season, latitude, cloud, panel temperature, shading, orientation, dirt, system losses, available battery capacity, tunnels and covered moorings. Solar generation varies with weather and time, so storage is needed when generation and demand occur at different times, as explained by the US Department of Energy.

Priscilla’s design should be tested against poor winter generation, not merely an attractive summer average.

Shore power

Shore power could provide the most dependable means of fully recharging a large battery bank, where a suitable supply is available.

Replacing 40kWh from a 16A, 230V connection would theoretically take 40kWh ÷ 3.68kW = 10.9 hours. Charging losses, onboard loads and reduced charging power near full capacity would extend this.

The design must consider common marina connection ratings, charging alongside domestic use, shore-power isolation and galvanic protection, residual-current and overcurrent protection, cable ratings, automatic reduction of charging current, metering and charging-point availability.

ISO 13297:2020 covers the design, construction and installation of extra-low-voltage DC and single-phase AC systems on small craft.

Onboard backup generation

A generator could provide resilience where solar and shore power are insufficient, although it would make Priscilla less completely dependent upon renewable electricity.

The comparison should consider fuel and storage, noise, vibration, exhaust, ventilation, emissions, maintenance, efficient loading, charging rate, space, weight, safety and intended operating hours. A small generator could take many hours to recharge a large bank; a larger one may be inefficient when lightly loaded. Any backup should be sized around a defined operational need.

Heating and hot water

Using electricity for propulsion does not automatically mean that every onboard service should be electric at all times. Resistance heating is energy-intensive: one hour from a 2kW heater consumes 2kWh, while sustained winter space heating could overwhelm the domestic energy budget.

Hot water and space heating therefore require separate seasonal modelling. Possible sources include shore-powered or solar-surplus immersion heating, recovered heat, diesel-fired heating, solid fuel, heat-pump technology or a combination of systems. Even if Priscilla ultimately uses electrical heating, the consequences for winter generation and battery capacity must be stated honestly.

Cooking and discretionary loads

Cooking is well suited to managed electrical use because it is intermittent and can often be scheduled around charging or strong solar generation. Induction cooking is efficient, but creates high short-term loads.

Practical management could include cooking during strong solar generation, avoiding simultaneous high-power appliances, insulated or pressure cooking, heating only the required water, selecting efficient appliances, using automatic load controls, and deferring washing and water heating until energy is plentiful.

Charging speed can be as important as capacity

A large battery is of limited value if charging sources cannot replenish it within the time available. For each source, the design should establish maximum and sustainable output, conversion efficiency, battery charge limit, temperature restrictions, hours available, simultaneous domestic demand and the time needed to restore a normal day’s use.

The combined system should also prevent chargers from exceeding the permitted current when solar, shore power and onboard generation operate together.

Build several operating scenarios

One average day will not provide a sufficient design basis. Priscilla’s audit should model at least:

Summer stationary day: domestic loads, strong solar and no propulsion.
Summer cruising day: domestic loads, several hours of propulsion and useful solar.
Winter stationary day: higher lighting and heating demand with very limited solar.
Winter cruising day: propulsion, domestic demand, heating and poor solar.
Demanding river day: sustained higher propulsion with reserve for currents, wind or delay.
Shore-power day: full domestic use while recharging within the supply limit.
Failure scenario: loss of one charging source, restricted battery capacity or major-system failure.

Monitoring the finished boat

The original power audit should become a living operational tool once Priscilla is commissioned. Monitoring should record battery state of charge, energy consumed, propulsion energy by journey, domestic energy by category, solar and shore generation, charging and discharging power, battery temperature, inverter load, alarms, and health trends.

A monitor based solely on instantaneous voltage would be inadequate for a large lithium system. Reliable measurement should integrate current over time and communicate with battery-management equipment.

The data should compare actual performance across speeds, rivers and canals, seasons, stationary and travelling days, cooking and heating patterns, battery age and equipment changes.

Commissioning trials

Before extended cruising, the system should be tested under controlled conditions:

Measure domestic consumption over representative days.
Record propulsion energy at different speeds and water conditions.
Test simultaneous high-power loads.
Confirm automatic load shedding.
Measure charging rates from every source.
Test operation with one charging source unavailable.
Confirm alarms and battery-management shutdowns.
Check emergency isolation.
Verify reserve capacity under realistic conditions.
Compare actual results with the design model.

Any discrepancy should lead to a change in the model, operating practices or equipment—not an assumption that the monitoring must be wrong.

A provisional design philosophy for Priscilla

Model propulsion and domestic energy separately.
Express capacity and consumption primarily in kWh.
Size the system for realistic operating scenarios.
Protect a defined propulsion and emergency reserve.
Treat solar as variable generation rather than guaranteed output.
Match battery capacity to achievable charging rates.
Manage simultaneous high-power loads automatically.
Keep essential systems operating if the main inverter fails.
Use marine-suitable equipment and competent professional design.
Measure actual performance during commissioning.
Allow for future battery degradation and equipment additions.
Design for safe failure, isolation and recovery.

Questions for the boatbuilder and electrical designer

What propulsion-energy assumptions support the proposed battery capacity?
What measured data from comparable boats are available?
What is the battery’s nominal and routinely usable capacity?
How much energy is protected as an emergency propulsion reserve?
What are the continuous and peak discharge limits?
What happens if the battery-management system disconnects?
Which essential services remain available after a main-system failure?
What is the realistic summer and winter solar yield?
How long will recharging take from typical 16A and 32A shore supplies?
Can charging current adapt automatically to the shore connection?
How are high-power domestic loads managed?
Can domestic use compromise the propulsion reserve?
How are batteries protected from excessive heat, cold and water?
What fire detection, isolation and emergency procedures are provided?
Which standards will the design and installation follow?
How will system performance be tested during commissioning?
What monitoring data can be exported and retained?
How is the system repaired if a proprietary component fails abroad?
What battery capacity should remain after ten years?
How can the installation be expanded or upgraded later?

Energy planning before equipment selection

The purpose of Priscilla’s power audit is not to justify the largest possible battery bank. It is to create a balanced system in which generation, storage, charging, propulsion and domestic demand work together. Oversizing one component cannot correct weaknesses elsewhere.

A reliable all-electric narrowboat needs enough stored energy for its intended journeys, sufficient power for simultaneous and starting loads, charging sources capable of replacing what is consumed, a realistic winter allowance, protected propulsion and emergency capacity, automatic control of discretionary loads, safe installation and accurate monitoring.

Priscilla’s eventual system should allow us to live comfortably and cruise confidently without constantly studying the battery display.

Achieving that will require more than arithmetic—but a thorough, realistic power audit is where the design must begin.