SER Unit : SailSafe Energy Rating for Electrically Powered Sailboats
Sail Safe Energy Rating (SER): A Proposed Energy Safety Metric for Electric Sailboats
Electric propulsion is increasingly used on sailboats, but onboard energy is still usually described using battery capacity, motor power and theoretical range.
Those figures are useful, but they do not answer one of the most important questions a sailor may face:
This is the question behind the Sail Safe Energy Rating (SER).
SER is a proposed energy-planning metric developed by Baywatt to help sailors, boat builders, marine professionals and other stakeholders evaluate the usable propulsion-energy reserve of an electrically powered sailboat.
- Electric Sailboats
- Energy Autonomy
- Battery Reserve
- Hybrid Propulsion
- Marine Safety
Calculate your Sail Safe Energy Rating →
Why electric sailboats need an energy-autonomy metric
With a conventional combustion engine, available autonomy can usually be estimated from the amount of fuel on board and the engine's fuel consumption.
Electric propulsion introduces a more complex energy system. A vessel may combine a battery bank, solar panels, hydro-generation, a generator, DC/DC converters, chargers and many onboard electrical consumers.
Battery capacity alone therefore does not describe the vessel's usable propulsion reserve.
A 30 kWh battery may appear substantial, but its practical value depends on:
- the propulsion power required by the vessel;
- the amount of battery energy that is genuinely usable;
- the electrical loads that must remain active;
- the amount of electrical generation available while motoring.
SER attempts to express these factors through one understandable measure of energy autonomy at a defined propulsion requirement.
What does the Sail Safe Energy Rating measure?
SER estimates how long a vessel can maintain a specified propulsion power while also supporting the electrical systems that must remain operational.
The SER result is represented by the stylized Poseidon W symbol.
This Poseidon W represents the SER unit and should not be confused with the conventional electrical unit watt, which is also written W.
For example, a rating of 4.75 W corresponds to approximately 4 hours and 45 minutes of calculated autonomy at the selected propulsion power.
Range is not the same as escape capability
Electric boats are frequently compared using maximum range figures. Range is useful, but maximum range and safety autonomy are not the same thing.
A sailboat may travel efficiently for many hours at low propulsion power in calm water. This can produce an impressive theoretical range.
An emergency may require considerably more power.
Low propulsion power, favourable weather and an emphasis on maximum efficiency.
Higher sustained propulsion power may be required to overcome wind, waves, tide or adverse current.
A vessel attempting to enter a harbour against a strong current, clear a lee shore or reach shelter before conditions deteriorate may need several times its normal cruising power.
It asks: How long can the boat maintain the propulsion power required to make meaningful progress toward safety?
The SER energy model
The revised SER methodology uses five principal values.
The electrical energy genuinely available from the battery bank, expressed in kWh.
The propulsion power considered necessary to maintain useful progress toward safety, expressed in kW.
VHF, AIS, navigation instruments, autopilot, navigation lights and other safety-related equipment.
Refrigeration, pumps and other equipment that remains powered during the scenario.
Electrical power genuinely available during the scenario from a generator, solar installation or another source.
The SER formula
Total Demand = Propulsion Power + Safety Loads + Other Continuous Loads
Net Battery Draw = Total Demand − Continuous Available Generation
SER = Usable Battery Energy ÷ Net Battery Power Draw
When battery energy is expressed in kWh and net battery demand in kW, the result is expressed in hours.
Why continuous generation must be treated as power
One important revision to the original SER methodology concerns onboard electrical generation.
A generator is a source of continuous power. A 2.2 kW generator does not simply add 2.2 kWh to the battery once.
If it continuously provides 2.2 kW while the propulsion system is operating, it continuously reduces the amount of power that must be supplied by the battery.
If the vessel consumes 8.1 kW while a generator supplies 2.2 kW, the battery supplies:
8.1 kW − 2.2 kW = 5.9 kW
This distinction is especially important for hybrid electric propulsion.
Not all generation is equally reliable
The generation value entered in SER should represent the power that can realistically be expected during the scenario being analysed.
Can potentially provide relatively predictable continuous output while sufficient fuel and cooling are available.
Output depends on sun angle, weather, shading and time of day. At night, solar output is zero.
Output generally depends on boat speed and may not remain available when propulsion is being used.
Generator or charger ratings should also be adjusted when conversion losses mean that less power actually reaches the propulsion DC bus.
A practical SER calculation
Consider an electric sailboat with the following configuration:
- Battery: 28 kWh usable
- Propulsion: 7.4 kW
- Safety: 500 W
- Other loads: 200 W
- Generator: 2.2 kW
7.4 kW + 0.5 kW + 0.2 kW = 8.1 kW
28 kWh ÷ 8.1 kW = 3.46 hours
Approximately 3 hours and 28 minutes.
Net battery demand:
8.1 kW − 2.2 kW = 5.9 kW
Revised autonomy:
28 kWh ÷ 5.9 kW = 4.75 hours
Approximately 4 hours and 45 minutes.
In this example, the generator increases full escape-power autonomy by approximately 1 hour and 17 minutes.
What happens after the usable battery reserve is depleted?
This is one of the most important distinctions made by the SER concept.
Reaching the usable battery discharge limit does not necessarily mean that all electrical power aboard disappears immediately.
If generation remains available, a limited amount of propulsion may still be possible.
Generator output: 2.2 kW
Safety electronics + other loads: 0.7 kW
Potential remaining propulsion:
2.2 kW − 0.7 kW = 1.5 kW
Assuming the electrical architecture allows generator power to feed the propulsion system, the vessel may therefore retain approximately 1.5 kW for propulsion.
But the escape scenario was calculated using 7.4 kW.
Three energy states of an electric sailboat
The battery and generation system can provide the propulsion power considered necessary to make meaningful progress toward safety while supporting essential onboard electrical systems.
Propulsion remains available, but at a lower power than the selected escape requirement.
The boat may still manoeuvre or make progress in favourable conditions, but this should not automatically be considered sufficient to overcome adverse wind, waves or current.
Remaining power may only be sufficient to maintain essential systems such as communications, navigation instruments, AIS and lighting, with little or no propulsion available.
How to determine usable battery capacity
SER should use usable energy, not simply the nominal capacity printed on the battery.
A nominal 30 kWh battery bank may not provide 30 kWh of realistically usable energy.
Usable capacity can be affected by:
- battery-management-system discharge limits;
- the owner's chosen state-of-charge reserve;
- battery ageing and state of health;
- temperature;
- high-current discharge behaviour;
- system voltage limits;
- electrical conversion losses.
If only 28 kWh of a nominal 30 kWh battery bank is realistically available for the analysed scenario, use 28 kWh.
How to choose escape propulsion power
The propulsion value is one of the most important assumptions in an SER calculation.
It does not necessarily have to equal the motor's maximum rated power.
A 10 kW motor may push a displacement hull at 5.5 knots at full power, while 7.5 kW might already produce 5 knots.
If an additional 2.5 kW creates very little additional boat speed, 7.5 kW may represent a more rational continuous escape-power setting for that vessel.
Ideally, this value should be established from actual sea-trial data at different propulsion-power settings.
What should be included in safety loads?
- VHF
- AIS
- GPS
- Chartplotter
- Autopilot
- Navigation Lights
Other continuous loads can include refrigeration, pumps and equipment that cannot reasonably be switched off during the scenario.
Short-duration high-power equipment such as a windlass or bow thruster is not represented perfectly by a simple continuous-power model and should therefore be considered separately when determining the energy reserve.
The SER Safety Scale
The SER scale provides a simple planning framework for comparing different levels of propulsion-energy autonomy.
| SER Rating | Energy autonomy interpretation |
|---|---|
| Less than 1 W | Very Dangerous — Less than one hour of full escape-power autonomy leaves very little reserve, including for harbour manoeuvres in adverse wind, tide or current. |
| 1 to 3 W | Dangerous — Limited margin to reach a nearby harbour or shelter in an emergency. |
| 3 to 6 W | Good for the Day — A more useful reserve for daytime navigation and unexpected motoring. |
| 6 to 9 W | Acceptable Risk — Suitable for coastal navigation where safe harbours remain reasonably accessible. |
| 9 to 12 W | Suitable for Coastal Cruising — Substantial autonomy where regular access to safe shelter remains available. |
| 12 to 16 W | Acceptable for the Weekend — Larger reserve for short cruises and periods requiring extended propulsion. |
| 16 to 20 W | Long Motoring Capability — Significant flexibility when propulsion must be maintained for extended periods. |
| 20 to 24 W | Good for Keeping Schedule — High propulsion autonomy for passages where sustained motoring may be required. |
| More than 24 W | Ideal for Long Cruises — Very high energy autonomy and substantial operational flexibility. |
These categories are planning guidance. They are not regulatory safety classifications, and the meaning of a particular SER value depends strongly on the vessel and its navigation area.
The same SER value can mean different things in different waters
A three-hour propulsion reserve may be adequate in a small coastal area containing several nearby ports and sheltered anchorages.
The same reserve may be inadequate offshore, along a lee shore, in a region with strong tidal streams or where safe shelter is many hours away.
What SER does not calculate
SER is deliberately a simple energy metric.
It does not determine whether a particular vessel can physically overcome a specific storm, current or sea state.
Actual escape capability may be influenced by:
- wind strength and direction;
- wave height and period;
- tidal and ocean currents;
- vessel displacement and loading;
- windage;
- hull resistance and fouling;
- propeller efficiency;
- motor and controller thermal limits;
- battery BMS current limits;
- battery condition and temperature;
- conversion losses;
- generator reliability;
- actual renewable-energy production;
- distance to a safe harbour or shelter.
SER measures energy availability at an assumed propulsion requirement. It does not guarantee seaworthiness or the ability to escape a particular maritime condition.
SER is a proposed metric, not a regulatory standard
It is not an IMO, ISO, classification-society or national regulatory standard.
The purpose of SER is to encourage discussion around a question that becomes increasingly important as marine propulsion electrifies:
How should the usable emergency propulsion-energy reserve of an electric vessel be communicated?
Motor power alone cannot answer this question.
Battery capacity alone cannot answer it either.
The relationship between propulsion requirement, usable energy, essential onboard consumption and continuous generation determines how long meaningful powered navigation can continue.
Why a common energy-autonomy metric could help
A simple answer to the question: if sustained propulsion is suddenly required, how many hours are realistically available?
A more transparent relationship between installed battery energy, motor power and usable propulsion autonomy.
An additional reference when discussing energy reserve and propulsion capability.
A possible contribution to the wider discussion about emergency propulsion-energy reserve as electric vessels become more common.
SER and hybrid electric propulsion
SER also illustrates why a hybrid system can behave very differently from a purely battery-powered vessel.
The battery continues to discharge, but more slowly.
Battery state of charge can theoretically remain approximately stable while those conditions continue.
Surplus power may become available for charging, subject to the electrical architecture and conversion limits.
The key parameter is therefore the net power flowing out of the battery.
SER and electric outboards
The same energy-autonomy principle applies whether the vessel uses an electric inboard motor or an electric outboard.
Motor power should therefore be considered together with usable battery capacity and the vessel's real operating profile.
1.5 kW electric outboard for lightweight boats and sailboats requiring modest propulsion power.
3 kW electric outboard providing increased propulsion capability for larger or more demanding applications.
The appropriate propulsion system depends on hull resistance, displacement, required speed and operating conditions. SER can then be used to evaluate the energy reserve available at the selected propulsion power.
Calculate your Sail Safe Energy Rating
The Baywatt SER calculator automatically applies the continuous-generation power model.
- SER Rating
- Escape Autonomy
- Total Demand
- Net Battery Draw
- Post-Battery Propulsion
Enter the usable battery capacity, escape propulsion requirement, essential loads and continuous available generation.
An open proposal
SER should not be considered the final answer to electric-vessel safety.
It is intentionally a simple model designed to make one important aspect of electric propulsion easier to understand and compare:
The methodology can evolve as practical experience with electric and hybrid vessels grows.
Feedback from sailors, boat builders, naval architects, electrical engineers, surveyors, insurers, classification organisations and maritime authorities is therefore welcome.
If a better common method emerges, the objective remains the same: make the available energy reserve of electrically powered vessels easier to understand before that reserve is needed at sea.
