This text is the second in a series on electrical systems. It enables you to assess your electrical requirements and compare them with your production capacities. By reading this text, you will be able to assess your energy balance. You will then be able to determine whether you need to change your production systems, or whether it is better to revise your requirements.
I used this diagnostic tool to assess the needs on Jean-du-Sud. Of course, each boat’s needs are different. As a result, the balance sheet shown therein does not make much sense for a boat that is docked every evening. Jean-du-Sud has long distance ambitions, so the onboard production of energy has to meet its needs.
However, the needs assessment methodology presented below may help you establish your needs in your own context. In other words, the method therein is more useful than the findings pertaining to Jean-du-Sud.
No Need to Be an Electrician to Perform an Electric Balance Sheet
Making an electrical budget requires an understanding of the concepts of Watt, Watt-hour and ampere-hour. It is also useful to know that the operational range of a battery (other than lithium) is the 50% to 80% charge interval. In other words, the daily electrical budget should not discharge the batteries below 50%, but should futher not require a state of charge greater than 80%. A person seeking to understand why may find the “Introduction to batteries” text useful.
The upper limit may vary according to your sailing style. A boat that is frequently in a marina, where batteries can be charged overnight, may not need to worry about the 80% upper limit (and planning with the 50% to 100% range is fine). The same applies if one is constantly motoring (!). However, for long passages, it is better to plan with an 80% upper bound, as the last 20% of charge is too time demanding for the production side to deliver.
This budgeting approach is adapted from a publication by Sailboat-Cruising.com (n.d.). The original text focuses on needs assessment. In addition, this text complements the original idea by adding a production evaluation to establish a balance sheet.
Watt, Watthour and Ampere-hour
Electrical appliances detail their power in Watts, while battery or production systems detail their capacities in Ampere-hours. For example, a Starlink antenna requires an average of 50-75 Watts of power, but an “ordinary” battery will display its energy capacity in ampere-hours (Clarke, 2023; Canadian Tire, n.d.). Being able to convert these units helps comparing apples with apples.
Watt is a measure of power (Électricité de France, n.d.). For example, a 100-Watt toaster requires “100 units of power” to operate. More powerful appliances will require more watts, and less powerful appliances will require less. You really do not have to be a “100 Watt” to understand!
The Watthour (Wh) is a unit of energy: it calculates the quantity of Watt required to operate a device for a given period of time (id.). Thus, 100 Wh represents the energy required to operate a 100 Watt device for one hour (100 x 1 = 100). Alternatively, it also refers to the energy required to operate a 50 Watt device for two hours (50 x 2 = 100), or a 25 Watt device for four hours (25 x 4 = 100).
For example, if a VHF radio requires 25 Watts of power during a radio call, and you make roughly five minutes of radio calls over the course of a day, then your energy requirements for your VHF calls are roughly 2.08 Wh per day (25 x 5/60≈2.08). That is a first element of your electric needs.
On boats, the notion of energy is often expressed in terms of Ampere-hours (Ah). As the standard for North American boats is 12 volts, one ampere-hour corresponds to one watt-hour divided by 12. In other words, 1 Ah = 1 Wh / 12.
Assembling the Balance Sheet
The previous three paragraphs give us three simple ideas for building a balance sheet:
- Identify the wattage of the appliances (on their labels or online description) and estimate how long they are run in a day. This gives an energy requirement in Wh for one day.
- Convert these Wh into Ah by dividing by twelve to obtain the number of amp-hours required for these appliances.
- Calculate the sum of requirements in Ah and compare it with the sum of production in Ah.
The process of identifying needs by device is relatively easy to do with a spreadsheet (e.g. Excel). A few lines of this spreadsheet would look like this:
| Item | Power (W) | Time (h) | Energy (Wh) | Energy (Ah) |
| Starlink Antenna | 75 | 3 | 225 | 18.75 |
| Radio VHF | 25 | 0.08 | 2 | 0.17 |
Note that the last column is nothing other than the fourth column divided by 12. Note also that the fourth column is nothing but the product of the second and third columns. Once the relationship between Watt, Watt-hour and Ampere-hour is understood, it is easy to build a balance sheet using a spreadsheet program!
Time assessment does not have to be accurate to the minute. A reasonable estimate is enough to establish an order of magnitude. Ultimately, to factor in the uncertainty of these estimates, as well as other factors such as energy lost in the system, adding a safety margin of 15% is a good practice.
Time to Inspect the Boat
Now comes the tedious task of going through all the onboard electrical systems in order to find out their energy requirements. Two practical tips will can save time.
First, it is a good idea to focus on the appliances that consume the most energy: fridge, watermaker, air conditioner, windlass, radar, personal computer, stereo, TV and electric cooking system (…if anything on this list is onboard). These are the big budget items. It is worth taking a hard look at these onboard devices by digging through their technical manuals and taking more time to estimate their usage time. For example, a fridge is not constantly working, but only when its thermostat detects a temperature above a threshold point. A little bit of research is in order!
Second, many components are almost identical and consume little energy. The best example is LED lighting (cabin lights, etc.). They consume next to nothing, and there are many of them. It is perfectly acceptable to make one reasonable estimate of power and then simply count the lamps onboard. It is faster and the estimation error will not make much difference to the balance.
What Ifs
In contrast with a direct amp-hour reading from the electric system, the listing performed above can easily be extended to estimate the impact of prospective gadgets on the electrical budget. For instance, what is the impact of adding a Starlink antenna onboard? Are larger batteries required?
Underway or at Anchor?
Electricity needs under sail are not the same as under anchor. When underway, the radar will most likely be on, but the TV may not be as much in demand. Conversely, the anchor light will only be on at anchor… and so will be an air conditioner (quite fictitious for Jean-du-Sud!). So it’s a good idea to estimate the energy needs according to two boat’s operating modes, adapting consumption and needs accordingly.
Production Evaluation
Evaluating onboard energy production is relatively straightforward. Energy sources are limited: solar panels, alternators, wind turbines, water turbines or a diesel generator (depending on what is installed). So the production capacity of each of these elements must be assessed through their technical manuals. It is the same language: Watt, Watthours and Amp-hours, although some generators may provide power curves rather than a single measure. It is perhaps worth pointing out that the production capacity reported by manufacturers should be taken with a grain of salt, i.e. the actual production capacity of the components may be lower than what is advertized under « idealized conditions ».
This is particularly true for alternators and solar panels (Pacific Yacht Systems, 2019). For example, a 40-amp alternator assumes that the engine is running at full speed and that the electrical wires are cold enough to neglect thermal losses. Similarly, a 200-Watt solar panel will produce according to the sun intensity and hours under the sun.
I am not aware of one hard and fast rule, but it is best to plan production conservatively. As an estimation rule, I use a factor of 80% for solar panels. I also estimate an average of 6 hours of sunlight. This means that a 200 W solar panel can produce around 960 Wh per day, or 80 Ah per day (200 x 0.8 x 6 / 12 = 80).
Batteries Capacity
Assessing storage capacity is very simple. One needs to look at the batteries installed and then count the total ampere-hours associated with these batteries. A range of 30% of this capacity should then be retained, i.e. the range from 50% to 80% (with the nuances detailed in the introduction).
As an example, Jean-du-Sud currently has two lead-acid batteries, each rated at 90 Ah. As they are connected in parallel, the storage capacities add up to a total of 180 Ah. Thus, Jean-du-Sud‘s effective storage capacity is 54 Ah (0.3 x 180, i.e. very little!).
Building a Balance Sheet Is an Iterative Process
Once prospective requirements, storage capacity and production capacity have been established, it is time to examine the first resulting balance sheet. Do requirements exceed storage capacity? Do they exceed daily production capacity? If the answer is yes to either of the two questions, then either storage capacity must be increased, production capacity must be increased (e.g. run the engines longer, or charge the batteries ashore), or ambitions with respect to the use of electrical gadgets must be revised. Is it really needed? This back-and-forth process helps to refine the balance sheet and clarify which elements are essential to a project.
Jean-du-Sud‘s Balance Sheet
Demand Side
The current balance sheet for Jean-du-Sud can be found here (electric items are listed in French). It is an evolving document, and can be a good starting point for a similar analysis on your boat.
The conceptual starting point for Jean-du-Sud‘s review is a need to modernize her on-board instruments (notably adding a Starlink antenna). I also explored the possibility of using an induction cooking system (Sailing SV Delos, 2019; Sailing Tritea, 2022), but opted for an alcohol stove, not least to moderate electrical requirements. The total balance is shown in the table below and includes a 15% margin.
For planning purposes, Jean-du-Sud needs 221 Ah per day. Electrically, she is a relatively simple sailboat. There’s no stereo on board, no watermaker, no water heater, no TV or similar gadgets. On the other hand, it is a boat with offshore ambitions. As such, the ability to supply this power demand with onboard systems is crucial.
Electricity Supply and Storage
Jean-du-Sud‘s operational storage capacity is 54 Ah, four times less than the identified needs. In addition, it is currently powered by two 100-Watt solar panels, producing an estimated average of 80 Ah per day.
| Under Passage (Ah/day) | At Anchor (Ah/day) | |
| Demand | 221 | 196 |
| Solar Supply | 80 | 80 |
| Residual Engine Supply | 141 | 116 |
| Engine time (estimate) | 1h10 | 2h00 |
Analysis
The current balance sheet suggests a significant energy deficit based on current systems and prospective needs. There is certainly still room to refine the demand estimate, but even a 50% reduction in demand would still lead to an energy deficit with respect to solar production. This first observation, a rough order of magnitude of the deficit, led me to explore an increase in solar production, as well as a possible replacement of batteries.
It also suggested that a refined estimate of energy-hungry systems such as the fridge compressor and the onboard computers is needed. Given the snow and weather, these estimates will have to wait!
Conclusion
It is easy to perform an energy balance assessment on a sailboat with a few simple electrical notions, a good understanding of what is onboard and an Excel spreadsheet. The result is an assessment of energy requirements and production capacity. The difference between the two is used to identify the energy deficit/surplus.
Based on the prospective needs established, Jean-du-Sud has an energy deficit, even if these needs turn out to be overestimated by 50%. The next steps are therefore to better estimate the boat’s energy demand, as well as to evaluate how to increase its solar production. These two considerations are the subject of forthcoming texts.
References
Canadian Tire (s.d.). Batterie à double usage MOTOMASTER NAUTILUS de groupe 2, webpage retrived online in december 2023 from this address.
Clark N. (2023). How Much Power Does Starlink Use?, webpage retrived online in december 2023 from this adress.
Électricité de France (s.d.). Volt, watt, ampère : les unités en électricité, webpage retrived online in december 2023 from this address.
Pacific Yacht Systems (2019). How To: Marine Electrical Seminar – Alternators, YouTube video retrived online in december 2023 from this address.
Sailboat-cruising.com (s.d.). Boat Electrics: The Demands of the Domestic System, webpage retrived online in december 2023 from this address.
Sailing SV Delos (2019). Induction Cooking on A Sailbot: One Year Review, YouTube video retrived online in december 2023 from this address.
Sailing Tritea (2022). New Electric Galley, YouTube video retrived online in december 2023 from this address.
