Po co robić bilans energetyczny na jachcie?

Bilans energetyczny pozwala dokładnie określić dobowe zużycie prądu przez wszystkie urządzenia, co jest niezbędne do prawidłowego dobrania pojemności akumulatorów oraz mocy paneli solarnych lub alternatora.

Energy balance of a yacht – how to calculate it before renovation?

Renovating the electrical system on a yacht is an exciting moment. The vision of new, shiny devices, reliable lighting, and complete comfort on board drives action.

However, without a solid plan, this enthusiasm can quickly collide with frustrating reality: insufficient power, dead batteries at the least opportune moment, or blown fuses due to poorly chosen components.

The key to success and avoiding costly mistakes is creating an energy balance – and do this before you buy the first meter of cable or a new battery!

What is an energy balance and why is it crucial?

An energy balance is nothing more than a detailed summary of the expected electricity consumption of all devices on the yacht over a specified time (usually within a day) and a comparison of this value with its production (charging) and storage capabilities (battery capacity).

Doing this before starting work and purchases is absolutely fundamental, as it allows you to:

Precisely select the capacity of the battery bank: You will avoid situations where the batteries are too small and constantly undercharged, as well as unnecessary spending on an oversized, heavy, and expensive bank.
Plan appropriate charging sources: You will assess whether the engine's alternator alone is sufficient or if you need solar panels, a wind generator, or perhaps a more efficient shore charger. And what power should it be!
Choose the right wire cross-sections: Knowing the maximum currents flowing in each circuit is essential for selecting cables of appropriate thickness, which minimizes voltage losses and the risk of overheating.
Select appropriate protections: Fuses and switches protect your installation. Their values must be perfectly matched to the load in a given circuit.
Avoid costly modifications: It's better to calculate something five times on paper or in a spreadsheet than to rip out freshly installed fittings to add an additional, overly thick cable.

How to create an energy balance step by step?

Prepare a spreadsheet or a piece of paper. Let's get started!

Step 1: Inventory of future power consumers

This is the most important stage. List absolutely all electrical devices you plan to have on your yacht. Be a visionary – include not only the current equipment but also future plans on the basis of "maybe someday" (e.g., an additional pump, a stronger audio system, Starlink, or a watermaker).

Step 2: Determine power consumption and operating time

For each device on the list, you need to establish two values:

Power consumption in Amperes (A): You will find this on the device's nameplate or in its manual. Sometimes the power is given in Watts (W) – in that case, divide it by the installation voltage (usually 12V) to get Amperes: Amperes (A) = Voltage (V) / Watts (W)
Estimated operating time per day in hours (h): This requires a realistic approach to how you use the yacht:
- Compressor refrigerator: It does not run continuously. Its cycle depends on the ambient temperature and insulation. It is safe to assume that it effectively operates for about 6 to 12 hours a day.
- Autopilot: Consumption depends on sea conditions (waves and wind). Assume an average operating time for a typical day of your sailing.
- VHF radio: Primarily operates in standby mode (low consumption), plus a short transmission time (very high consumption). It's reasonable to assume, for example, 24 hours of standby and 15 minutes of transmission.
- 230V inverter: Remember that the inverter, even when not powering any device, draws power for its own operation (so-called no-load current). Check this parameter in the technical data sheet!

Step 3: Calculate daily energy consumption (Ah)

For each device, multiply its power consumption (A) by the estimated operating time per day (h). The result is the daily energy consumption in Ampere-hours (Ah).

Step 4: Sum total daily demand

Add together the Ah/day values for all items. You will get the total estimated daily energy demand for your yacht.

Example: After summing all the consumers on your yacht, the result was 85 Ah/day. This is your compensation target.

Step 5: Plan charging sources

Now it's time for the other side of the coin – how to replenish this energy?

Engine alternator: Check its output. Remember that it only provides maximum current at higher RPMs. If you are charging LiFePO4 batteries through a DC/DC converter, the matter is simple – the device enforces a constant, full charging current. With traditional lead-acid batteries, the current drastically drops at the end of charging, significantly extending the engine's running time while stationary.
Solar panels (Photovoltaics): Determine the total power of the panels in Watts (Wp). In Polish climatic conditions, during summer, a safe yield of approximately 3 to 4 Ah per day can be assumed for every 10 Wp of panel power (when using an efficient MPPT controller and no shading).
Wind generator and grid charger: Allow for balancing at anchor in good weather or while resting in a marina.

Sum the estimated daily energy yield from all available sources.

Step 6: Determine the required battery capacity

The bank capacity must be sufficient to cover the demand when the charging sources are not operating (at night, on cloudy days without wind).

First, determine the desired autonomy – that is, how many days you want to stay "off-grid" without starting the engine or connecting to shore power (e.g., 1, 2, or 3 days). Then account for a safe depth of discharge (DoD), which varies significantly depending on the technology:

Lead-acid batteries (AGM, GEL): They should not be discharged below 50% of their nominal capacity if you want to enjoy their long lifespan.
LiFePO4 batteries: They can be safely and harmfully discharged up to 80-90% of their capacity.

The formula for the required nominal capacity of the bank is as follows:

Bank capacity (Ah) = Maximum allowed DoD × Daily demand (Ah) × Days of autonomy

Let’s see the difference in our example of a demand of 85 Ah/day with an assumed 2-day autonomy:

Step 7: Balancing and adjustments

Finally, combine three key values: daily consumption, daily yield, and the usable capacity of the batteries.

If your daily balance turns out negative (you consume more than you can produce during the day), you need to return to the design and make decisions:

Increase production: Add solar panels or replace the alternator with a stronger one.
Increase storage: Expand the battery bank or switch to LiFePO4 technology.
Reduce consumption: Replace the old refrigerator with a new energy-efficient compressor model or optimize crew habits.

Let’s summarize

Creating an energy balance during the design phase of a yacht installation is an absolute foundation for success. It is an investment of time that will pay off multiple times in terms of reliability, a sense of security on the water, and pure financial savings. Don't take shortcuts – first calculate and plan, and only then build!

If you feel unsure about calculations or selecting advanced components, consult your project with an experienced marine electrician. It is better to ask for advice once too many on shore than to regret and search for a fault blindly in the middle of the sea.

Energy balance BEFORE the electrical installation renovation on the yacht