Onboard Power Consumption: How to Calculate and Manage It

Why energy on board is a resource to be managed

At home, electricity is a utility: you flip the switch and it's there. On board, it's a supply — just like water or diesel. It's finite, it runs out faster than you'd think, and it recharges more slowly than you'd like. The difference between a relaxed cruise and a week spent switching off the fridge to make the batteries last almost always comes down to a calculation nobody made before casting off.

Most onboard power autonomy problems don't come from faulty batteries: they come from a battery bank undersized for actual consumption, insufficient charging sources, or hidden loads nobody ever measured. The good news is that it's basic arithmetic, and once you've done it, it stays valid for years.

The power budget: the calculation you only need to do once

The unit of measurement for onboard energy is the amp-hour (Ah): the current drawn by a load multiplied by the hours it's switched on. A 1 A light left on for 3 hours uses 3 Ah. The power budget is simply the sum of all your loads over a typical day, compared against what the batteries can deliver and what the charging sources can actually put back.

The process is this:

• List every onboard load with its current draw in amps. You'll find it on the nameplate or in the manual; if it's given in watts, divide by nominal voltage (W ÷ 12 = A).
• Estimate real operating hours over 24 hours, not the theoretical ones. The fridge doesn't run continuously: the compressor cycles on and off, with a duty cycle that can reach 40–50% of the time in summer.
• Multiply and add up to get your daily Ah.
• Add a 20% margin for the unexpected, battery aging, and any loads you forgot.

Typical onboard loads

Indicative figures at 12V, useful as a starting point if you don't have exact data:

• Compressor fridge: 3–5 A while running, with a variable duty cycle. In practice, 30–60 Ah per day in summer. Almost always the single biggest consumer on board.
• Autopilot: from 0.5 A in calm seas with a well-balanced boat up to 4–5 A in a seaway with the rudder constantly working. The difference is huge and depends heavily on how well the sails are trimmed.
• Plotter, instruments, AIS: 1–2 A combined for a typical navigation station.
• VHF: 0.3–0.5 A on receive, 5–6 A while transmitting. Left on 24 hours a day, it adds up more than you'd expect.
• Radar: 2–4 A in continuous operation. Use it intermittently when range is tight.
• LED lighting: 0.1–0.5 A per fixture. LED navigation lights draw under 1 A combined; old incandescent ones can draw 4–5 A.
• Freshwater pump and bilge pumps: 5–8 A but short, intermittent runtime: typically 3–8 Ah per day.
• Inverter: beyond the load it powers, it also draws current at idle (0.3–1 A). An inverter left on overnight "for convenience" can cost you 15 Ah without powering anything at all.

A concrete example

10-metre boat, summer cruise, four people on board, daytime sailing with one overnight leg:

• Fridge: 45 Ah
• Autopilot (6 hours): 12 Ah
• Instruments and plotter (6 hours): 9 Ah
• VHF on receive (24 hours): 10 Ah
• Navigation lights (8 hours): 8 Ah
• Interior lighting: 6 Ah
• Pumps: 5 Ah
• Charging phones, tablets, headphones: 10 Ah

Total: roughly 105 Ah per day, which with the 20% margin becomes 125 Ah. A number that surprises most owners — and explains why a 200 Ah nominal battery bank runs flat in a night and a half.

From consumption to capacity: sizing the battery bank

The point that trips up most calculations is this: a battery's nominal capacity is not its usable capacity. How much you can actually draw depends on the technology.

• Lead-acid, AGM and gel: shouldn't be discharged beyond 50% of nominal capacity if you want them to last. A 200 Ah bank gives you 100 Ah of real, usable energy. Deeper discharges drastically shorten cycle life. You also need to factor in the Peukert effect: at high discharge currents, available capacity drops further.
• Lithium (LiFePO4): tolerates discharges down to 80–90% of nominal capacity, accepts much higher charge currents, and weighs considerably less for the same energy. It costs more upfront, but the cost per cycle is generally lower. It does require a suitable BMS and, almost always, an alternator regulator compatible with lithium: wiring a lithium battery to an alternator designed for lead-acid is a mistake that can damage the alternator.

Going back to the 125 Ah/day example: with AGM batteries you'd need 250 Ah nominal for a single day of autonomy, 500 Ah for two days without recharging. With lithium, around 155 Ah nominal covers the same day. This is where the difference in size and weight between the two solutions becomes clear.

Two rules always apply, whatever the technology: the starter battery must be separate from the house bank, with a charge management system that protects starting power under all conditions; and the house bank's batteries must be identical in type, capacity and age, because in a mixed bank the weakest battery dictates how the whole bank behaves.

Charging sources: putting back what you use

A big battery bank solves nothing if you can't recharge it. The budget has to balance on the input side too, and each source has its own characteristics.

• Engine alternator: the most powerful source, but it requires the engine to be running. The classic mistake is thinking an hour of motoring fully recharges the batteries: the alternator only delivers peak current in the first phase, after which the current the batteries will accept drops off quickly. Taking a lead-acid bank from 50% to 100% on the alternator alone can take many hours. An external multi-stage regulator improves output significantly.
• Solar: the most effective solution for anchoring and slow cruising, since it produces energy every day with no noise and no maintenance. In the Mediterranean summer, a 100 W panel with an MPPT controller realistically delivers 30–40 Ah per day if it's not shaded. Shadows from the boom, rigging and antennas hurt output significantly: in panels wired in series, one shaded cell reduces the output of the entire string.
• Wind generator: worth considering at anchor in windy areas, and it produces power at night too, when solar is idle. Weigh it against noise and vibration, which are more noticeable on board than you'd expect.
• Hydro generator (towed): a solution for offshore passages, producing well above 5–6 knots of boat speed through the water, at the cost of added drag.
• Shore power charger: must be multi-stage and sized to the battery bank — roughly 10–20% of total capacity in amps. A charger that's too small never completes a full charge; one with the wrong curve for your battery technology slowly damages them.

Measure, don't guess

The voltmeter on your panel doesn't tell you how much energy you have left. A battery's voltage varies with temperature, with the load applied, and stays artificially high right after charging: to get even a rough reading you'd need to measure it at rest, with no load and no charging, after several hours. It's not a figure you can act on while underway.

The right tool is a shunt-based battery monitor: installed on the negative side of the house bank, it actually counts the amps going in and out, giving you state of charge as a percentage, Ah consumed, and estimated remaining runtime. It's the single piece of equipment that changes how you manage power on board the most, because it turns a feeling into a number.

With a monitor installed, you'll almost always discover two things: that your real consumption is higher than what you'd estimated, and that there are permanent loads you didn't suspect existed — some electronics left in standby, a stereo that's never truly off, an inverter forgotten switched on.

Cutting consumption: where the real gains are

Before adding batteries or panels, it's worth working on consumption first. These are low-cost interventions with a high return:

• Focus on the fridge, not everything else: it's the dominant load. Better insulation, good ventilation for the condenser, fewer door openings, and pre-chilling drinks before loading them do more than any other single optimisation. A condenser working inside a closed, hot locker can double the unit's consumption.
• Switch to LED everywhere: the payback is immediate, especially on navigation lights and courtesy lights left on for long stretches. Make sure replacement LEDs are marine-rated and, for navigation lights, compliant with the applicable regulations.
• Turn off the inverter when it's not needed: and consider whether small AC loads could run directly off 12V instead. Every DC-AC-DC conversion throws away 10–20% of the energy.
• Eliminate standby loads: a dedicated switch for circuits not needed underway or at anchor recovers dozens of amp-hours a week.
• Balance a sailboat properly: an autopilot constantly correcting course draws three to four times what it would with properly trimmed sails.
• Check for voltage drop: undersized cables or oxidised connections dissipate energy as heat along the way — energy you've paid for and that never reaches the load, and, as covered in our piece on electrical safety, a potential ignition point too.

The most common mistakes

• Adding one new battery to old ones: the bank levels down to the weakest battery. House bank batteries are always replaced together, as a set.
• Sizing the charger "by eye": too small and it never completes a full charge, leaving the batteries chronically sulfated.
• Switching to lithium without touching the rest of the system: the alternator, charger and solar regulator all need compatible charging curves, and you need a properly integrated BMS.
• Relying on the alternator for the bulk of charging: it's meant to bring the batteries back up to a safe level quickly, not to complete the full charge cycle.
• Installing solar panels without checking for shading: position matters as much as rated power.
• Ignoring temperature: heat in the battery compartment shortens lead-acid battery life; sub-zero cold prevents lithium from charging without heating. Temperature compensation on the charger isn't optional.

The seasonal checklist for your power system

• Before the season: check the resting voltage of every battery in the bank, check terminals for tightness and cleanliness, and revisit your power budget in light of any loads added over winter. Every new piece of equipment changes the calculation.
• Mid-season: compare the battery monitor's data against your original estimate. If real consumption is much higher, hunt for the hidden load by isolating circuits one at a time.
• Every month in use: clean the surface of the solar panels — salt spray and dust can noticeably cut output — and check that the regulator is actually charging.
• End of season: never leave lead-acid batteries discharged during winter storage. Discharged lead sulfates and degrades within weeks. Maintain charge with a maintainer or the solar array, or remove the batteries and store them charged in a dry place.
• Every 2–3 years: a load test on the battery bank under controlled conditions. A battery can read a perfect voltage and still have lost half its capacity — only a load test will reveal that.

Conclusion: autonomy is a design choice, not a matter of luck

A power system that works isn't the one with the biggest batteries: it's the one where consumption, storage and charging are all matched to each other and to how you actually use the boat. Someone doing short daily hops with the engine running every day has different needs from someone spending three days at anchor, and the very same setup can be generous in one case and inadequate in the other.

Do the calculation once, install an instrument that tells you where you stand, and size the system to the numbers instead of to a feeling. That's the difference between a boat where power is never a worry, and one where it becomes the topic of conversation every evening.