Boat Electrical System Design: A Step-by-Step Guide
A boat's electrical system works only as well as its weakest part. Batteries, charging sources, inverter, wiring and protection have to be sized together, for the way you actually use the boat. This guide walks through the same steps our engineers follow when we design a system, with the formulas and rules of thumb behind each one.
1. Measure your daily energy use
Everything else is sized from one number: how many amp-hours (Ah) your boat uses in 24 hours. List every DC load with its current draw and the hours it runs per day, separately for time at anchor and under way. Loads that run through an inverter count too: divide their watts by the battery voltage and add about 10% for inverter losses.
Use our energy usage calculator for DC loads and the power consumption guide for typical AC appliance wattages.
| Example load (12 V boat at anchor) | Amps | Hours/day | Ah/day |
|---|---|---|---|
| Refrigerator (compressor duty cycle) | 5 | 12 | 60 |
| Anchor light (LED) | 0.2 | 12 | 2.4 |
| Cabin lights (LED) | 1.5 | 4 | 6 |
| Chartplotter, VHF, instruments | 3 | 4 | 12 |
| Laptop and phones through the inverter (100 W) | 9.3 | 3 | 28 |
| Total | about 110 Ah/day |
These figures are an illustration; use the ratings of your own equipment.
2. Size the battery bank
A battery bank must cover your daily use for the time between full charges, without discharging below what the chemistry tolerates.
Bank size (Ah) = daily use (Ah) × days between charges ÷ usable fraction
- AGM and flooded lead-acid: plan to use about 50% of rated capacity to get a reasonable cycle life.
- Lithium iron phosphate (LiFePO4): about 80% of rated capacity is a common design value, and lithium holds its voltage through the discharge.
For the example above, 110 Ah/day for 2 days needs about 440 Ah of AGM (110 × 2 ÷ 0.5) or about 275 Ah of LiFePO4 (110 × 2 ÷ 0.8). Our marine battery guide compares the chemistries, and the battery selector guide helps match a battery to the job. Browse marine batteries.
3. Choose the system voltage
Power is voltage times current, so the same load at 24 V draws half the current it does at 12 V, and at 48 V a quarter. Lower current means smaller cables, less voltage drop and less heat.
- 12 V suits most sailboats and small power boats, and most marine electronics run on it.
- 24 V makes sense for larger boats, long cable runs, big windlasses and thrusters, or inverters of about 3 kW and up.
- 48 V is used for electric propulsion and very large house banks.
Mixed systems are common: a 24 V or 48 V house bank with a DC-DC converter to feed 12 V electronics.
4. Plan the charging sources
Charging must replace what you use, plus losses. Most cruising boats combine several sources:
- Alternator. A stock alternator with an internal regulator charges slowly. An external smart regulator, see alternator regulators, or a higher-output alternator shortens engine run time. With lithium, protect the alternator from overheating with a regulator set up for lithium, or charge the house bank through a DC-DC charger such as the Victron Orion-Tr Smart.
- Solar. In good sun (about 4–5 peak sun hours), each 100 W of panel produces very roughly 25–35 Ah per day on a 12 V system with an MPPT controller. Shading cuts output sharply, so separate controllers for panels in different positions help. See choosing marine solar panels and solar charge controllers.
- Wind. Charges at night and in cloudy, windy weather; output rises with the cube of wind speed. See marine wind turbines.
- Hydrogenerator. Produces power from boat speed on passages. See hydrogenerators.
- Shore power and generator. A charger or inverter/charger sized at roughly 10–25% of bank capacity for lead-acid, or what the lithium battery maker allows, refills the bank quickly.
5. Size the inverter
Add up the watts of the AC loads you will run at the same time, then allow for the start-up surge of motors and compressors. Choose a pure sine wave inverter for sensitive electronics, see pure sine vs modified sine. An inverter/charger adds a battery charger and an automatic transfer switch for shore power or a generator.
Check the DC side too: battery current is about inverter watts ÷ (battery voltage × 0.9). A 2,000 W inverter running at full load on 12 V draws roughly 185 A, which sets the cable and fuse size. Browse marine inverters and inverter/chargers.
6. Wiring and protection
In the US, ABYC Standard E-11 (AC and DC Electrical Systems on Boats) is the reference for marine wiring. Key points:
- Voltage drop. ABYC allows a 3% drop for critical circuits such as navigation lights and electronics, and 10% for others. Size cables for the full round-trip length, positive and negative.
- Overcurrent protection. Put a fuse or breaker close to the battery, generally within 7 inches of the terminal, rated to protect the cable it feeds. Large lithium banks need fuses with a high interrupt rating, such as Class T fuses.
- Cable. Use stranded, tinned copper marine-grade cable, such as our marine grade wire.
- Shore power. A galvanic isolator or isolation transformer protects underwater metals from galvanic corrosion through the shore power ground.
7. Monitor the system
Battery voltage alone does not tell you the state of charge, especially with lithium. A shunt-based monitor such as the Victron SmartShunt or BMV-712 counts amp-hours in and out. A Victron Cerbo GX brings batteries, chargers, solar and inverter onto one screen, with remote monitoring.
When to have the system designed
A simple solar or battery upgrade can be done with the steps above. Lithium conversions, high-output alternators, multiple charging sources, or systems above a few kilowatts are worth engineering as a whole. Our team designs complete systems. One example: a Fountaine Pajot 57 with over 3 kW of solar on five MPPT controllers feeding a 25.6 V, 1,200 Ah lithium bank monitored by a Cerbo GX.
See our marine electrical system design service, or contact us with your boat, your loads and how you cruise.