The essentials in 30 seconds
- 5-step workshop method: (1) 24-hour consumption inventory by load, (2) simultaneity coefficient 0.6-0.75 depending on usage, (3) safety margin 30% for rough seas/overcast skies, (4) useful DoD 80%, (5) final capacity = demand × margin / DoD.
- Typical 40-foot coastal cruising sailboat: demand 80-110 Ah/24 h → gross lithium capacity 150-200 Ah. With continuous autopilot + fridge + Starlink, increase to 280-350 Ah.
- Offshore transatlantic without solar: double the capacity. Fridge and autopilot run 24/7, AIS stays on, no shore power for charging. Plan 400-600 Ah for 14 days of autonomy.
- Lead → LFP conversion: 200 Ah AGM (50% usable) = 100 Ah usable ≈ 130 Ah LFP (80% usable). Multiply lead capacity by 1.3 to get equivalent usable LFP capacity.
- Mistake #1: sizing based on average consumption instead of the 24-hour peak. A fridge running 16 h/24 at 35 °C in the Mediterranean weighs 4× more than its manufacturer's "average consumption" rating.
Choosing a lithium battery bank for a sailboat comes down to three questions, in order: how many Ah, which brand, which BMS. 90% of failed refits skip the first and start with the second. Result: a bank that’s either too small (discharges in 2 days at anchor) or too large (80 kg heavier than necessary and €1,500 over budget).
This article covers ONLY the first question: how many Ah for your sailboat, your program, your real consumption. For brand/BMS selection, see our 2026 marine lithium BMS comparison. For plug-and-play pitfalls, see switching to lithium.
Why size before choosing a brand
Three technical reasons make prior sizing non-negotiable before selecting a brand:
- Price per usable kWh varies by 50% between a Victron Smart 12.8 V 200 Ah (€1,183 for 1.6 kWh usable at 80% DoD) and a Victron 25.6 V 200 Ah (€2,263 for 4.1 kWh usable). Without knowing how many usable Ah you need, you cannot compare.
- Weight and footprint must fit the existing technical compartment. A MG Energy 24 V 304 Ah weighs 49 kg and measures 545 × 290 × 230 mm — check before ordering.
- The BMS depends on max current, which depends on capacity and program. For 100 Ah usable, a Smart BMS CL 12-100 at €179 is enough. For 400 Ah usable in offshore racing, you need the Lynx Smart 500 at €1,019.
Step 1 — 24-hour consumption inventory by load
List ALL 12 V (and 24 V if your boat is 24 V) loads on the boat, with their average current and 24-hour usage time under typical sailing conditions. Not ideal, not worst-case — typical.
Typical magnitudes observed in the workshop (40-foot cruising sailboat 2026):
| Load | 12 V current (A) | Hours / 24 h | 24-hour consumption (Ah) |
|---|---|---|---|
| Autopilot (calm seas) | 1-3 A | 24 h | 24-72 Ah |
| Autopilot (rough seas) | 3-6 A | 24 h | 72-144 Ah |
| Indel/Vitrifrigo fridge (cruising) | 3-5 A (cycling) | 12-16 h | 36-80 Ah |
| Cruising fridge at 30°C+ | 3-5 A (cycling) | 16-20 h | 48-100 Ah |
| Navigation displays (1 9-12" chartplotter) | 1-2 A | 12 h | 12-24 Ah |
| VHF standby + AIS Class B | 0.3 A | 24 h | 7 Ah |
| Starlink Mini (see dedicated article) | 2-4 A | 8-16 h | 16-64 Ah |
| LED interior lighting | 0.5-1 A | 4 h | 2-4 Ah |
| Water pump + bilge pump | 5-8 A (cycling) | 0.5 h | 3-4 Ah |
| USB charger / laptop | 2-4 A | 4 h | 8-16 Ah |
| Continuous AC inverter (coffee machine, etc.) | 8-15 A | 1 h | 8-15 Ah |
For precise measurement, two options:
- From datasheet: each device publishes its typical consumption. Multiply by actual 24-hour usage hours.
- Via battery shunt: install a Victron BMV-712 Smart monitor on the current bank, sail for 1-2 weeks under typical conditions, and read the 24-hour average consumption in the history. This is the most reliable workshop method: you measure the real, not the theoretical.
Step 2 — Simultaneity coefficient
Not all loads run at the same time. The autopilot works at sea, not at anchor. The fridge cycles, it doesn’t run continuously. Displays turn off at night. The simultaneity coefficient corrects the gross total.
- Coefficient 0.75 for summer coastal cruising (fridge, water, ventilation, alternating displays).
- Coefficient 0.65 for offshore cruising (continuous autopilot, continuous fridge, continuous AIS, rotating crews).
- Coefficient 0.55 for seasonal comfort at anchor (continuous fridge, lights, computer, less sailing).
Example: if you add up 145 Ah of theoretical gross consumption, multiply by 0.7 (typical coefficient) → real demand ~100 Ah / 24 h. This is the 100 Ah you need to cover, not the 145 gross.
Step 3 — Safety margin for rough seas and overcast skies
The previous calculation is valid under typical conditions. Reality includes atypical days:
- Rough seas: the autopilot consumes 2-3× more to hold course (permanent rudder correction). Add +50% to the autopilot load for the 20% of days in heavy weather.
- Heatwave in the Mediterranean: the fridge cycles 18-20 h / 24 instead of 12-14 h. Add +30% to the fridge load for tropical and summer zones.
- Several days of overcast skies: solar panels deliver 0.2-0.4× their nominal peak output. If your sizing relies on solar to cover 50% of demand, plan 3-5 days of autonomy without sun.
Workshop rule of thumb: increase the demand from Step 2 by 25-35% overall safety margin. On our example 100 Ah / 24 h → increased demand 130-135 Ah / 24 h.
Step 4 — Useful depth of discharge (DoD)
A lithium LFP battery technically supports 100% discharge, but commercial marine BMS typically cut at 90-95% to preserve cycle life. The industry standard is 80% useful DoD, which maximizes the ratio of usable capacity to longevity.
The calculation is simple: gross capacity = increased demand ÷ 0.8.
In our example, 130 Ah / 24 h ÷ 0.8 = gross capacity 162 Ah. You therefore buy 175-200 Ah gross to have 130 Ah usable after safety margin and depth of discharge.
Step 5 — Final capacity and product selection
With the calculated gross capacity, choose the most suitable battery combination. A few rules:
- 12 V is the pleasure-craft standard. Beyond 400 Ah gross, consider 24 V (lower cable losses, smaller cross-sections).
- Prefer 1-2 large batteries over a bank of 4-6 small ones. Fewer connections = fewer failure points.
- Check weight and dimensions before ordering. A Victron 12 V 330 Ah Smart weighs 45 kg.
Numbered examples: 30 / 40 / 50-foot sailboats
Three concrete workshop cases, with step-by-step calculations. Conditions: summer coastal cruising in the Mediterranean for the 30 and 40-footers, offshore transatlantic for the 50-footer.
30-foot sailboat — weekend cruising and 1-2 weeks in summer
- Gross 24-hour consumption: autopilot (0.5 A × 8 h) + fridge (3.5 A × 14 h) + display (1 A × 6 h) + VHF (0.3 A × 24 h) + LED + USB ≈ 65 Ah/24 h gross
- With simultaneity 0.75: 49 Ah/24 h real
- With +30% safety margin: 64 Ah/24 h increased demand
- Gross capacity (DoD 80%): 80 Ah
- Workshop solution: 1 × Victron SuperPack 12 V 100 Ah or 2 × Smart 12 V 200 Ah if shared engine + house bank
- Pack + BMS + accessories budget: €1,500-2,500 VAT excluded installed
40-foot sailboat — summer cruising 2-4 weeks + coastal hopping
- Gross 24-hour consumption: autopilot (2 A × 16 h) + fridge (4 A × 16 h) + 2 displays (1.5 A × 10 h) + VHF/AIS (0.3 A × 24 h) + Starlink (3 A × 12 h) + LED + laptop ≈ 145 Ah/24 h gross
- With simultaneity 0.7: 102 Ah/24 h real
- With +30% safety margin: 133 Ah/24 h increased demand
- Gross capacity (DoD 80%): 167 Ah
- Workshop solution: 1 × Victron Smart 12 V 200 Ah (€1,183) if no electric motor, or 1 × Victron 12 V 330 Ah (€1,765) for comfortable margin with occasional AC inverter
- Pack + BMS + accessories budget: €3,500-5,500 VAT excluded installed
50-foot sailboat — transatlantic + offshore cruising
- Gross 24-hour consumption: continuous autopilot in rough seas (4 A × 24 h) + fridge + freezer (5 A × 18 h) + 3 displays (1.5 A × 18 h) + VHF/AIS + Starlink + nav computer + watermaker (8 A × 2 h) ≈ 270 Ah/24 h gross
- With simultaneity 0.65: 175 Ah/24 h real
- With +35% safety margin: 236 Ah/24 h increased demand
- Gross capacity (DoD 80%): 295 Ah
- Workshop solution: 1 × MG Energy 24 V 304 Ah (€2,990) or 2 × Victron 24 V 200 Ah Smart-a in parallel (€4,526)
- Pack + BMS + accessories budget: €6,500-9,000 VAT excluded installed
Lead → LFP conversion: practical rule
If you’re replacing an existing lead bank that meets your needs, do NOT calculate the gross equivalent. Lithium is used differently.
- Lead AGM/Gel: recommended DoD max 50% (beyond that, cycle life drops by 4×).
- Lithium LFP: 80% DoD standard, i.e., 1.6× more usable per Ah gross.
Workshop rule: multiply the gross lead capacity by 0.65-0.75 to get the equivalent LFP capacity. Example: 300 Ah AGM (150 Ah usable at 50% DoD) → 200 Ah LFP (160 Ah usable at 80% DoD).
Important: NEVER mix lead and lithium in parallel or series. Charge voltages and profiles differ — risk of overcharging lithium and sulfation of lead.
5 workshop pitfalls to avoid
Sizing mistakes — seen in the workshop
- Sizing for the absolute worst case. "What if I want to boil an electric kettle 230 V at anchor..." → you end up with 800 Ah that you’ll never use more than 200 Ah of. Direct cost: +€3,500 + 60 kg of weight. Prefer typical sizing + safety margin.
- Ignoring engine start surge. The starter draws 200-400 A for 2-3 seconds. Many lithium BMS cut at 200 A continuous, so it’s impossible to start the engine from the house bank. Solution: keep a dedicated lead starting battery (40-80 Ah AGM is enough) + DC-DC to recharge from lithium.
- Forgetting BMS self-discharge. Marine BMS draw 0.3-1 A continuously for their electronics (sensors, communication, cell balancing). Over 30 days without recharge, that’s 7-25 Ah lost. Plan for this if the boat is at anchor for long periods.
- Not checking alternator compatibility. A standard 80 A alternator won’t handle lithium demanding 100 A for long. See the plug-and-play article for DC-DC solutions or external regulator.
- Buying "round up to the next unit" without checking compartment dimensions. A 200 Ah Victron Smart measures 197 × 410 × 321 mm. If your compartment is 180 × 400 × 300 mm, it won’t fit. Always measure BEFORE ordering.
FAQ — Sizing a lithium battery bank for a sailboat
How do I measure my real consumption before sizing?
Install a battery monitor like the Victron BMV-712 Smart (€215) with a 500 A shunt on the battery negative. Sail for 1-2 weeks under typical conditions. Read the 24-hour average consumption in the VictronConnect app (history). This is the most reliable method because it captures all real-world usage, including things you forget on paper (USB chargers left plugged in, cable losses, etc.). Plan 1 hour of workshop installation time.
12 V or 24 V for my sailboat?
12 V below 350 Ah gross demand. 24 V above 400 Ah gross, or if electric propulsion. 24 V reduces cable losses (current is 2× lower for the same power) but requires conversion to 12 V for N2K equipment, standard fridges, displays. The Victron Orion-Tr 24/12-20A isolated converter handles this (€171). Above 600 Ah gross, 48 V becomes relevant (yachts, polar expeditions).
Should I plan a separate starting battery?
Yes in 95% of cases. The diesel starter draws instantaneous currents (200-400 A) that often exceed the continuous capacity of the lithium house BMS. Standard solution: 1 dedicated lead starting battery (40-80 Ah AGM, €80-150) for the engine, recharged from the lithium bank via an isolated DC-DC. This redundancy also protects against BMS failure — you can still start the engine no matter what.
How much solar should I plan for a given lithium bank?
Workshop rule: 1 Wp of panel for 1 Ah of gross lithium capacity in summer cruising. So 200 Wp minimum for a 200 Ah bank, 400 Wp for 400 Ah. With a Victron SmartSolar MPPT 100/30 controller (€137), a 140 W Victron panel produces 50-80 Ah/24 h in the Mediterranean in June-August. Double this for offshore cruising without daily engine recharge.
How many years will my lithium bank last?
At 80% useful DoD in typical cruising (200-300 cycles/year): 15-20 years for Victron (5,000 cycles datasheet), 12-15 years for Mastervolt (4,000 cycles), 20-25 years for MG Energy (8,000 cycles). In intensive offshore use (400-500 cycles/year), halve these lifespans. The #1 factor that reduces real lifespan: heat. Cell at a constant 45 °C = -40% cycle life vs cell at 25 °C. Ventilating the battery compartment is non-negotiable.
Can I add an LFP battery later to expand the bank?
Yes in the first 6-12 months, after that it’s complicated. LFP cells are factory-matched for precise characteristics (real capacity, internal resistance). Mixing a new cell with one that’s 2 years old in parallel creates permanent imbalance that reduces usable capacity and stresses the BMS. If future expansion is planned, buy the maximum capacity envisaged from the start — for brands that allow it (MG Energy), expansion kits may still be available later.
What cable diameter between battery and switchboard?
Standard calculation: 1 mm² for 5 A continuous at 12 V, with margin. For 200 A continuous (Lynx Smart 500 output), plan 35-50 mm² copper. Length also matters: 1 m = 50 mm², 2 m = 70 mm². Voltage drop must stay under 3% between battery and fuse. For Lynx distributions, the Lynx 1000 DC Distributor (€226) neatly centralizes the outputs with M8 busbars.
Skysat has distributed Victron Energy, Mastervolt, and MG Energy Systems since 2018. This sizing method is the one we apply systematically to the 25-35 sailboat energy refits performed annually in our workshop. Consumption figures are based on shunt measurements on over 80 sailboats between 2022 and 2026. 2026 prices are indicative distributor prices excluding VAT, wiring, and workshop labor.
