The essentials in 30 seconds
- 5-step workshop method: (1) 24 h consumption inventory per load, (2) simultaneity coefficient 0.6-0.75 depending on usage, (3) safety margin 30 % in 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, this rises to 280-350 Ah.
- Offshore transatlantic without solar: double the capacity. Fridge and autopilot run 24/7, AIS stays on, no shore power to recharge. Plan for 400-600 Ah for 14 days of autonomy.
- Lead-acid → LFP conversion: 200 Ah AGM (usable 50 %) = 100 Ah useful ≈ 130 Ah LFP (usable 80 %). Multiply lead-acid capacity by 1.3 to get the equivalent usable LFP capacity.
- #1 mistake: sizing on average consumption instead of the 24 h peak. A fridge running 16 h/24 at 35 °C in the Mediterranean consumes 4× more than its manufacturer’s "average consumption" rating.
Choosing a lithium battery bank for a sailboat boils down to three questions, in order: how many Ah, which brand, which BMS. 90 % of failed refits skip the first question and start with the second. Result: a bank that’s either too small and dies in 2 days at anchor, or too large, adding 80 kg of unnecessary weight and costing €1,500 extra.
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 & play pitfalls, see switching to lithium.
Why size before choosing a brand
Three technical reasons make pre-sizing non-negotiable before brand selection:
- 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 h consumption inventory per load
List ALL 12 V (and 24 V if your boat is 24 V) consumers on board, with their average current and 24 h usage time in typical sailing conditions. Not ideal, not worst-case — typical.
Orders of magnitude observed in the workshop (40-foot cruising sailboat 2026):
| Load | Current 12 V (A) | Hours / 24 h | 24 h consumption (Ah) |
|---|---|---|---|
| Autopilot (calm seas) | 1-3 A | 24 h | 24-72 Ah |
| Autopilot (rough seas) | 3-6 A | 24 h | 72-144 Ah |
| Cruising fridge (Indel/Vitrifrigo) | 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 chartplotter 9-12") | 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 piece of equipment publishes its typical consumption. Multiply by actual hours of use over 24 h.
- Via battery shunt: install a Victron BMV-712 Smart monitor on the existing bank, sail for 1-2 weeks in typical conditions, read the average 24 h 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 are off at night. The simultaneity coefficient corrects the gross total.
- Coefficient 0.75 in summer coastal cruising (fridge, water, ventilation, alternating displays).
- Coefficient 0.65 in offshore cruising (continuous autopilot, continuous fridge, continuous AIS, rotating crews).
- Coefficient 0.55 at seasonal comfort anchorages (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 100 Ah is what you need to cover, not the 145 gross.
Step 3 — Safety margin for rough seas and overcast skies
The previous calculation is valid for typical conditions. Reality includes atypical days:
- Rough seas: the autopilot consumes 2-3× more to hold course (correction from permanent helm). 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 for 3-5 days of autonomy without sun.
Workshop rule of thumb: increase the demand from Step 2 by 25-35 % global safety margin. On our 100 Ah/24 h example → 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.
On 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 relevant battery combination. A few rules:
- 12 V is the pleasure-craft standard. Above 400 Ah gross, consider 24 V (lower cable losses, smaller cross-sections).
- Prefer 1-2 large batteries over 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.
Worked examples: 30 / 40 / 50-foot sailboats
Three real 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 h 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 safety margin +30 %: 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 ex-works installed
40-foot sailboat — summer cruising 2-4 weeks + coastal hopping
- Gross 24 h 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 safety margin +30 %: 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 ex-works installed
50-foot sailboat — Atlantic transatlantic + offshore cruising
- Gross 24 h 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 safety margin +35 %: 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 ex-works installed
Conversion from lead-acid to LFP: practical rule
If you are replacing an existing lead-acid bank that works for you, do NOT calculate the equivalent gross capacity. Lithium is used differently.
- Lead-acid AGM/Gel: recommended DoD max 50 % (beyond that, cycle life drops by 4×).
- Lithium LFP: standard DoD 80 %, i.e. 1.6× more usable Ah per gross Ah.
Workshop rule: multiply the lead-acid gross 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-acid and lithium in parallel or series. Charge voltages and profiles differ — risk of overcharging the lithium and sulfating the lead-acid.
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 beyond 200. Direct cost: +€3,500 + 60 kg extra weight. Prefer typical sizing + safety margin.
- Ignoring engine start-up surge. The starter draws 200-400 A for 2-3 seconds. Many lithium BMS cut at 200 A continuous, so you cannot start the engine from the house bank. Solution: keep a dedicated lead-acid start battery (40-80 Ah AGM is enough) + DC-DC charger to recharge from the lithium bank.
- Forgetting BMS self-discharge. Marine BMS draw 0.3-1 A continuously for electronics (sensors, communication, cell balancing). Over 30 days without recharge, that’s 7-25 Ah lost. Budget for this if the boat is at anchor for long periods.
- Not checking alternator compatibility. A standard 80 A alternator won’t tolerate a lithium bank that demands 100 A for long. See the plug & play article for DC-DC or external regulator solutions.
- 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 in typical conditions. Read the average 24 h consumption in the VictronConnect app (history). This is the most reliable method because it captures all real-world usage, including what you forget on paper (USB chargers left plugged in, cable losses, etc.). Allow 1 h 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 DC-DC handles this conversion (€171). Above 600 Ah gross, 48 V becomes relevant (yachts, polar expeditions).
Should I plan a separate start 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-acid start battery (40-80 Ah AGM, €80-150) for the engine, recharged from the lithium bank via an isolated DC-DC charger. 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 per 1 Ah of gross lithium capacity in summer coastal 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), expect 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-world life: 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 on precise characteristics (real capacity, internal resistance). Mixing a new cell and a 2-year-old cell in parallel creates permanent imbalance that reduces usable capacity and stresses the BMS. If expansion is planned later, buy the maximum capacity from the start — for brands that allow it (MG Energy), expansion kits may still be available later.
What cable cross-section 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 for 35-50 mm² copper. Length matters too: 1 m = 50 mm², 2 m = 70 mm². Voltage drop must stay below 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 on the 25-35 sailboat energy refits carried out annually in our workshop. Consumption figures come from shunt measurements on more than 80 sailboats between 2022 and 2026. 2026 ex-works prices are indicative distributor prices, excluding cabling and workshop labor.
