Battery sizing for electric boat range

People often size motors first and batteries second. But the real question is:

How many kWh do you need for your actual use?

→ Calculate your kWh + charge time: /electric-boat-spec

Step 1: Decide your runtime at your chosen power

Energy is simple:

Energy (kWh) = Power (kW) × Time (hours)

If you cruise at 8 kW for 2 hours:

• shaft energy ≈ 16 kWh

But that’s not your pack size yet.

Step 2: account for system efficiency

Battery-to-shaft efficiency includes:

• inverter/controller losses
• motor losses
• drivetrain losses

If efficiency is 90%:

• battery energy ≈ shaft energy / 0.9

Step 3: choose a usable DoD

You rarely want to use 100% of a lithium pack.

A typical usable DoD target might be 70–90% depending on chemistry and lifecycle goals.

Step 4: add reserve

Reserve is what keeps you from getting trapped when:

• wind dies
• current increases
• conditions change

Reserve is often 10–20% for sane auxiliary planning.

Step 5: convert kWh to Ah (only after choosing voltage)

Ah depends on nominal voltage:

Ah ≈ (kWh × 1000) / V_nom

That’s why you shouldn’t “pick Ah” without voltage context.

Charging reality check

Charge time ≈ pack kWh / charger kW (plus losses and taper).

If your pack is large and your charger is small, you may wait days—not hours.

→ Use the wizard to see charge time immediately: /electric-boat-spec