How to Calculate Your Boat's Daily Power Use
Calculate daily Wh and kWh for fridges, Starlink, autopilot, laptops, watermakers, washing machines, inverters and air conditioning.
Do not start by asking, "How many batteries should I buy?"
Start with:
What does the boat actually use in 24 hours?
Core formula
Daily Wh = Watts x hours used per day x duty cycle
Then add all loads.
If an appliance runs through an inverter, allow for inverter and standby losses.
Example liveaboard load audit
| Appliance | Assumption | Daily energy |
|---|---|---|
| Fridge | 50W, 35% duty, 24h | 420Wh |
| Freezer | 55W, 35% duty, 24h | 462Wh |
| Navigation electronics | 35W x 12h | 420Wh |
| Autopilot | 45W x 8h | 360Wh |
| Starlink Standard | 85W x 24h | 2,040Wh |
| Two laptops | 140W x 3h | 420Wh |
| TV | 50W x 4h | 200Wh |
| Lights and pumps | 60W x 5h equivalent | 300Wh |
| Induction cooking | 1,500W x 0.5h | 750Wh |
| Washing machine | 600Wh cycle every 3 days | 200Wh/day average |
| Watermaker | 300W x 1.5h | 450Wh |
Example total: approximately 6.0kWh/day.
Add a 700W small air conditioner for three hours:
700W x 3h = 2.1kWh
The same boat is now near 8.1kWh/day before extra losses or unusually heavy use.
Why measured data is better
Measure:
- fridge duty cycle in hot weather
- autopilot draw in real sea conditions
- watermaker consumption
- inverter idle draw
- Starlink actual consumption
- laptop charging
- pumps
- electric cooking
- air conditioning startup and steady load
Apply this guidance to your boat
Use this page as one input to a vessel-specific plan. Record the actual equipment, cable runs, charging sources and operating pattern aboard before choosing hardware or changing the installation.
- Measure the real load and its operating time instead of relying on a label or a generic boat profile.
- Check every proposed current against the battery, BMS, protection, cable and connection limits.
- Confirm the result against current manufacturer instructions and the rules that apply to your vessel.