How Much Battery Capacity Do You Need on Your Boat?
Once you know how much solar you need, it’s time to figure out how much battery storage you’ll need to keep everything running — even when the sun isn’t out.
Think of your batteries like your fuel tank. Solar charges them, your equipment drains them. It’s all about balance.
Step 1: Know Your Daily Energy Use
Refer back to your energy audit. How many watt-hours do you use in a typical day?
Example: 1500Wh/day.
Step 2: Decide How Many Days You Want to Last Without Sun
Are you out just for a weekend? Or do you want enough backup to sail through 3–5 days of cloudy weather?
Multiply your daily usage by your target days of autonomy:
1500Wh/day × 3 days = 4500Wh of usable battery capacity
Step 3: Adjust for Battery Type
Different batteries give you different usable capacity.
Not sure if you're working with lithium, AGM, or something else? We break that down here.
Just make sure your calculation is based on usable energy — not just the sticker rating.
Step 4: Align With Your Solar Setup
Your solar setup needs to keep up with your daily energy use and recharge your batteries. There’s no point having massive battery storage if you can’t fill it. Likewise, don’t skimp on battery space if you plan to go off-grid for days.
Bottom line:
Your battery bank should cover your energy needs for your target number of sunless days. Plan based on your usage, your cruising style, and your goals for self-sufficiency.
Next up: how much solar can you actually fit on your boat?