How to size a lithium battery bank for your boat

The most common mistake with lithium upgrades is guessing the size. Too small and you run out on the second evening at anchor. Too big and you pay for capacity you never use and cannot recharge. Sizing properly takes about half an hour with a notepad.

Step 1: List every consumer

Write down everything that uses power, its power draw in watts and how many hours a day it effectively runs. The draw is on the label or in the manual. For things that cycle on and off, like a fridge, estimate the time the compressor actually runs.

Here is an example for a 12 V sailing yacht spending a summer day at anchor and an evening on board. Replace the numbers with your own.

Consumer	Power (W)	Hours per day	Energy (Wh)
Fridge (compressor running about a third of the time)	50	8	400
Navigation instruments and plotter	30	6	180
Anchor light	3	10	30
Interior LED lights	20	4	80
Water pump	60	0.5	30
Phones, laptops and speaker	60	3	180
Small inverter loads	300	0.5	150
Total per day			1,050

Step 2: Convert to amp-hours

Divide watt-hours by the battery voltage. A 12 V LiFePO4 bank has a nominal voltage of 12.8 V.

1,050 Wh ÷ 12.8 V ≈ 82 Ah per day

Step 3: Decide how many days without charging

How long do you want to last if there is no sun and you do not run the engine? For day charters returning to a marina each night, one day may be enough. For cruising, one and a half to two days is a common target.

82 Ah × 2 days = 164 Ah

Step 4: Add a margin for usable capacity and ageing

Although LiFePO4 can deliver nearly all its rating, keeping a reserve protects the battery and leaves a buffer for ageing, cold and mistakes. Dividing by 0.8 is a sensible margin.

164 Ah ÷ 0.8 ≈ 205 Ah

In this example a 200 Ah LiFePO4 bank would be a good fit, for instance one 200 Ah battery or two 100 Ah batteries in parallel.

Step 5: Check your charging can keep up

A bank is only as good as your ability to refill it. Compare your daily use with what you can put back:

• Solar: in summer in Mallorca, a rough planning figure is four to five equivalent full-sun hours per day. A 300 W array could then deliver in the order of 1,000 Wh on a good day, less with shading from the boom, mast and bimini.
• Alternator: usually limited by a DC-DC charger to protect it, for example 30 to 50 A while the engine runs.
• Shore power: easiest, if you return to a marina.

If your charging cannot cover your daily use, either reduce consumption, add charging, or accept that you will need shore power more often. A bigger battery alone does not solve that.

Step 6: Think about peak loads

Check the battery's continuous and peak current ratings against your largest loads. An inverter running a kettle or coffee machine can draw well over 100 A at 12 V for a short time. This can decide whether you need a larger battery, more batteries in parallel, or a higher system voltage.

A quick sanity check

• Daily use and bank size should be in proportion: a bank of around two days' consumption is a good starting point for most cruising boats.
• Your daily charging should be able to replace a day's use.
• Your peak loads should stay within the battery's continuous rating.

If all three are true, you are close to the right size. If you are in Palma and want help with the numbers, the Sail Mallorca team, who write this guide, offer energy system consultations.