The Offshore Voyaging Reference Site

Cruising Boat Electrical System Design, Part 1—Loads and Conservation

Two core decisions we must make when designing a cruising boat’s electrical system for living aboard full-time and making offshore voyages are:

  • the size of the battery bank, and
  • which charging sources we will need: generator, solar, main engine, wind, etc.

But the first thing we need to do, before getting into all that fun stuff, is think about electrical consumption and how to keep it reasonable.

That’s what I’m going to cover in this chapter, and then in the next we will move on to battery bank specification.

It’s Worth It

Whatever your situation, do make the effort to go through this process, particularly if you have not yet got out there cruising and/or done a multi-day passage, since unrealistic assumptions about power usage, leading to undersize battery banks and charging sources, is one of the most common cruise and/or voyage ruiners.

And even for me, after all my years of cruising successfully on the same boat, this exercise has proved both interesting and useful.

The Old Way

Typically, when designing an electrical system, we are supposed to fill out a form listing every single load on the boat in order to calculate our expected consumption.

But there are several problems with that approach:

  • It pushes us into thinking about the details ahead of the big picture—pretty much always a bad idea.
  • It’s difficult to estimate electrical usage for a given piece of kit. Sure we can look up the specification, but that doesn’t help much unless we can also make an accurate assessment of how much of the time the machine in question will be running, and how its consumption may vary over time; not always, or even often, easy.
  • On a modern cruising boat, with scores of machines that use electricity, this kind of bottom-up analysis will take a huge amount of time.

A Better Way

Given that, I’m going to come at this from the top down, by looking at the loads that really matter, along with rules of thumb for estimating them based on my some-25 years of living on boats.

Will this approach be dead-nuts accurate? No, but it will let us get as close as we need to. And, anyway, the inaccuracies in the bottom-up approach are probably as bad or worse—just because we write down a lot of numbers doesn’t make ’em right.

Do We Even Need To Fix It?

The other advantage of this approach is that we can reverse the process by using the same rules of thumb to understand what level of electrical consumption the system we already have will support (more on how in Part 2). Increasing battery bank size and charging sources is a huge project, at least when done right, that can seriously screw with our cruising budget both money- and time-wise.


When thinking about at-sea loads, I will be primarily focusing on sailboats, since a motorboat, at least one with proper alternators, does not need to worry about loads while underway. But I will also cover loads at anchor, which will be of interest to both sail and power owners.

A couple of conventions:

  • I will be expressing usage, load, and capacity in amp hours (Ah) for a 12-volt system. If you have a 24-volt system just divide my numbers by two.
  • When I write “day” I’m referring to 24 hours, unless otherwise stated.

Power Suckers

Let’s start by looking at the four horsemen of the flat battery: 

More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. 11 Steps To Better Lead Acid Battery Life
  33. How Hard Can We Charge Our Lead-Acid Batteries?
  34. How Lead Acid Batteries Get Wrecked and What To Do About It
  35. Equalizing Batteries, The Reality
  36. Renewable Power
  37. Wind Generators
  38. Solar Power
  39. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  40. Battery Monitors, Part 1—Which Type Is Right For You?
  41. Battery Monitors, Part 2—Recommended Unit
  42. Battery Monitors, Part 3—Calibration and Use
  43. Battery Containment—Part 1