Since starting sailing offshore singlehanded, I’ve received a number of questions about electrical setups. When I purchased Idefix, she had the minimalist electrical system of an inshore racer: running lights, a VHF radio, speed and wind instruments, and a small battery. This clearly wasn’t sufficient for what I planned to do, so I had a lot of number crunching and wiring to do. Here’s how I went about it.

The Basics

There are a few basic things you need to know about electricity before reading this. If you’re an electrical genius, you might want to skip to the next part. First of all, units:

  • Electrical potential is measured in Volts (V) – this is used to measure the electrical charge on a battery. Boats usually use a 12 Volt electrical system. This is only a nominal voltage, a typical battery will show somewhere between 14.4 and 12.5 Volts. More on this later, but if it’s showing 12 V, you need to charge it ASAP!
  • Electrical current is measured in Amperes (A), also known as « amps ». This is the measure of flow of electricity.
  • Electrical power is measured in Watts (W).
  • Electrical resistance is measured in Ohms (Ω). This is typically a property inherent to electrical circuits. An open circuit (no electrical connection) is infinite Ohms. A short circuit (perfect connection) is zero ohms.
  • Battery capacity is measured in Ampere-hours (Ah), also known as « amp-hours ». 1 Ah is equivalent to a 1A current drawn for one hour, or a 2A current over half an hour.

Electricity follows a couple basic laws that you might remember from your high school or college physics classes:

  1. Ohm’s Law: Current = Voltage / Resistance
  2. Joule’s Law: Power = Voltage * Current

We won’t deal with resistance much here, so focus is really on the second one. For example, a 20 Watt incandescent light at 12V draws 20/12 = 1.67 Amps. Finally, most things on boats (batteries or devices) are connected in parallel (positive to positive and negative to negative), which means that their currents add up, but voltages do not. Putting two 12V, 110Ah batteries in parallel means that you have 220Ah of battery capacity at 12V. Wiring them in series would mean that you have 110Ah at 24V.

Electrical Budget

The first step to equipping any boat’s electrical system to be independent offshore is to make a budget of all the electrical power consumed. It helps to be conservative here. Once that’s done, one can choose a scheme for power generation that adequately covers the losses for the time spent offshore. Here is my electrical budget for Idefix:

amps hours/day Ah/day
power budget: Autopilot 1 18 18
AIS 0.25 10 2.5
laptop (7.4W) 0.9 10 9
VHF radio 2.08 0.2 0.416
nav lights 0.5 10 5
instruments 0.32 24 7.68
music 0.42 4 1.68
HF radio 2.5 5 12.5
Ah/day 56.77
Ah for 16 days 908.41

Power consumption for each item on board can be easily measured using an ammeter or battery monitor, or estimated from specifications. The hardest part is to have a conservative but realistic estimate of how much you will use each item every day. The one I’m showing above was my « rainy day » budget, and most of the time I ended up running the AIS, laptop and music almost 24 hours a day. As you see, I had to figure out how to make up a little under 60 Ah per day. The total use for a 16 day trip (about how long a transpac could expect to take for a boat with an intact hull and rig) was over 900 Ah. To give you an idea how much power that is, running your standard 60W incandescent light bulb for the same amount of time would consume about 2000 Ah. the average car battery holds about 70 Ah. So on average, Idefix at sea runs at less than half a light bulb. I also came up with a second electrical budget for the 400-mile qualifier I had to do. Since this was going to be a much shorter trip, I was going to push the limits a little more, hand steer the boat most of the way, and rely more on VHF for weather:

amps hours/day Ah/day
power budget: Autopilot 1 10 10
AIS 0.25 3 0.75
laptop 2 3 6
HF radio 2 0.2 0.4
nav lights 0.5 9 4.5
instruments 0.32 24 7.68
other 0.42 3 1.26
Ah/24 30.59
Ah total 143.39

The planned daily power consumption for my qualifier was going to be almost half of what it would be during a transpac.


My choice of batteries was driven by a couple constraints:

  • Capacity: I wanted to be able to run my electrical system for the four days it would take me to do the 400-mile SHTP qualifier without recharging or damaging the batteries. Most flooded lead-acid batteries can only be discharged to about 50% without damage. This means I need a capacity of about 143/0.50 = 286 Ah. An AGM battery can withstand a discharge of up to 80%, so I could get an AGM battery of only 143/0.80 = 179 Ah.
  • Space: To keep the batteries low, centered, secured, and covered, I was limited to the space under the benches in the middle of the cabin. This meant a maximum battery size of about 16″ x 9″.
  • Safety: for redundancy, I needed two batteries that I could isolate in case one failed. I also wanted a sealed lead-acid or AGM battery that wouldn’t spill acid when the boat heeled.
  • Budget: you’ve probably understood by now that I’m a cheapskate. I wanted the least expensive batteries that met my criteria.
  • Weight: I wanted my batteries as light as possible for a given capacity.

With these criteria, I ended up buying two Universal UB121100 110Ah AGM batteries. They’re about 70lbs each, and fit nicely under the benches. They ended up providing plenty of power, and with my meager electrical consumption I could run for 4 or 5 days without any charging.

Solar Power

Idefix with solar panels - ©2010 Latitude 38 - LaDonna Bubak

Idefix with solar panels – ©2010 Latitude 38 – LaDonna Bubak

The batteries alone wouldn’t suffice for crossing the Pacific, so I had to think of a way to charge them. The Olson 30 has no inboard engine, and therefore no alternator. A gasoline-powered generator would’ve been about $1000, and I didn’t like the idea of flammable fuel and a loud and stinky mess on the boat. Wind generators were a little more expensive, and I ruled them out because they’re also noisy, heavy, and don’t put out enough power downwind. I pretty quickly focused on solar power, as it was quiet, clean, low weight, and cheap compared to other options. There are a couple things to know about solar panels. For one, they rely on weather, which is a notoriously unpredictable thing. Clouds will cut down on solar power production dramatically. So will the shadow of a sail, mast or rigging. So will a solar panel that’s not pointed directly at the sun. One can dedicate a lot of effort to calculating what the output of a particular solar installation would be on a given day in a given latitude, but since I was going to be cruising from the cloudy environs of 38 North to the mottled skies of the tropics, I simply came up with some rough figures: an efficiency of about 50% to account for controller losses, shadows and angles, and an illumination of 8 hours per day to account for nighttime and low sun angle. A solar panel typically isn’t wired directly to a battery, but through a device called a charge controller. There are several types of charge controllers, the most popular of which are PWM and MPPT. I selected an MPPT charge controller as they are significantly more efficient than PWM controllers. The charge controller losses were factored in my 50% efficiency.

generation: installed P 175
Actual P 87.5 0.5 efficiency
Ah/24 58.3 8 hours sun/day

If we go back to our transpac power consumption of 56.77 Ah/day, that gives us a requirement of 56.77 * 12 = 681 Wh/day. With 8 hours of power-generating sun a day we need 681/8 = 85 Watts, and at 50% efficiency that means 170 W of installed solar panels. I found a reasonable deal on a 135W panel and a 40W panel, so I ended up with 175W installed, with the two panels in parallel. I wanted to have two separate panels in case one cracked, and the 40W panel would’ve allowed me to run running lights and send out a message. I didn’t really know where to put it, so it ended up forward of the companionway under the boom, which means it was in the shade most of the time.

Interestingly, solar panels have become much more affordable and much better since I first equipped Idefix in 2010. The lightweight, flexible panels in particular are very attractive, and it’s only a matter of time before we see sails capable of gathering power.

Other Means of Generation

Solar was the clear choice for what I wanted to do, but there are other options to consider. Of course if your boat has a diesel engine, you have a fair bit of power at your disposal already. Fuel cells can be attractive too, being clean, quiet and powerful, although they are expensive and methanol fuel can be difficult to find. Finally, hydrogeneration seems to make leaps and bounds, with many of the IMOCA 60s now using hydrogenerators for power. Watt&Sea makes one that sells for a little under $4k.

Battery Monitor

A good battery monitor is a critical part of the installation. The monitor simply counts the current flowing in or out of the batteries and integrates current flow over time. It can therefore display the state of charge of the batteries at any given point in time, which is extremely useful. Like I mentioned earlier, most lead-acid batteries cannot tolerate being discharged past 50% of their capacity. I have seen several sailors deep-discharge their batteries and pointlessly ruin them. Most monitors have alarms set to prevent this. It’s also critical that everything be wired through the monitor to get an accurate picture of the state of charge of the batteries.


Our final power budget adds the power generation and subtracts the consumption to come up with a daily figure. Obviously we want this to be positive.

amps hours/day Ah/day
consumption: Autopilot 1 18 18
AIS 0.25 10 2.5
Laptop (7.4W) 0.9 10 9
VHF radio 2.08 0.2 0.416
nav lights 0.5 10 5
instruments 0.32 24 7.68
music 0.42 4 1.68
HF radio 2.5 5 12.5
Ah/day 56.77
generation: installed P 175
Actual P 87.5 0.5 efficiency
Ah/24 58.33 8 hours sun/day
in-out Ah/24 1.56
Ah total (16 days) 24.92

Tips for saving power

A red LED trailer light bathes the cabin and saves your night vision.

A red LED trailer light bathes the cabin and saves your night vision.

One of the greatest things about sailing is that we have virtually unlimited range. However, on a small boat with no inboard engine, electricity can be a bit of a stretch. If you’re singlehanding, the amount of power burned by an autopilot running over twelve hours a day starts to make things tricky. But it’s also a great opportunity to show that you can do more with less, and there are quite a few benefits to being frugal with energy. Lowering the power draw helps lighten the boat (panels and batteries add up), free up real estate (panels take up a ton of space), preserve your wallet (panels and batteries are expensive), keep your boat from looking like a solar farm, avoid the noise of a wind generator and the smoke and fuel of gas or diesel, keep you independent of marinas and fuel docks, and most importantly, keep you from ending up in the dark offshore. For that you have to be willing to forgo some of the power-intensive habits that are so easy to acquire on land. The most important part is to spend a little bit of time thinking about where your electrons are going, and discipline yourself while underway, keeping things off unless you really need them, and hand steering when you can. Here are a couple tips for lowering your power consumption:

  • Ditch all your incandescent bulbs for LED lights. LEDs use up a fraction of the power. This is especially important for running lights, which are on all the time. As for cabin lighting, now you can find warm white LEDs that have that pleasant glow, so there’s no reason to stick to incandescents. I used a $5 red trailer LED to light up Idefix‘s chart table and cabin while underway.
  • Ditch refrigeration. It is the single most power-hungry piece of equipment on a boat (except for a windlass, but you don’t run that all day while underway). I lived on Idefix for many months without refrigerated food. Here are some of the things I ate: cured meats, dried sausage, hard cheeses, eggs, bread, tortillas, cabbage, citrus, bananas, potatoes, onions, peppers, pineapple, fruit juice, UHT milk, dehydrated veggies of all kinds, pasta, rice, every canned food imaginable, and of course fresh fish! When you get back to land your loved ones will think you a rock star for keeping your eggs out of the refrigerator and eating things you left on the kitchen counter overnight.
  • Look for mechanical alternatives to some of your electrical systems. I’ve never used a wind vane before, but I’ve heard they’re like a magical autopilot, and they use no power! Where many boats have a depth sounder, I got by with a lead line.
  • Laptops are big power hogs. Look at battery life and power rating to get an idea before you buy. The netbook I used for AIS, chartplotting, email and weather was the least power-hungry on the market. I swapped out the hard disk with a solid state drive and brought the power consumption to under two-thirds of an amp.
  • Inverters waste a lot of power. Get a 12V power supply for your laptop, MP3 player or cellphone.
  • Make it a ritual to turn off running and instrument lights when the sun comes up.
  • Understand your system and know what your big consumers are. Autopilot and laptop were mine, and I shut these down whenever I felt stressed about power.
  • Most importantly, keep an eye on your power consumption. Use the battery monitor, and note the battery state of charge in your logbook. This will help you track trends, and tell if you’re running a deficit before things get hopeless.

On the 2012 SHTP we were in overcast for ten days and I noted early on that I was never coming up to full charge and started rationing power.  I also kept an eye on battery voltage and realized at one point that my MPPT controller had been set to the wrong output voltage and wasn’t topping off my batteries, which allowed me to reset it to get even more power out of it. Not far away, my friend Ronnie on US101/Hope for the Warriors was in the dark, hand steering for 20 hours a day and unable to check in with his position. In the weeks before the race he had installed some high-tech lithium batteries that required a significantly higher voltage than lead-acid batteries, with a lightweight flexible solar panel that put out a little over 100W, and had a top-of-the-line but power-hungry autopilot. Unfortunately he was running short on time and hadn’t had a chance to test his setup, and the weather caught him off-guard and left him without power. Being a stupendous badass, he simply shut everything off, leaving just enough power to send the message that we wouldn’t hear from him for a while, and pulled out every ounce of human endurance to hand-steer his boat to a division win. While it makes for a great story, thoroughly planning and testing your electrical system is a good start to having a more relaxing trip.