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# Start With the Number: Energy Budgets for Cruising Boats
- URL: https://svsabado.com/start-with-the-number-energy-budgets-for-cruising-boats/
- Published: 2026-09-26T22:48:31.000Z
- Updated: 2026-09-26T22:48:31.000Z
- Description: Before you buy more batteries or solar, you need to know how much energy your boat actually uses each day. That number is the foundation for everything else.
- Author: Ray Hendricks
- Tags: Boat Life

It seems like there is a common conversation that we hear in anchorages or on docks.

Someone is unhappy with their power situation. The batteries don't last through the night, or the refrigerator quit in the tropics, or they run the engine for hours and the battery never seems to get full. So they ask what size battery bank they need to have. Or what size solar array. Or whether lithium will fix it.

It's often hard to answer that question, because the question skips a step. The step is knowing how much energy the boat uses in a day.

Holly and I went through Sabado's refit with that number in our hands, and the electrical upgrade was planned around it. Not the battery capacity, not the solar wattage. The daily consumption. Everything else was built from it.

## What an energy budget actually is

It's a list of every electrical thing on the boat, how much power each one draws, and how many hours a day each one runs. Multiply and add, and you have your daily energy.

That's the whole idea. The complications are in the details, and that's where people often go wrong.

**Hours are hours of running, not hours of being turned on.** The refrigeration is powered up twenty four hours a day, but the compressor only runs a fraction of that. In a temperate climate maybe fifteen percent of the time. In Fiji in February, closer to forty. That fraction is the duty cycle, and getting it wrong will skew the whole list, because refrigeration is usually the largest single consumer group on a boat without air conditioning.

**Use watts, not amps.** An amp figure only means something alongside the voltage it was quoted at. A fridge that pulls 5 amps at 12 volts pulls 2.5 amps at 24, and it's the same fridge doing the same work, because 60 watts is 60 watts either way. Write down watts wherever you have them. If all you have is an amp rating, write down the voltage it was quoted at as well, or the number becomes unusable the moment anything about the system changes.

**Everything is per day.** Hours per day, watt hours per day, solar per day. How long you want to go without starting the generator is a separate question, and an important one, but don't mix it into the load list. Nothing on that list can be more than 24 hours.

**Two scenarios, not one.** At anchor and underway are different boats electrically. Underway you have autopilot, radar, instruments, nav lights. At anchor you have refrigeration and people. Then run each of those twice, once for a normal day and once for a bad one. Bad means the fridge working hard, restricted visibility with the radar on all day, everyone aboard, the watermaker running, and a week of clouds cutting your solar in half. You size the bank so a normal day is comfortable and a bad day still works.

> **That list is easier on paper** than on a screen when you’re upside down behind a panel with a flashlight in your teeth. The Load Inventory Worksheet is the same structure this method uses: device, bank, watts or amps, the voltage that amp figure was quoted at, then hours and duty cycle for anchor and underway, normal and worst case. Six pages. [Download the Load Inventory Worksheet](https://svsabado.com/power-usage-inventory-sheet/) (free with a Sabado account)>

## Why it matters more than the gear

Four separate decisions come out of this one number, and all four are really just guesses without it.

### **Battery capacity**

You need to know your daily draw before you can choose how much bank to put in the battery box. Then it's arithmetic: subtract what your solar puts back on a bad day, and you're left with the deficit the bank has to carry. Multiply that by how long you want to go without the generator, divide by the share of capacity you're willing to use, and there's your bank. If you undersize it, you discharge deeper than you meant to, and lead acid punishes that quickly. Oversizing costs money and weight but is otherwise kind to the batteries.

### **Charging capacity**

Consumption tells you what has to go back in, and it is always more than what came out, because charging is not free. Flooded lead acid is the worst of it, with losses typically in the 10 to 20 percent range, and some sources put it even higher under real world conditions. A good AGM is better, usually in the 5 to 10 percent range. Lithium is the outlier, with losses of 5 percent or less, often closer to 2.

That loss is one reason lead banks take so long to charge. The bigger reason is the shape of the charge curve itself. Bulk charging is efficient, with nearly all the energy going into the battery. But once the charger switches from bulk to absorption, efficiency drops off fast, and that is why the last fifth of a lead acid charge takes as long as the first four fifths. Lithium does not have this problem to nearly the same degree, which is a big part of why it charges so much faster start to finish.

Nobody has ever been happy to discover this at anchor.

### **Conductors and overcurrent protection**

This is the part of the audit that has nothing to do with energy. Alongside the daily consumption you collect the peak instantaneous load, which is the worst the system can be asked to deliver at once. Windlass and bow thruster together, or the windlass while the inverter is running the kettle. That number sets your main conductors and your fuse ratings, and on a lithium boat it also has to stay inside what the BMS will pass. Get it wrong and the fuse blows, or the BMS disconnects, at the moment you least want either.

**Expectations.** Knowing you use six and a half kilowatt hours a day, and that your array makes seven in July and four in May, tells you what kind of life the boat supports. Don't argue with the math.

## Measured beats estimated

This is where I may do things a little different that others.

The classic method, the one laid out in most of the books and guides, is where I learned all of this, and it is built for a boat you cannot measure. You estimate every load because estimating is all you have.

If your boat has a shunt and any history at all, you are in a better position than that. You already know the answer. Sabado's number came from a Victron VRM export covering several months, integrated properly rather than averaged, and it says the boat used 6.5 kilowatt hours a day through the current stretch. Earlier in the season with both of us aboard and living normally it ran between seven and a half and eight, with spikes to fourteen.

That doesn't make the load list pointless. The shunt tells you the total and nothing else. It can't tell you that the fridge is a third of it, or that the network gear you never think about is quietly eating four hundred watt hours a day. Only the list can do that.

So build the list, then compare it to what the shunt says. If the list adds up to five and the boat uses six and a half, you have a kilowatt and a half a day you cannot account for, and finding it is the actual work. It is nearly always standby draws, or a duty cycle guessed low, or something nobody mentioned because it has been running since the boat was launched.

## Where it earns its keep: upgrades

Every addition to a boat that takes power or produces power is an energy decision.

A watermaker is 1500 watts for two hours before it is a plumbing project. Induction cooking is a peak load and inverter question before it is a galley preference. Air conditioning at anchor changes the whole system. A lithium conversion changes your usable window, your charge efficiency and your peak current limits at once.

The value of having the budget already built is that you can test any of these on paper in ten minutes. Add the load, set the hours, look at what it does to the daily total, to the peak, and to how many days you can sit at anchor before something has to run. Then ask whether the existing charging can cover it, or whether the honest answer is another two hours of generator time each day, which is a different purchase than the one you thought you were making.

I would rather find that out before the box arrives.

## The tool

[I built one](https://svsabado.com/energy-audit/), because I was tired of doing this in spreadsheets. [We work with clients](https://svsabado.com/consulting/) and energy budgets are a very common part of many of the projects we assist with. I’ve been using this tool myself to help with those projects.

The existing planners are fine at the arithmetic and awkward everywhere else. Separate sheets for every battery bank, no way to compare a load list against what the boat really uses, and a peak load figure that assumes every device on the boat runs flat out simultaneously, which produces numbers no one can wire. I wanted something I could sit down with in front of a stranger's electrical panel and fill in as we went.

It is a single page that runs entirely in your browser. Nothing is sent anywhere and it works with the internet off, which matters when the electrical panel you are standing in front of is three hundred miles from a cell tower.

One thing it won't do is guess. If a field it needs is blank, it says so and names the field instead of quietly substituting something reasonable. A confident number built on an assumption you never made is worse than no number at all.

![](https://storage.ghost.io/c/43/0b/430bbca4-113f-4e23-b72d-96a7c0bcf98d/content/images/2026/07/IMG_0773-1.png)

### When to use it

- Before buying batteries, solar, a high output alternator or an inverter
- Before adding any significant load, to see what it costs you per day
- When the boat is not behaving and you want to know where the energy is going
- When you are looking at a boat to buy and want to know what its systems will demand
- When you need to size conductors and fuses for a new circuit

### How to use it

**Set up the boat.** Add a bank for each independent house bank: voltage, chemistry, charge efficiency, the capacity you have installed, and the state of charge you refuse to go below. Chemistry prefills a typical charge efficiency, but it's your number, so put in what you've measured if you know it. Skip the cranking batteries, they barely cycle and they don't move the number. Set your inverter efficiency and its surge rating.

Then the assumptions that decide whether the whole thing is realistic. How many hours you want to run without starting the generator, which is what drives the bank size. A worst case solar factor, where 0.5 means a bad run of weather cuts your array in half. And the simultaneity factor for peak load, which is how much of the theoretical worst case can genuinely happen at once. If your boat has a shunt, put the measured daily figure in here too.

**List the loads.** One row per device, in watts or amps. If you use amps, set the voltage that amp figure was quoted at in the column beside it, and the tool converts to watts and carries watts through everything after that. It's what lets you change a bank from 24 volts to 12 and watch the amp hours double while the energy stays where it was.

Then hours and duty cycle for a normal day, at anchor and underway. Worst case columns inherit the normal values automatically, so you only fill in what changes. There is a list of common boat loads with typical figures to get you moving, but replace them with nameplate ratings or a clamp meter reading before you trust the output. Typical is not your boat.

![](https://storage.ghost.io/c/43/0b/430bbca4-113f-4e23-b72d-96a7c0bcf98d/content/images/2026/07/IMG_0774-1.png)

**Look at the breakdown.** The results page shows the day as a single bar, biggest consumer first. If you entered a measured figure, it draws that as a line across the bar and shows the unaccounted portion as a hatched block. That block is your homework.

**Read the bank size and runtime.** All four scenarios side by side: consumption, solar in, the net deficit, how long the bank you already have will last, and how big a bank each scenario would need to cover the hours you asked for. The recommendation comes from whichever scenario bites hardest, which on most boats is a bad day at anchor. If solar covers the load in a scenario, it says so instead of pretending there's a deficit.

![](https://storage.ghost.io/c/43/0b/430bbca4-113f-4e23-b72d-96a7c0bcf98d/content/images/2026/07/IMG_0775-1.png)

**Do the wiring.** Turn on the wiring columns and enter the round trip cable length for each circuit. Check the critical ones, which get a three percent voltage drop target instead of ten, and check anything running through an engine space so the ampacity gets derated. You can work in metric or in AWG, and the sizes come from the appropriate table either way.

**Do the charging.** Alternators and chargers go in as amps for a number of hours. Solar and wind go in as kilowatt hours a day, which is the number people actually know about their array. It gives you the daily balance per bank, how many days you can run on solar alone, and then the run it takes to recover, netting your house load off the charger output because the charger has to carry the boat as well as fill the bank. Pick which source does the recovering, because a generator and a pair of alternators are very different answers. Lead acid banks include the acceptance taper, so the answer is honest rather than flattering.

**Print the report.** One page with the headline figures, all four scenarios, the bank sizing and runtime, the ten biggest consumers, the wiring table and every assumption listed underneath. Save the audit to a file and it is portable, so you can reopen it in a year or hand it to somebody.

### What it will not do

It is a planning tool and the output is only as good as what you put in. The ampacity tables are transcribed for planning, so confirm them against the current published standard before you buy cable. It does not do AC circuit design or power factor, it does not know about your particular conduit or bundle temperatures beyond a single engine space derate, and it will not catch a load you forgot to enter. It gives ballpark numbers with the arithmetic done correctly, which is useful and is not the same thing as an engineered design.

I tested it against the two projects I helped with and have found it working well. It reproduces their consumption, recharge and peak figures. Several bugs surfaced doing that, all now fixed, which is a decent argument for a close evaluation by someone knowledgeable about these audits.

## Go find your number

If you do one thing after reading this, put a clamp meter on your fridge compressor and time how long it runs in an hour. That one measurement will tell you more about your boat's power than a nameplate rating will.

> **Then build the list**, compare it to your shunt, and go looking for the gap. It is in there.

> [The Load Inventory Worksheet](https://svsabado.com/power-usage-inventory-sheet/) is available to anyone with an account on our blog (including free accounts). Print it, walk the boat, fill it in as you go.

> [The energy audit tool](https://svsabado.com/energy-audit/) is available to anyone with an account on our blog (including free accounts). It runs in your browser, saves nothing on our end, and exports each boat as a file you keep.

*Questions about your own numbers? Leave a comment. This is the kind of thing I enjoy digging into, and the answers usually help somebody else too.*