How to Size a Marine Inverter

Last spring a client called me from a marina in Annapolis, frustrated. He had bought a 3,000-watt inverter for a 28-foot cruiser because the salesman told him “bigger is safer,” and now his battery bank was dying overnight and his cables were running warm. He did not have a power problem. He had a sizing problem.

Picking the right inverter is not about buying the biggest number you can afford. It is about matching one box to the appliances you actually run, the battery bank you actually have, and the cable you can actually fit. Get that match right and the whole system runs cool, quiet, and cheap.

This walkthrough shows you how to size a marine inverter step by step, with real numbers, a worked example, and the mistakes I see most often at the dock.

Tools and materials you will want on hand

You do not need much to size an inverter correctly. A notepad or phone, the wattage labels off your appliances, and five minutes of honest thinking do most of the work.

A clamp meter (a tool that reads current by clamping around a wire, roughly $40 to $90) is the one instrument worth owning. Nameplate wattage is often optimistic, and a clamp meter lets you measure what an appliance really draws instead of trusting the sticker.

Keep your battery bank’s specs nearby too. You need to know the amp-hour rating and the chemistry, because a lithium bank and a flooded bank of the same size deliver very different usable power.

Step 1: List every AC load you might run

Start by writing down the household appliances you want to power at anchor. These are the 120V things: coffee maker, microwave, laptop charger, blender, a small TV, maybe a hair dryer.

Next to each one, write its running wattage. Most appliances print it on a label near the plug or on the bottom. If you only see amps, multiply amps by 120 to get watts, so a 5-amp blender is about 600 watts.

Do not include anything that runs off 12V directly, like your cabin lights, bilge pump, or chartplotter. Those never touch the inverter. If you are still mapping out which loads live on which side of the boat, the Marine Electronics Setup Guide lays out the whole DC and AC split.

Step 2: Find your real peak, not your total

Here is the step people get wrong, and it is the single most common mistake I see on weekends. They add up every wattage on the list and buy an inverter to match. That is how a 28-footer ends up with a 3,000-watt monster it never needs.

Instead, ask what you would honestly run at the same instant. A coffee maker plus a phone charger, yes. A microwave plus a hair dryer plus a space heater, almost never. Circle the largest single load, then add anything that genuinely runs alongside it.

That circled number is your peak continuous load. For most cruising boats it lands somewhere between 800 and 1,500 watts, even though the paper total might read 4,000.

Step 3: Add headroom and account for surge

Once you have your honest peak, add about 20 to 25 percent so the inverter is not running flat out every time it is on. An inverter run constantly at its ceiling gets hot and dies young.

Then think about surge, the brief spike of current a motor pulls in the first fraction of a second when it starts. A blender, a pump, or a small compressor can demand two to three times its running watts for a heartbeat.

Most inverters publish two numbers: a continuous rating and a surge rating that holds for a few seconds. Your continuous peak plus headroom should fit the continuous rating, and your worst motor start should fit under the surge rating.

Step 4: Check your battery bank can feed it

An inverter is only as good as the bank behind it. This is where the Annapolis client went wrong: a big inverter starved by a small bank sags, browns out, and shortens battery life.

The DC current an inverter pulls is large. Divide the AC watts by roughly 11 to estimate the 12V amp draw once you include conversion losses, so a 1,500-watt load pulls about 135 amps from the battery.

A practical rule I use: your inverter’s continuous watts should stay under about 10 times your bank’s usable amp-hours. Chemistry matters here, and if you are weighing an upgrade, our pure sine vs modified sine inverters guide pairs naturally with a look at your bank’s real capacity.

Note that “usable” is the key word. A 100 Ah flooded battery only gives you about 50 Ah before you damage it, while a 100 Ah lithium (LiFePO4) battery gives you 80 to 90 Ah. Two banks with the same label can support very different inverters.

Step 5: Size the DC cable to the draw

The cable between battery and inverter carries all that current, and it is not the place to save a few dollars. A boat I rewired last spring had a 2,000-watt inverter fed by 6-gauge wire the previous owner had lying around. It browned out on every coffee brew, and the cable was warm to the touch.

Size the cable to the inverter’s maximum amp draw, not its rated output, and keep the run short. For a 2,000-watt inverter pulling up to 200 amps over a 3-foot run, you want 2/0 cable, with an appropriately rated fuse within 7 inches of the battery.

A worked example: a 30-foot weekend cruiser

Let me put real numbers on it. Say you run a coffee maker (900 W), charge two laptops (130 W together), and occasionally use a small blender (600 W running, 1,400 W surge). You never run the coffee maker and blender at the same time.

Your peak continuous load is the coffee maker at 900 W plus the laptops at 130 W, so 1,030 W. Add 25 percent headroom and you are at about 1,290 W. The blender surge of 1,400 W is brief and fits under a typical surge rating.

A 2,000-watt pure sine inverter covers this comfortably: 1,290 W continuous is well inside its ceiling, and its surge rating (usually 3,000 to 4,000 W) swallows the blender start. On the DC side, full load draws roughly 180 amps, so you fit 2/0 cable and a 250-amp fuse.

For the battery, that 2,000-watt inverter wants at least 200 Ah usable behind it. A 200 Ah lithium bank or a 400 Ah flooded bank keeps voltage steady without sagging.

Where the inverter fits in the bigger system

An inverter never lives alone. It shares wiring, fusing, and battery space with your charger and your shore connection, so it pays to plan them together rather than one at a time.

If you charge from the dock, read Shore Power Charging Without Cooking Cells so your refill routine matches your battery chemistry. If your alternator does the charging underway, Installing a DC-DC Charger on a Boat shows how to keep a lithium bank fed without frying the alternator.

For the full picture of how the inverter, charger, and bank work as one system, our Boat Inverter and Charger Guide ties every piece together.

Getting it right the first time

Sizing an inverter is mostly honesty. Be honest about what you run at the same time, honest about what your bank can deliver, and honest about the cable you need, and the box you end up buying is almost always smaller and cheaper than the one the showroom pushed.

Walk the four numbers before you spend a dollar: peak load, headroom, DC draw, and usable amp-hours. Those four figures point straight at the right inverter, and they keep your batteries, your wiring, and your weekend running the way they should.