Marine Inverter Buying Guide: How to Size, Power and Install One on Your Boat
Quick answer: Add up the watts of everything you want to run at the same time, add 20 to 25 percent headroom, and buy a pure sine wave inverter at or above that number. Plan on about 10 amps of 12V battery current for every 100 watts you run, and wire it with heavy cable and a Class T fuse within 7 inches of the battery.
An inverter turns your boat's 12V (or 24V) DC battery power into 120V AC, the same power as a wall outlet at home. It is what lets you brew coffee at anchor, charge laptops and camera batteries, run a blender at the sandbar, or power a TV without firing up a generator. Getting the size, type, and installation right is the difference between a system that just works and one that kills your batteries or trips every time someone turns on the microwave.
Step 1: Add up your loads
List everything you want to run at the same time, not everything on the boat. Here are typical draws for common onboard gear:
| Device | Typical running watts | Notes |
|---|---|---|
| Phone or tablet charger | 10 to 30W | Easy load for any inverter |
| Laptop | 60 to 100W | Pure sine recommended |
| TV (32 to 43 inch LED) | 50 to 120W | Pure sine avoids buzzing |
| Blender | 300 to 700W | Motor startup surge 2 to 3x |
| Coffee maker | 600 to 1,200W | Short run time, high draw |
| Microwave (700W cooking) | 1,000 to 1,500W | Input watts are higher than the label |
| Hair dryer | 1,200 to 1,875W | One of the biggest loads aboard |
Check the label on your actual devices. Microwaves are the classic trap: a "700 watt" microwave often pulls 1,100 watts or more from the outlet.
Step 2: Check surge (peak) watts
Anything with a motor or compressor, like a blender, fridge, or power tool, pulls 2 to 3 times its running watts for a split second when it starts. Every inverter lists a continuous rating and a higher surge rating. Your continuous rating covers what runs all the time, and the surge rating has to cover your biggest motor starting up.
Step 3: Choose pure sine wave
- Pure sine wave: Clean power identical to shore power. Safe for laptops, TVs, chargers, motors, and anything with electronics. This is the right choice for nearly every boat.
- Modified sine wave: Cheaper, but can make motors run hot, cause audio buzz, and damage sensitive chargers. Not worth the savings on a boat.
Step 4: Make sure your batteries can keep up
This is where most inverter setups fail. A rough rule at 12V: DC amps = AC watts ÷ 10. That builds in typical inverter losses of 10 to 15 percent.
| AC load | 12V draw | 24V draw |
|---|---|---|
| 500W | about 50A | about 25A |
| 1,000W | about 100A | about 50A |
| 2,000W | about 200A | about 100A |
| 3,000W | about 300A | about 150A |
To estimate how much battery you use, multiply amps by hours. A 1,200W coffee maker running 6 minutes uses about 12 amp-hours. A laptop at 80W for 4 hours uses about 32 amp-hours. Remember that flooded and AGM batteries should only be drawn down to about 50 percent, while lithium (LiFePO4) batteries can safely deliver 80 to 90 percent of their rating. For big inverters, a lithium house bank makes a dramatic difference.
Above about 2,000W, many builders move to a 24V system. Half the current means smaller cables and less voltage drop.
Step 5: Inverter or inverter/charger?
- Standalone inverter: Converts DC to AC only. Great for smaller loads and boats that already have a good battery charger.
- Inverter/charger: Combines an inverter, a multi-stage battery charger, and an automatic transfer switch. Plug into shore power or start the generator and it switches over automatically and recharges your batteries. Unplug and it seamlessly goes back to battery power. For boats with built-in AC outlets, this is the cleanest setup.
If you run lithium batteries, make sure the charger side has a lithium charging profile.
Step 6: Install it safely
Inverters move a lot of current, so installation details matter. Key points that marine electricians and the ABYC guidelines stress:
- Fuse close to the battery. Put the fuse on the positive cable within 7 inches of the battery or positive bus (up to 72 inches if the cable is sheathed or in conduit).
- Use a Class T fuse. Most manufacturers specify Class T over ANL fuses because of their much higher interrupt capacity, which matters with large battery banks, especially lithium.
- Add a disconnect switch on the DC supply for service and for emergencies.
- Size the cable to the current and the run. Inverter cables are often thumb-thick or larger. Follow the manufacturer's cable chart for your length.
- Ground the chassis properly. The chassis ground conductor generally needs to be the same size as the DC positive cable, a detail many installers miss.
- Mount it where the manufacturer allows. Never mount directly above batteries, keep it ventilated and dry, and follow the orientation rules in the manual.
- Look for UL 458 listing and GFCI-protected outlets for marine and RV use.
If the inverter will feed your boat's AC panel, have a qualified marine electrician do or check the install.
Our picks by boat and budget
All of these are pure sine wave units from brands marine electricians trust, and all are available at Boat Supplies Direct.
Phones, laptops, and small electronics (up to about 350W)
Victron Phoenix 12/500 Inverter with GFCI outlet. Compact, efficient, UL 458 listed, and easy to mount in a console or cabin.
Coffee maker, blender, and TV (about 1,000W)
Victron Phoenix 12/1200 Inverter with GFCI or the Xantrex Freedom X 1000. Both are true sine wave and sized for the most common onboard loads.
Hardwired to your boat's outlets (1,000W with transfer switch)
Xantrex Freedom X 1200 Hardwire with Transfer Switch. Feeds your existing AC outlets and switches to shore power automatically.
Microwave, full galley, and recharging (2,000 to 3,000W inverter/charger)
Xantrex Freedom XC 2000 Inverter/Charger (80A charger), the Victron MultiPlus 12/3000 Inverter/Charger (120A charger), or the Xantrex Freedom XC Pro 3000 (150A charger). These are full power systems that run big loads and recharge your bank whenever you're on shore power or generator.
Frequently Asked Questions
What size inverter do I need for my boat?
Add the running watts of everything you'll use at the same time and add 20 to 25 percent. Most boats running electronics, a TV, and a coffee maker land at 1,000 to 1,500W. Add a microwave and you're usually at 2,000W or more.
Can a 1,000 watt inverter run a microwave?
Usually not. Most microwaves draw 1,000 to 1,500 watts from the outlet even when labeled 700 or 900 watts. Plan on at least a 2,000W inverter for a microwave.
How long will my batteries run an inverter?
Divide your AC watts by 10 to get 12V amps, then divide your usable amp-hours by that number. Example: a 100Ah lithium bank (about 85Ah usable) running an 850W load (about 85A) lasts roughly an hour.
Pure sine or modified sine for a boat?
Pure sine. It's safe for electronics, chargers, and motors, while modified sine can cause buzzing, overheating, and damage.
Do I need an inverter/charger or just an inverter?
If you have shore power or a generator and want your outlets to switch over automatically while your batteries recharge, get an inverter/charger. For small portable loads, a standalone inverter is enough.
Does an inverter drain the battery when nothing is plugged in?
Yes. Most draw a small idle current just being on. Turn it off or use its power-save mode when you're not using it.
This guide is general information. Always follow your equipment manufacturer's installation manual, and use a qualified marine electrician for permanent AC installations.
