The short answer. Add up the watts of everything that runs at the same time. For anything with a motor or compressor, use the maker’s start-up factor (Samlex says 5 times running watts for a fridge). Pick an inverter whose continuous rating covers that total. Then check the DC amps it will pull. That number sets your cable and your fuse.
You’ll need
- A list of what you want to run at the same time, with the watts from each label
- An inverter sized for that: Samlex NTX-1000-12, Victron Phoenix 12/1200 120V GFCI, Samlex PST-1500-12, or the Victron MultiPlus 12/2000/80-50 120V if you also want a shore charger
- A battery bank that can supply the current: Battery bank size calculator
- A cable and fuse sized for the inverter’s DC draw: Wire gauge calculator
- A way to see what it’s really pulling: Victron SmartShunt 500A
Size your inverter, step by step
- List what runs at the same time. Read the watts off each label.
- Add the start-up factor for anything with a motor or compressor.
- Pick the inverter on its continuous rating, not its peak.
- Work out the DC amps it pulls from your battery.
- Size the cable and fuse from the inverter maker’s table.
- Leave the AC side to a licensed electrician if it ties into a panel, a shore inlet or a generator.
Or let the Inverter size calculator run these steps for you.
You can do this. Most people size an inverter by the biggest thing they want to run, and then the DC side surprises them. A 12V inverter running hard can pull well over 100 amps from the battery, and the cable, fuse and battery all have to handle it.
Every number here comes from the inverter makers’ own datasheets and manuals. Where a maker doesn’t print a number, we don’t guess one.
How do I add up the watts?
Write down everything you’d run at once. Not everything you own. The coffee maker and the microwave probably won’t run together. The fridge and the TV might.
Read the label on each device. Watts on the label? Use that. Only amps? Samlex’s manual says to multiply the amps by the AC voltage (115 for its NTX inverters). That gives you VA, a slightly different number. More on that below.
Watch for devices that change their draw. Samlex’s own example is a coffee maker that pulls 500 W while it brews for about 5 minutes, then about 100 W to keep the pot warm. Size for the 500 W. That’s what the inverter has to deliver.
How much extra for a fridge, pump or air compressor?
This is the step people skip, and it’s an easy one to get right. A motor pulls far more for a moment at start-up than it does running. Samlex’s NTX manual says induction motors may need 2 to 6 times their power rating to start. The hardest are compressors and pumps that start under load.
Appliance labels only give the running number. Samlex says to get the start-up surge from the appliance maker, test it, or estimate it. For the estimate, both its NTX and PST manuals give a “sizing factor”. Multiply the device’s running watts by the factor. The answer is the continuous rating your inverter needs.
| Device | Samlex sizing factor |
|---|---|
| Refrigerator or freezer (compressor) | 5 |
| Air conditioner | 5 |
| Air compressor | 4 |
| Sump, well or submersible pump | 3 |
| Furnace fan | 3 |
| Circular saw, bench grinder | 3 |
| Microwave (rated at cooking power) | 2 |
So a fridge rated at 100 W (Samlex’s own example size) calls for an inverter rated at 500 W continuous by this rule.
Victron doesn’t publish a factor. Its Phoenix manual explains what the inverter does when a motor asks for too much. It lowers its output voltage to limit the current. If the overload keeps up, it shuts down, waits 30 seconds and tries again. After 3 restarts and another overload within 30 seconds, it stays off until you switch it off and on.
Continuous, peak and surge: which number counts?
Every inverter lists at least two numbers. Size on the continuous one. The peak only lasts seconds:
| Inverter | Continuous at 25°C | Peak or surge |
|---|---|---|
| Samlex NTX-1000-12 | 1000 W | 2000 W, under 1 second |
| Victron Phoenix 12/1200 | 1200 W (1100 W at 40°C) | 1600 W for 15 seconds |
| Samlex PST-1500-12 | 1500 W | 3000 W, under 0.8 seconds |
| Victron MultiPlus 12/2000/80-50 | 2000 VA / 1600 W (1400 W at 40°C) | 3500 W (no time given) |
Mind the heat. Victron prints lower ratings for 40°C. A hot engine room or a closed van in summer sits closer to that column than the 25°C one. Samlex states all its ratings at 25°C.
VA vs W: why the MultiPlus says 2000 and 1600
Watts are the power that does work. VA (volt-amps) is the voltage times the current actually flowing. For a heater or a kettle they’re the same. For motors and electronics, the current runs higher than the watts suggest. Samlex puts the power factor of those loads anywhere from 1 down to 0.6.
Victron rates the MultiPlus at both: 2000 VA and 1600 W at 25°C. Treat them as two limits and stay under both. Samlex’s PST manual says to size the inverter, the AC wiring and the breakers on VA, the bigger number.
Do I need pure sine wave?
For a boat, van or RV running modern gear, yes. Samlex’s manual lists what can misbehave on a modified sine wave: laser printers, clocks that won’t keep time, dimmers and fan speed controls, some battery chargers. It also says motors and compressors run hotter on one. Samlex’s PST-1500-12 spec sheet gives under 3% distortion, and Victron’s datasheet calls the MultiPlus a true sine wave inverter.
How many amps will the inverter pull from a 12V battery?
Here’s the math: AC watts ÷ battery volts ÷ efficiency. Using each maker’s printed maximum efficiency at full continuous output on a 12V bank:
| Inverter | Math | DC current |
|---|---|---|
| NTX-1000-12 | 1000 ÷ 12 ÷ 0.905 | about 92 A |
| Phoenix 12/1200 | 1200 ÷ 12 ÷ 0.91 | about 110 A |
| PST-1500-12 | 1500 ÷ 12 ÷ 0.85 | about 147 A |
| MultiPlus 12/2000 | 1600 ÷ 12 ÷ 0.93 | about 143 A |
Two things to keep in mind. Maximum efficiency isn’t full-load efficiency, so real current runs higher. Samlex’s own spec for the NTX-1000-12 is 100 A at rated load, not 92. And a battery sitting below 12V means more amps for the same watts. Samlex’s quick rule is simpler: divide AC watts by 10 for 12V amps. That gives 100 A for a 1000 W load.
Idle draw counts too. With nothing plugged in, the MultiPlus uses 10 W (3 W in search mode) and the Phoenix 11 W (about 1 W in ECO mode). The NTX draws under 0.8 A and the PST under 1.0 A. Left on all day, 10 W adds up to 240 watt-hours. On a small bank that’s a big bite, so switch the inverter off when you’re not using it. Put your idle draw in the Battery bank size calculator.
What that current means for cable and fuse
At 100-plus amps, an undersized cable gets hot. This is where good cable and the right fuse pay you back. Start from the maker’s table:
- Victron Phoenix 12/1200: 25 mm² cable for runs up to 1.5 m, 35 mm² up to 3 m. Its internal 200A fuse protects the inverter, not your cable.
- Samlex NTX-1000-12: a 150 A fuse, with AWG 4 cable up to 4 feet from the battery and AWG 2 up to 6 feet.
- Samlex PST-1500-12: a UL class T fuse (or equivalent) with at least 10,000 A interrupting capacity, within 7 inches (178 mm) of the battery positive. Samlex also accepts MRBF fuses. Its manual is clear that the fuses inside the inverter will not blow for a short in the battery cables.
- Victron MultiPlus: the datasheet lists no internal DC fuse, so the external fuse at the battery is the only protection on that cable.
Longer runs need bigger cable. Put your real length, out and back, through the Wire gauge calculator. For which fuse type goes where, see Fusing a lithium bank: Lynx, MRBF and class T. A bigger inverter also asks more of the battery: Lithium vs lead-acid batteries for boats, vans and RVs covers how each type handles high current.
Inverter-charger or inverter plus a separate charger?
A plain inverter only turns DC into AC. An inverter-charger like the Victron MultiPlus 12/2000/80-50 120V also charges the battery from shore power or a generator (80 A to the house bank, plus 1 A to a start battery). It passes shore power through its 50 A transfer switch. Victron’s PowerAssist feature also lets it add battery power when a shore or generator supply is too small for a short peak.
A standalone inverter, like the Victron Phoenix 12/1200 120V GFCI, plus its own charger, keeps charging if the inverter is down for repair, and the other way round. MultiPlus vs Phoenix plus a separate charger walks through the trade-offs.
Does this guide fit your setup?
This applies if: – Your boat, van or RV has a 12V house bank and you want 120V AC from it. – You’re choosing an inverter size, or checking that a planned one fits your battery and cable.
This doesn’t apply if: – Your system is 24V or 48V. The same math works, but the products above are the 12V models. – You need to wire the AC side into a panel, add a shore inlet or tie into a generator. Samlex warns never to connect an inverter’s output to a panel that’s also fed by shore power or a generator without a transfer switch. It can destroy the inverter and start a fire. That part of the job needs a licensed electrician (on a boat, look for ABYC certification too).
Products used in this guide
| Product | Why it’s here |
|---|---|
| Samlex NTX-1000-12 | 1000 W pure sine with GFCI outlets. Remote included. For small loads. |
| Victron Phoenix 12/1200 120V GFCI | 1200 W, with an ECO mode near 1 W idle. Pair with a separate charger. |
| Samlex PST-1500-12 | 1500 W pure sine, GFCI outlets plus hardwire terminals. |
| Victron MultiPlus 12/2000/80-50 120V | Inverter, 80 A charger and 50 A transfer switch in one. |
| Victron SmartShunt 500A | Shows the real amps the inverter pulls, on your phone. |
Prices show live from the store. To run the numbers for your own list, use the Inverter size calculator.
Quick answers
What size inverter do I need to run a fridge on my boat? Samlex’s rule is 5 times the fridge’s running watts as the inverter’s continuous rating. Check the fridge maker’s start-up figure if they publish one, and add whatever else runs at the same time.
How many amps does a 1000 watt inverter draw on 12V? About 100 A at full load. Samlex rates its NTX-1000-12 at 100 A DC input at rated load, and its manual’s rule is AC watts divided by 10.
Is a bigger inverter always safer? No. It idles higher, pulls more current when loaded, and needs bigger cable and a bigger fuse. Size for what you run together, plus start-up surge.
Can I size on the peak rating? No. Peak lasts seconds or less (Samlex’s NTX: under 1 second; Victron’s Phoenix 12/1200: 15 seconds). Size on continuous watts, and treat the peak as headroom for motor starts.
Do I need an electrician to install an inverter? For the AC side, yes: any connection to a panel, shore inlet or generator goes to a licensed electrician. The DC side you can do yourself, following the inverter manual’s cable and fuse table.
Sources
- Victron Inverter VE.Direct 120V manual (HW15): https://www.victronenergy.com/upload/documents/Inverter_VE.Direct_120V_-_HW15/106668-Inverter_VE_Direct__Smart_and_SUN-pdf-en.pdf
- Victron MultiPlus 2000VA 120V datasheet: https://www.victronenergy.com/upload/documents/Datasheet-MultiPlus-2000VA-120V-EN.pdf
- Samlex NTX owner’s manual: https://samlexamerica.com/wp-content/uploads/2025/02/11002-NTX-1000-1500-2000-3000-3000S-12-24-0524_R1.pdf
- Samlex NTX-1000-12 spec sheet: https://samlexamerica.com/wp-content/uploads/2026/08/12002-NTX-1000-12-24-0826.pdf
- Samlex PST-1500/2000 owner’s manual: https://samlexamerica.com/wp-content/uploads/2026/07/11001-PST-1500-2000-12-24-48_0726_Lrez.pdf
- Samlex PST-1500 spec sheet: https://samlexamerica.com/wp-content/uploads/2026/07/12001-PST-1500-12-24-48-0826.pdf
- Victron SmartShunt manual, installation: https://www.victronenergy.com/media/pg/SmartShunt/en/installation.html
