The short answer. Add the running watts of everything that will be on at the same time. Anything with a motor or compressor needs more, because it pulls a surge to start: Samlex multiplies a fridge’s running watts by 5 and a pump’s by 3. The total is the continuous rating to look for.
Add the appliances you’ll run together.
12V systems. Anything wired into the boat’s or RV’s AC panel goes to a qualified marine or RV electrician. Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
How the math works
Only count what runs together. A coffee maker and a microwave you never use at the same moment don’t add up; a fridge that’s always on does.
For plain loads, like laptop chargers, LED lights and most electronics, the running watts on the label are enough. Motors are different. Samlex’s manuals say an appliance’s label shows running power only, and give a rule of thumb for the start-up surge: multiply the running watts by a sizing factor to get the inverter’s continuous rating.
| Load | Samlex sizing factor |
|---|---|
| Fridge or freezer (compressor) | 5 |
| Air conditioner | 5 |
| Air compressor | 4 |
| Pump, furnace fan, power saw or grinder, incandescent or halogen lamp | 3 |
| Microwave (use its cooking watts), switch-mode power supply without PFC | 2 |
Example: a 600 W coffee maker, a 65 W laptop charger and a 150 W fridge. The fridge counts as 750 W, so the total is 1,415 W. That rules out a 1,000 W or 1,200 W inverter and points to the Samlex PST-1500-12 or the MultiPlus.
Victron doesn’t publish a multiplier. Its manual says that when a motor’s inrush is too much, the inverter drops its voltage, cuts out, waits 30 seconds and tries again.
The battery side
An inverter pulls a lot of current from a 12V bank. The tool divides your AC watts by 12 volts and by each inverter’s maximum efficiency from its datasheet: 665 W through the NTX-1000-12 is about 61 amps. Real draw runs a little higher than that, since the inverter isn’t always at peak efficiency.
That current sets three things: the cable (use the inverter maker’s table, or the Wire gauge calculator), the fuse (Fuse size calculator), and whether your battery can deliver it at all. Lithium makers list a maximum continuous current. To see how long the bank lasts at that draw, use the Run time calculator.
Ratings drop in the heat
Makers rate continuous power at 25°C. At 40°C, Victron lists the Phoenix 12/1200 at 1,100 W instead of 1,200 W, and the MultiPlus 12/2000 at 1,400 W instead of 1,600 W. A hot engine room in summer is closer to the 40°C numbers.
Quick answers
What size inverter do I need to run a fridge? Samlex’s rule is the fridge’s running watts times 5, plus everything else running at the same time. A 150 W fridge alone points to a 750 W continuous inverter.
What’s the difference between watts and VA? Victron rates the MultiPlus 12/2000 at 2,000 VA and 1,600 W at 25°C. Watts are the number to compare with your loads. Samlex’s PST manual says to size the inverter’s wiring and protection on VA, which is the bigger number.
How many amps does a 1,000 W inverter draw from a 12V battery? At full load, Samlex lists the NTX-1000-12 at 100 A of DC input. Lower loads draw proportionally less.
Should I get an inverter-charger instead? If you plug into shore power, an inverter-charger like the MultiPlus 12/2000/80-50 also charges the bank at up to 80 A. See MultiPlus vs Phoenix plus a separate charger.
Sources
Victron Inverter VE.Direct 120V manual and MultiPlus 2000VA 120V datasheet; Samlex NTX and PST owner’s manuals and spec sheets. Links are in each product’s spec table and in What size inverter for a boat, van or RV.
