Inverter Sizing Calculator: Continuous Watts, Surge and DC Current
Size an inverter on the largest set of AC loads that run at the same time, plus about 20 percent headroom, and confirm the surge rating covers the biggest motor start. A laptop and a 1,200W induction burner together is 1,265W continuous, which needs a 2,000W inverter, and that inverter pulls roughly 185A from a 12V battery at full output: 2/0 cable on a 10 ft run and a 250A class T fuse at the battery post.
An inverter is sized by three numbers, and only the first one is printed on the box. Continuous rating covers what you run at the same time. Surge rating covers the instant a motor starts, which can be three to six times its running draw. And DC current on the battery side is the number that decides what the installation costs, because it sets the cable, the fuse and the terminations. This calculator produces all three.
Inverter sizing calculator
Tick only what genuinely runs at the same time. The single most common mistake here is adding up everything you own rather than everything that is switched on at once.
| Running now | Appliance | Running watts | Start surge |
|---|---|---|---|
| Runs far more efficiently from a 12V USB-C PD socket, which skips the inverter entirely. | 1x | ||
| Resistive in effect, so no start surge, but it is the load that sets most inverter sizes. | 1x | ||
| Brief but brutal. Runs the battery hard for two minutes and then stops. | 1x | ||
| Input power is far higher than the cooking figure on the door, typically 1.4 to 1.6 times. | 1.3x | ||
| Universal motor. The surge is short and real, and it is what trips undersized inverters. | 3x | ||
| Small motor, large multiplier. Harmless on a big inverter, awkward on a 600W one. | 3x | ||
| Induction motor under load from the first revolution, which is the worst case for surge. | 3x | ||
| Switch mode supply. Some cheap ones dislike modified sine wave output. | 1x | ||
| Trivial on its own, but it is usually on while something else is running. | 1x |
- Minimum cable, 10 ft run
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- Fuse at the battery post
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- Amp-hours per hour at full load
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Why is bigger not simply better?
Because an oversized inverter costs money three times over. It costs more to buy. It idles higher, and idle draw runs all day whether or not anything is plugged in. And it demands cable and fusing sized for a current you will never actually use, which on a 12V system is the expensive part.
| Inverter | DC amps at 12V, 90 percent efficient | Cable, 10 ft run | Fuse | Typical idle draw | Idle cost per day |
|---|---|---|---|---|---|
| 600W | 56 A | 4 AWG | 70 A | 6 to 10 W | 11 to 19 Ah |
| 1000W | 93 A | 2 AWG | 125 A | 8 to 14 W | 15 to 26 Ah |
| 1200W | 111 A | 2 AWG | 150 A | 8 to 15 W | 15 to 28 Ah |
| 1500W | 139 A | 1 AWG | 175 A | 10 to 18 W | 19 to 34 Ah |
| 2000W | 185 A | 2/0 AWG | 250 A | 12 to 22 W | 23 to 41 Ah |
| 3000W | 278 A | 3/0 AWG | 350 A | 15 to 30 W | 28 to 56 Ah |
| 4000W | 370 A | 2 x 3/0 parallel | 500 A | 20 to 40 W | 38 to 75 Ah |
Read the last column. A 2,000W inverter left switched on all day can eat 23 to 41 amp-hours doing nothing at all, which is a compressor fridge's worth of consumption for the privilege of having an outlet ready. This is not an argument against inverters. It is an argument for a switch, or for a unit designed to be left on. A Victron Energy Phoenix 1200VA 12V Pure Sine Wave Inverter costs more than a budget 2,000W unit and delivers less peak power, and it is still the better buy for a full-time build precisely because its idle draw is low enough to leave running.
The cable and fuse columns assume a 10 ft one-way run and a 3 percent voltage drop target. Shorter runs allow smaller cable and longer runs demand more, so check your own figure in the wire gauge calculator. The 3000W row is instructive: 278A continuous requires a 350A fuse, and a 350A fuse may only protect a conductor rated for at least 350A, which forces 3/0 cable even though 1/0 would have passed both the ampacity and the voltage drop tests on its own.
The 4000W row is the point where 12V stops making sense. At 370A a single 4/0 conductor cannot hold a 3 percent drop over 10 ft and no standard fuse fits inside its ampacity, so the run has to be split into two paralleled 3/0 conductors with matched lengths. The same inverter on a 24V bank draws 185A and runs on a single 2/0 cable. If your build is heading toward 3,000W or more of continuous AC, decide the system voltage before you buy any copper.
What is surge, and which appliances have it?
Surge is the brief overload an inverter tolerates while a motor comes up to speed. An induction motor at rest is close to a short circuit as far as the supply is concerned, so it draws three to six times its running current for a fraction of a second before back electromotive force builds and the current falls to normal.
| Load type | Surge multiplier | Example, running watts | Momentary demand | Notes |
|---|---|---|---|---|
| Resistive heat, kettle or hob | 1.0x | 1,200 | 1,200 W | No surge at all. The load is the load. |
| Switch mode charger, laptop or tool | 1.0 to 1.2x | 90 | 108 W | Small inrush as capacitors charge. |
| Microwave | 1.3x on input | 1,400 | 1,820 W | Input power is 1.4 to 1.6 times the cooking figure on the door. |
| Universal motor, blender or grinder | 3x | 500 | 1,500 W | Short and sharp. Usually within a good inverter's surge window. |
| Induction motor under load, compressor | 3 to 5x | 480 | 1,440 to 2,400 W | Worst case, because it starts against pressure. |
| Air conditioner compressor, no soft start | 4 to 6x | 900 | 3,600 to 5,400 W | A soft start module cuts this by roughly two thirds. |
Most inverters surge to about twice their continuous rating for a few seconds, so a 2,000W unit handles a momentary 4,000W. The failure mode when it does not is not subtle: the inverter shuts down and usually needs resetting, which is annoying with a blender and genuinely disruptive with a compressor that has to restart against pressure. Check the surge figure in the manufacturer's specification rather than assuming twice the rating, because budget units vary.
The other half of surge is the battery, not the inverter. A 4,000W surge at 12V is roughly 370A for a moment, and the battery management system on a single 100Ah LiFePO4 pack with a 100A BMS will simply disconnect. Surge capability is a whole-system property: inverter, cable, fuse and battery all have to be able to pass the current. The battery bank calculator covers the storage side.
Which AC loads should not go through an inverter at all?
The ones with a 12V equivalent. Every watt-hour that goes through the inverter pays a 10 to 15 percent conversion tax, and several of the most common loads in a vehicle do not need to pay it.
| Load | Through the inverter | Native 12V | Daily saving | How |
|---|---|---|---|---|
| Laptop, 60W for 4 hours | 21.3 Ah | 18.8 Ah | 2.5 Ah | 12V USB-C Power Delivery socket at 65W |
| Phones and tablets, 20W for 3 hours | 5.3 Ah | 4.7 Ah | 0.6 Ah | 12V USB socket rather than a wall plug |
| Fridge cooling, 528Wh a day | 46.9 Ah | 41.3 Ah | 5.6 Ah | A 12V compressor fridge instead of an AC one |
| Lighting, 14W for 4 hours | 5.0 Ah | 4.4 Ah | 0.6 Ah | 12V LED strip on a fused circuit |
| Roof fan, 18W for 6 hours | 9.6 Ah | 8.4 Ah | 1.2 Ah | 12V roof vent fan |
| Water pump, 90W for 15 minutes | 2.0 Ah | 1.8 Ah | 0.2 Ah | 12V demand pump, which is the normal product anyway |
| Inverter idle, 15W for 24 hours | 28.1 Ah | 0 Ah | 28.1 Ah | Switch the inverter off, or do not fit one at all |
Figures assume 88 percent inverter efficiency and 12.8V nominal. The conversion loss on the six real loads adds up to about 10.7 amp-hours a day, which is meaningful but not dramatic. The seventh row is the one that matters: an inverter left switched on to serve a 60W laptop spends 28 amp-hours a day doing nothing, which is nearly three times the entire conversion loss above it. That is the whole argument for native 12V, and it is why an 12V USB-C Panel Mount Socket, 65W PD and QC3.0 (83W total) at about twenty dollars is one of the highest return purchases in a build. The inverter is then reserved for what genuinely needs 120V, which is usually cooking, tools and occasional appliances, and it stays off the rest of the time.
How do you install the DC side safely?
Mount the inverter as close to the battery as the heat and ventilation allow, because every foot of that cable costs money and voltage. Five feet instead of ten halves the drop and can save a whole cable size. Give it air: inverters derate when hot, and a 2,000W unit in a sealed cabinet is a 1,400W unit in practice.
Fuse at the battery, not at the inverter. The fuse exists to protect the cable against a fault anywhere along its length, including a chafe six inches from the battery post, so it goes at the source of power within a few inches of the terminal. A Blue Sea Systems 5191 MRBF Terminal Fuse Block (30 to 300A) bolts directly to the post for smaller inverters, and above roughly 200A a class T holder is the right answer for its DC interrupt rating. A Blue Sea Systems m-Series Battery Switch, On/Off with Knob (6006) on the same run lets you isolate the whole thing by hand, which no fuse can do for you.
Terminate properly. A poor crimp on a 185A cable is a heater. Use a Brileine 10 Ton Hydraulic Lug Crimping Tool (12 to 2/0 AWG, 9 dies) and tinned TKDMR Copper Wire Lug and Ring Terminal Kit (2 to 12 AWG, 160 pieces) , seal with adhesive lined heat shrink, and torque the inverter terminals to the manufacturer's specification rather than to feel. Then check them again after the first trip, because they will relax. A Victron SmartShunt 500A Battery Monitor on the negative side lets you watch what the inverter is actually pulling instead of guessing, which is how most people discover their idle draw for the first time.
Everything on this page is arithmetic drawn from published manufacturer specifications and standards documents. It is researched guidance rather than an electrical certification, it does not replace ABYC E-11 or the installation manual supplied with your inverter, and any lithium installation should be inspected by a qualified installer before it carries load.
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Frequently asked questions
What size inverter do I need for a van?
Add up only the AC loads that will genuinely run at the same time, add about 20 percent headroom, then check that the inverter surge rating covers the largest motor start. A laptop plus a 1,200W induction burner is 1,265W continuous and needs a 2,000W inverter. Most van builds land between 1,000W and 2,000W. Going larger is not free: idle draw rises and the DC cable and fuse get considerably more expensive.
How many amps does a 2000W inverter draw from a 12V battery?
Roughly 185A at full output, calculated as 2000 watts divided by 12V times 90 percent efficiency. That is more current than a typical starter motor draws while cranking, running continuously through cable you installed yourself. It needs 2/0 cable on a 10 ft run to stay inside a 3 percent voltage drop, and a 250A class T fuse fitted within a few inches of the battery positive post.
What is surge rating and why does it matter?
Surge is the brief overload an inverter tolerates while a motor starts. An induction motor draws three to six times its running current for a fraction of a second as it comes up to speed, so a 500W blender can demand 1,500W momentarily. Most inverters surge to twice their continuous rating for a few seconds. An inverter that is adequate on continuous power and short on surge shuts down every time the compressor kicks in.
Does inverter idle draw matter?
Yes, more than most people expect, because it runs all day. A budget 2,000W inverter can idle at 15 to 25W with nothing plugged in, which is 30 to 50 amp-hours a day at 12V, comparable to a compressor fridge. A well designed unit idles under 10W and has a low power search mode. Either switch the inverter off when nothing is drawing from it, or buy one designed to be left on.
Should I run my laptop through the inverter?
Not if you can avoid it. Converting 12V DC to 120V AC and then back to DC inside the laptop charger loses roughly 15 percent of the energy every time, and it forces the inverter to idle all day for a 60W load. A 12V USB-C Power Delivery socket charges most modern laptops directly from the battery at far higher efficiency, and it lets the inverter stay switched off.
Do I need a pure sine wave inverter?
For almost every build, yes. Modified sine wave is a stepped approximation, and induction motors run hot and noisy on it, some laptop and tool chargers refuse it or degrade over time, microwaves lose effective power, and medical devices such as CPAP machines should never see it. The price gap has narrowed to the point where modified sine is only defensible for purely resistive loads on a very tight budget.
Electrical safety: 12V is low voltage but not low energy. A lithium house battery can push several hundred amps into a short circuit, and undersized or unfused cable is one of the most common causes of vehicle fires in DIY builds. Every circuit must be fused at the source of power, within a few inches of the battery terminal, at or below the ampacity of the smallest conductor it protects. Size cable for voltage drop as well as ampacity, torque terminals to specification, and have a lithium installation inspected by a qualified installer before it carries load. The figures on this page are researched guidance, not a substitute for ABYC E-11 or your component manufacturer's installation manual.
How we choose: we compare published manufacturer specifications, standards documents including ABYC E-11, and verified owner reviews. We do not test gear in person. Vehicle payload, roof load and charging limits vary by model and year, so confirm yours against the door jamb sticker and the owner's manual rather than any number published here.
Totalling your own build weight against your payload? The Overland Build & Power Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.