12V power

What size inverter do you need in a sleeper cab? Match watts to real loads

Short answer: add up the watts of everything you will run at the same time, multiply by 1.25, and buy that size in a hardwired pure-sine inverter. For most Canadian sleeper cabs that lands at 1,500–2,000W continuous. Anything under ~150W belongs on the 12V lighter socket with a plug-in unit; anything over it needs cable as thick as your thumb wired straight to the batteries.

Research-based. We did not test these hands-on. Scores come from published specs and driver reviews. This article contains affiliate links to Amazon.ca — same price to you, we may earn a commission. As an Amazon Associate I earn from qualifying purchases. Prices are in CAD and can change — check the live listing before you buy.

This is the question every driver asks after reading what the 12V outlet can safely run and realizing the answer is “not the microwave.” An inverter is simply a box that turns your truck’s 12V DC into 120V AC — the same power your house outlets deliver. The only hard part is buying the right size, and that is pure arithmetic.

Watts are volts times amps. Your batteries sit at roughly 12 volts, so every 120 watts of AC power you want pulls about 10 amps from the batteries — plus roughly 10–15% lost inside the inverter itself as heat. The quick mental math: AC watts divided by 12, then add a bit, equals the amps leaving your batteries. That is the number that ends the “I’ll just get the biggest one” fantasy. A 2,000W inverter at full tilt pulls roughly 190 amps from a 12V bank. That is welding-cable current, not extension-cord current — which is exactly why big inverters get hardwired with heavy-gauge cable and a properly sized fuse (your inverter’s manual has the exact cable/fuse chart for your install), never plugged into a lighter socket.

• ≈9.5A — 100W of AC gear pulls from the batteries
• ≈48A — 500W pulls from the batteries
• ≈95A — 1,000W pulls from the batteries
• ≈190A — 2,000W pulls from the batteries

Read the strip right to left and the sizing logic clicks: a phone charger is a rounding error, a kettle is a serious electrical event. Your inverter choice is just deciding which column your sleeper lives in.

Typical draws from published specs and manufacturer labels. “Pull from batteries” includes ~13% inverter loss — the number your bank actually feels.

Add up the watts of everything you would ever run at the same time, then multiply by 1.25 — that is the 25% headroom the old calculator added for you. Typical sleeper loads:

• Phone + tablet charging — 25W
• Laptop — 90W
• CPAP machine — 30–90W
• 24-inch LED TV — 40W
• 120V heated throw — 120W
• Coffee maker — 900W
• Kettle — 1,000W
• Truck microwave (700W cooking) — ≈1,050W
• Air fryer — 1,400W

Assumes ~87% inverter efficiency. Surge-hungry loads (microwave, air fryer) need extra startup headroom on top of the 25%.

Every inverter wears two wattage numbers. Continuous is what it can deliver all day. Surge (sometimes “peak”) is what it can survive for a few seconds — usually about double the continuous rating. Your buying decision uses the continuous number; the surge number just has to cover your hungriest single startup.

Microwaves are the classic surge trap. A 700W-cooking truck microwave pulls roughly 1,050W running, but the magnetron’s startup spike can briefly demand two to three times that. That is why the standard driver-tested answer for a 700W microwave is a 1,500–2,000W continuous inverter, hardwired — a 1,000W unit will fault or trip the moment the magnetron kicks in, even though “1,000” looks bigger than “700” on paper. The sizing method above already bakes this in: it sizes continuous at 125% of your simultaneous loads and flags surge at twice your biggest single load.

Inverters also come in two waveforms. Modified sine is the cheap, choppy imitation of wall power; pure sine is the real thing. For anything with a motor, a compressor, or sensitive electronics, the waveform matters.

Modified sine. Cheaper and fine for dumb resistive loads — kettles, coffee makers, basic heaters. But a microwave on modified sine can cook at roughly half power while still pulling full draw from your batteries, and some CPAP machines, laptop bricks, and variable-speed motors run hot or refuse to start on it.

Pure sine. Clean wall-equivalent power. Microwaves cook at full power, CPAP machines and laptops run as designed, and nothing buzzes or overheats that should not. The price gap over modified sine has shrunk to the point that it is the default recommendation.

Winner — rule of thumb: if your loads include a microwave, CPAP, laptop, or TV — pure sine, no debate. If you are genuinely only ever heating water, modified sine works, but check the price difference first; it is usually smaller than you expect.

A 12V outlet is typically fused at 10–15 amps. Fifteen amps times 12 volts is 180 watts in theory; in practice ~150W is the ceiling for a plug-in inverter, and even that deserves a look at your truck’s fuse rating first. Push past it and the fuse blows — or worse, the socket’s thin wiring and plastic housing get hot doing a job they were never built for. This is not a limitation to fight; it is a boundary to respect.

Phones, laptops, a CPAP, a TV — all of it fits under 150W. We covered the full socket-safe list in 12V outlet safety: what you can safely run. The inverter conversation only starts where that post ends: kettles, coffee makers, microwaves, air fryers.

Anything over ~150W gets wired directly to the battery bank, and this is where drivers get hurt skipping steps. The math is unforgiving: a 2,000W inverter can pull ~190 amps. Standard automotive wire cannot carry that. As a rough example, many 2,000W installs use 2/0 AWG for very short runs and 4/0 for longer ones, with a fuse or breaker (often in the ~250A range for a 2,000W unit) mounted as close to the battery as possible — but cable length, insulation rating, and ambient heat all change the answer, so follow your inverter manufacturer’s cable/fuse chart and your installer’s judgment, not a rule of thumb from the internet. Keep the run as short as you can manage — every extra foot is voltage drop and heat. Three non-negotiables:

1. Fuse at the battery. The fuse protects the cable, not the inverter. An unfused 190-amp cable chafing through its insulation is a fire, not a blown fuse.
2. Short, fat cables. Mount the inverter as close to the batteries as the install allows. Long thin cables starve the inverter, which then shuts down under load — the classic “my 2,000W inverter can’t run my microwave” complaint that is actually a wiring problem.
3. Get it installed by someone who does vehicle electrical. This is the one place on this page where DIY confidence is the wrong call. A shop that wires reefers and APUs does this weekly; the labor cost is trivial next to a melted harness or a denied insurance claim.

One more honest note: your alternator is part of this system, and it may not keep up the way drivers assume. A 2,000W AC load pulls roughly 190A from the 12V side — which can exceed the spare output of a typical 160A Class 8 alternator once the truck’s own electrical loads are counted. Alternator capacity varies by truck, so check your specs rather than assuming. Parked with the engine off, the battery bank carries the whole load alone — which is why the bank, not just the inverter, has to be sized for your loads. Big cooking loads are a drive-then-cook or idle-while-cooking affair, not an all-night one. (See how big loads drain a bank overnight for the parked math.)

The easiest watts to “buy” are the ones you never convert. Every pass through an inverter wastes ~10–15% as heat. A compressor fridge drawing 45W on 12V DC pulls roughly 52W when fed through an inverter — you are paying a conversion tax on every cold drink, all day, for no reason. Compressor fridges are natively 12V appliances; plug them into 12V.

Same logic applies in reverse: anything that is natively 12V (fridge, 12V blanket, USB charging) stays on 12V. The inverter exists for one job only — appliances that have no 12V version, like microwaves and kettles. See the best 12V coolers for truck drivers shortlist for fridges that sip power straight off the bank.

Buy a pure-sine inverter sized by the method above — for most drivers that is 1,500W or 2,000W continuous — from a brand that publishes real continuous and surge specs and honors warranties in Canada. Look for: a wired remote on/off switch (so it is not idling 24/7), low-voltage shutdown to protect the bank, and UL/CSA listing. Skip anything that only advertises “peak watts” in big type and hides the continuous rating in small print.

What not to buy: a 300W+ plug-in inverter for the lighter socket (it will blow the fuse on the first real load — why), a modified-sine unit if you run a microwave or CPAP, or a bigger inverter than your loads need. Oversizing is not free headroom — a 3,000W inverter idling all day wastes more standby power and costs more in cable and fusing than a right-sized 2,000W unit.

12V power

12V outlet safety — what you can safely run

The 150W lighter-socket ceiling and the socket-safe load list.

Read the guide →Winter power

How long can you run a 12V cooler overnight?

The parked battery math — amp-hours, cold, and cutoff settings.

Read the guide →Reviews

Best 12V coolers for truck drivers

Compressor fridges that sip power straight off 12V DC.

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