Van Inverter Sizing Guide: Pure Sine, Surge Ratings & Real Costs

The inverter is the component van owners most reliably get wrong, and they get it wrong in both directions. Half buy a 3,000W unit to run a laptop and then wonder why their battery drains overnight with everything switched off. The other half buy a 600W unit, plug in a kettle, and trip it within four seconds. Correct van inverter sizing is not about buying the biggest number you can afford — it is about matching continuous rating, surge headroom and idle consumption to loads you have actually measured.

What an Inverter Actually Does — and What It Costs You

An inverter converts 12V or 24V DC from your battery into 230V AC mains. That conversion is not free. Quality inverters run at 88–93% efficiency under load, so every 100W your laptop draws pulls roughly 110W from the battery. Cheap units run at 78–85%, and the difference compounds across a full day of use.

The more important cost is idle draw — the power an inverter consumes simply by being switched on with nothing plugged in. This is the figure that quietly destroys van battery banks, and it scales with inverter size:

• 300–600W inverter: 4–8W idle
• 1,000–1,500W inverter: 8–15W idle
• 2,000–3,000W inverter: 15–30W idle
• 5,000W inverter: 30–50W idle

A 3,000W inverter left on continuously at 22W idle consumes 528Wh per day — more than a compressor fridge, and more than a 200W solar array generates in a British December. Buying an oversized inverter and leaving it powered is one of the most expensive mistakes in van electrics, and it is entirely invisible until you watch your state of charge fall overnight.

Every inverter should be switched off at the unit or via a remote panel when not actively in use. If you cannot be disciplined about that, size down.

Continuous Rating vs Surge Rating

Inverters carry two numbers. The continuous rating is what the unit will deliver indefinitely. The surge rating — usually 2× continuous for 3–10 seconds — covers the inrush current when a motor or compressor starts.

Resistive loads (kettles, toasters, hair dryers, heaters) draw their rated wattage instantly and hold it. A 1,200W kettle needs 1,200W continuous, full stop.

Inductive loads (fridges, pumps, power tools, microwaves) draw a starting surge far above their running figure:

• Compressor fridge: 45W running, 150–250W surge
• Water pump: 60W running, 180–300W surge
• Microwave (800W rated output): 1,200–1,400W running, 1,800–2,200W surge
• Angle grinder or circular saw: 800–1,200W running, 2,000–3,500W surge

Size the continuous rating to your largest simultaneous running load plus 25% headroom, then verify the surge rating covers your worst-case inrush. In most vans the continuous figure is set by a kettle or microwave, and the surge figure is set by a power tool.

Pure Sine Wave vs Modified Sine Wave

Modified sine wave inverters approximate mains AC with a stepped square wave. They cost 40–60% less than pure sine units and they will run a kettle, a filament bulb or a resistive heater perfectly well.

They will also, over time, damage or refuse to run a long list of things van dwellers actually own: laptop chargers with active power factor correction, CPAP machines, variable-speed power tools, induction hobs, modern TVs, battery chargers with switch-mode supplies, and most medical equipment. Motors run hotter and less efficiently on modified sine and fail earlier. Some devices simply buzz audibly and work at reduced performance until they don’t.

The verdict is straightforward: fit pure sine wave. The price gap on a 2,000W unit is roughly £120–200, and a single replaced laptop charger erases the saving. Modified sine is defensible only in a dedicated tool-only circuit, and even then barely.

Calculating the Right Size — Worked Example

List every 230V device and identify what genuinely runs at the same time. Van owners consistently overestimate simultaneity — you do not boil a kettle while microwaving and running a hair dryer.

A typical touring van:

• Laptop and monitor: 90W
• Phone/camera chargers: 40W
• Kettle (occasional): 1,200W
• Microwave (occasional): 1,300W running, 2,000W surge
• Cordless tool charger: 90W

The largest realistic simultaneous load is the microwave at 1,300W plus background charging at 130W, totalling 1,430W. Add 25% headroom: 1,790W. The correct continuous rating is 2,000W, and the surge requirement of 2,000W is comfortably inside a 2,000W unit’s typical 4,000W surge capability.

Note what this calculation does not justify: a 3,000W inverter. The extra 1,000W of capacity you will never use costs you 8–12W of additional idle draw every hour it is switched on, plus heavier cable and a larger fuse.

If your van has no kettle and no microwave — many don’t, because both are brutally inefficient uses of stored battery — your largest load drops to around 200W and a 600W inverter is genuinely sufficient. Gas or induction on shore power handles cooking better than any inverter will.

12V or 24V: The Decision That Sets Everything Else

Inverter current draw on the DC side is the load divided by system voltage. A 2,000W load on a 12V system pulls approximately 185A after conversion losses. The same load on 24V pulls 93A.

That difference dictates cable, fusing and cost. 185A requires 50–70mm² cable and a 200A fuse; 93A needs 25–35mm² and a 100A fuse. Heavy DC cable is expensive, stiff, and difficult to route through a van.

The practical rule: any inverter above 2,000W belongs on a 24V system. Below 1,500W, 12V is simpler and keeps you compatible with the enormous range of 12V appliances, lighting and fridges. Between 1,500W and 2,000W either works, and the deciding factor is usually whether the rest of your system is already 12V.

Cable Sizing and Fusing — Where People Get Hurt

Inverter DC cables carry currents high enough to melt an undersized conductor in seconds. This is the single most dangerous part of a van electrical system and the part most often improvised.

Minimum cable cross-sections for a 1.5m run at 12V, keeping voltage drop under 2%:

• 600W inverter (≈55A): 16mm²
• 1,000W inverter (≈92A): 25mm²
• 1,500W inverter (≈138A): 35mm²
• 2,000W inverter (≈185A): 50mm²
• 3,000W inverter (≈275A): 70mm²

Keep the run as short as physically possible — mount the inverter within 1.5m of the battery bank. Every additional metre demands a larger conductor and wastes energy as heat.

Fit a Class T or MRBF fuse within 200mm of the battery positive terminal, rated at approximately 125% of the inverter’s maximum continuous DC current. Standard blade fuses are not rated for these currents and will not interrupt a fault safely. Use proper crimped and heat-shrunk lugs, torqued to the terminal manufacturer’s specification — a loose high-current connection generates heat, and heat in a van is a fire.

Standalone Inverter or Integrated Power Station?

A wired system — battery bank, separate inverter, separate charge controller, separate DC-to-DC charger — gives you the best cost per watt-hour at scale and full control over every component. It also demands correct high-current wiring, proper fusing, and enough confidence to do it safely.

An integrated power station puts battery, inverter, MPPT controller and AC charger in one certified box with no high-current DC wiring for you to get wrong. You pay a premium per watt-hour and you cannot upgrade individual parts, but the failure modes that injure people are engineered out.

For most van conversions the integrated route is the correct answer. The Bluetti AC200L delivers 2,400W continuous with 3,600W surge from 2,048Wh of LiFePO4 storage — which is precisely the 2,000W-class sizing the worked example above arrived at, with the cable and fusing already handled internally.

For higher-demand builds running induction cooking or workshop tools, the Bluetti Elite 400 provides 2,600W continuous with 3,900W surge from a 3,840Wh battery, covering many loads that would otherwise need a 24V wired system with heavy cable throughout.

Where a compact permanent installation matters more than peak output, the Bluetti Elite 300 sits between the two on both capacity and footprint.

Inverter-Chargers and Shore Power

An inverter-charger combines the inverter with a mains battery charger in one housing, switching automatically between the two. Plug into a campsite hookup and it charges the battery bank while passing 230V straight through to your sockets; unplug and it reverts to inverting from the battery without interrupting whatever is running.

The convenience is real, but so is the price — expect £400–900 for a 2,000W unit against £220–420 for a bare inverter plus £90–150 for a separate charger. The genuine advantages are automatic transfer switching, a single set of high-current cables rather than two, and proper charge profiles for lithium chemistry.

Buy an inverter-charger if you use campsite hookups more than a few times a month, or if your van is your permanent home and mains charging is a routine part of winter. For occasional hookup use, a separate mains charger left in a cupboard costs less and does the same job with one extra plug.

Three Inverter Configurations With Real Costs

Minimal — 600W pure sine, £90–160

Runs laptops, chargers, small tools and a TV. Idle draw 5–7W. Needs 16mm² cable and an 80A fuse. Suits vans that cook on gas and heat with diesel — which is to say, most efficient builds. This is a far more common correct answer than the market’s marketing would suggest.

Standard — 2,000W pure sine, £220–420

Adds kettle and microwave capability. Idle draw 15–20W, so switch it off between uses. Needs 50mm² cable and a 200A Class T fuse on 12V. This is the configuration most touring and full-time vans should target.

High output — 3,000W+ pure sine on 24V, £480–900

Supports induction hobs, air fryers and 110V power tools simultaneously. Idle draw 25–40W. Requires 24V architecture, 50mm² cable, and a battery bank capable of sustaining 130A+ discharge — typically 400Ah or more at 24V. Only justified if you genuinely cook by induction daily.

Four Mistakes That Cost Real Money

Sizing for a load you’ll use twice a year. A 3,000W inverter bought for an occasional heat gun costs you 300Wh a day in idle draw for the other 363 days. Rent or borrow for one-off jobs.

Leaving it switched on permanently. Fit a remote on/off panel where you can reach it. This single habit is worth more battery capacity than a second solar panel.

Undersizing cable to save money. The cable is not the place to economise. A 2,000W inverter on 25mm² cable will brown out under load, run hot, and eventually fail — usually the cable, sometimes the van.

Ignoring the battery’s discharge rating. A 2,000W load pulls 185A. Many budget lithium batteries limit continuous discharge to 100A, so the inverter shuts down under load regardless of its rating. Check the battery BMS discharge limit before choosing the inverter.

Verdict: Smaller Than You Think, Better Than You Planned

Correct van inverter sizing almost always lands lower than instinct suggests. Measure your real simultaneous loads, add 25%, buy pure sine, and spend the money you saved on cable and fusing rather than on unused capacity. For the majority of UK van builds the answer is a 2,000W pure sine unit on 12V with 50mm² cable and a 200A Class T fuse, switched off when idle — or an integrated 2,000W-class power station that removes the high-current wiring from the equation entirely.

The inverter you never notice is the correctly sized one. Oversizing announces itself every morning in your state of charge.