Watts in the Back: Setting Up a Work Van's Power Without the Generator Din

There is a point in many tradespeople's lives when the extension lead stops reaching, the site power gets locked off, and the generator becomes the default answer. It works, but it is noisy, thirsty, heavy and never quite where you left it. A properly thought-out van power setup can replace most of that faff, provided you understand what you are asking the vehicle to do.

This is not about turning a Transit into a mobile substation. It is about giving yourself enough 230V power for chargers, a laptop, a kettle, a site radio and the odd power tool, while keeping the van's starter battery for its actual job: starting the van.

Start With a Power Audit, Not a Shopping List

Before you buy an inverter, add up what you actually run. Look at the labels on your chargers and tools. A laptop charger might be 65W, a battery charger 120W, a site radio 30W, and a small grinder perhaps 500W to 900W. Add them together, then add a sensible margin. Inverters are rated for continuous output and often a short surge, so a 1,000W inverter may cope with a brief start-up spike from a tool but not run a 1,000W load all day.

Be honest about duty cycle. If you are only charging batteries between jobs, a modest inverter and a decent leisure battery will do. If you plan to run a heat gun or a small compressor, the numbers climb quickly and so does the cable size. Pure sine wave inverters are the safer choice for laptops, battery chargers and anything with a motor or sensitive electronics. Modified sine wave units are cheaper, but they can make some chargers buzz, run hot or refuse to play ball.

The Starter Battery Is Not a Power Bank

Running an inverter straight off the van's starter battery is the quickest way to a flat battery and an inconvenient call to a mate. A starter battery is designed for a big short burst of current, not for being slowly drained by a laptop and a kettle. Even a modest inverter can pull 40A or more at 12V, and that adds up fast.

The usual answer is a second battery, often called a leisure or auxiliary battery. It lives in the back or under a seat, gets charged by the van when the engine is running, and runs the inverter and 12V accessories. The starter battery stays isolated so you can still drive home. That isolation is the whole point, so do not skip it.

Split Charging in the Real World

Older vans often used a simple voltage-sensing relay to link the starter and leisure batteries once the alternator was charging. That can still work, but many modern vans have smart alternators that drop voltage when the vehicle thinks the battery is full. A voltage-sensing relay may then fail to charge the leisure battery properly.

A DC-DC charger is the more reliable modern option. It takes power from the van's electrical system and charges the leisure battery with a proper multi-stage profile. It also gives you a defined cable run and fuse protection rather than hoping the alternator is in the mood. If you are buying a used van for work, check whether it has a smart alternator and factor that into the plan.

Battery choice matters too. AGM and gel batteries are familiar and tolerate decent cycling, but they are heavy for the usable capacity. Lithium iron phosphate batteries are lighter and give more usable power, but they need a compatible charger, low-temperature charging protection and a bit more care. They are not a fit-and-forget upgrade for every van.

Cables, Fuses and the Boring Bits That Matter

Most van power problems are not mysterious. They come from thin cables, poor connections, missing fuses or a rushed install. Keep the cable run from the starter battery to the DC-DC charger short and thick enough for the current. Fuse at the battery end, not just at the charger end. Use grommets where cables pass through metal, secure everything so it cannot rub, and keep the inverter somewhere with airflow.

For the 230V side, treat it with respect. A fixed inverter installation with sockets in the back is not a Saturday morning bodge job unless you genuinely know what you are doing. Use an inverter with proper protection, keep mains cables tidy, and avoid daisy-chaining extension leads around a metal box. If in doubt, get an auto electrician who understands both vehicle wiring and mains installations. It is cheaper than a fire or a failed insurance claim.

Idling the Engine Is Not a Charging Strategy

It is tempting to think a diesel van idling for half an hour will top everything up. In practice it is a slow, expensive way to charge a leisure battery. The alternator is not a generator set, the engine is running well below its efficient zone, and modern diesels with particulate filters are not keen on long idle periods. Add noise, fumes and the growing number of council idling campaigns, and it stops looking clever.

If you need power on site, charge the leisure battery while driving, plug into mains at base when you can, or add solar if your van sits outside. A small solar panel will not run a grinder, but it will keep a battery topped up and slow the drain from chargers and alarms. Think of it as a trickle feed, not a power station.

A Sensible Setup for Most Trades

For plenty of UK work vans, a realistic setup looks like this: a 100Ah to 150Ah leisure battery, a DC-DC charger sized to the battery, a fused cable run from the starter battery, a 1,000W to 2,000W pure sine wave inverter for modest tools and charging, and a couple of well-placed 230V sockets. Add a battery monitor so you can see what is left before you plug in the kettle.

Keep the payload in mind, because batteries and inverters are not weightless. Check your van's Gross Vehicle Weight and axle limits once the racking, tools and power kit are in. A power setup that pushes you over the limit is not a bargain.

Done properly, van power is quietly useful. You charge batteries on the move, run a laptop at the roadside, and stop lugging a generator in and out of the back. Done badly, it is a flat battery waiting to happen. The difference is mostly in the sizing, the fusing and the decision not to treat the starter battery as a free-for-all.