Every device that plugs into a wall socket converts AC back to low-voltage DC. In a van, boat or off-grid building, that means your power goes DC → AC → DC, losing energy at each step. Feeding those devices directly from your DC system removes two conversions — and often an entire inverter — from the chain.

Start here

Read the label on the power supply you already use. It tells you the two numbers that matter: the output voltage (for example 12 V, 19 V or 5 V) and the output current in amps. Multiply them together to get watts, then add headroom. That is the specification of the converter you need — nothing more, nothing less.

Why DC to DC beats DC to AC and back

A typical setup converts twice before the power reaches your device:

  1. Battery 12 V DC → inverter → 230 V AC
  2. 230 V AC → the device’s own power brick → 5 V or 12 V DC again

Inverters are usually 85–93% efficient, and small power bricks are often worse, especially when they are running well below their rating. Each step turns the difference into heat. A direct DC-DC converter in the 94–98% band removes both conversions and the standby drain of an inverter that is running for one small load.

There is a second benefit that matters as much as efficiency: an inverter has to be sized for everything you might plug in, and one left running for a router wastes power all night. A small dedicated converter can stay connected permanently.

Match the voltage exactly

This is the part that damages equipment if you get it wrong. Low-voltage electronics are not tolerant the way a 12 V lighting circuit is.

  • 5 V devices (single-board computers, USB hubs, some cameras) expect a tightly controlled 5 V. Feeding 12 V into them destroys them instantly.
  • 12 V devices are usually comfortable across roughly 12–13.8 V, but check the label — some are 12 V ±5%.
  • 19 V laptops and similar need their exact adapter voltage. Use an adjustable-output model or one documented for that voltage, and set it before connecting.
  • Devices with a built-in mains supply — most desktop monitors, many audio devices — cannot simply be fed DC. If the mains lead goes straight into the case with no external brick, leave it on AC.

Polarity

DC has a positive and a negative. Get them the wrong way round and most devices are damaged immediately, often without any protection. Check whether the plug is centre-positive (the common convention) or centre-negative, and confirm it with a multimeter before connecting anything expensive.

Work out the current you need

Add up the devices you intend to run from one converter, then add headroom so it is not running at its limit:

Example loads — check your own labels, these are illustrative only
Device Typical adapter output Approx. watts
Small router or access point 12 V, 1–2 A 12–24 W
Network video recorder (NVR) 12 V, 2–5 A 24–60 W
Single-board computer 5 V, 2–3 A 10–15 W
USB charging hub 5 V, 4–10 A 20–50 W
Starlink-style terminal varies; often 48–57 V check the label

Aim to run the converter at roughly 60–80% of its rating for the load you expect most of the time. That keeps it cooler and leaves room for a device that draws more at startup than it does running.

Keep the cable drop in mind

At low voltage, a small resistance costs you a lot. A 1 V drop on a 12 V, 10 A feed is 10 W of heat in the cable — and your device sees 11 V instead of 12 V, which is often outside its tolerance. Short, thick cable solves this far more cheaply than a bigger converter.

Fuse every feed at the battery end. The fuse protects the cable, not the device, so size it for the cable’s current rating.

These models cover the common cases: a 5 V rail for small electronics, a regulated 12 V rail for network gear, and a step-up where you need a higher rail than your battery provides.

Converters for device rails

For a waterproof installation, or one where the converter sits in a damp locker, check the model’s own page for the documented enclosure and protection details rather than assuming.

What about Power over Ethernet?

Power over Ethernet is not simply “48 V on the cable”. A PoE device negotiates with the injector or switch before power is applied, and not every device accepts a fixed supply. This shop sells step-up converters that can produce a 48 V DC rail, and they are useful for DC distribution generally, but do not assume that feeding a PoE injector with a fixed 48 V supply will work with every device. Confirm the injector’s input requirements first.

A sensible order of work

  1. List the devices and read each adapter’s output voltage and current.
  2. Group them by voltage. Do not mix a 5 V device and a 12 V device on one converter unless the converter has documented separate outputs.
  3. Add the watts per group, then add 25–30% headroom.
  4. Choose the converter whose documented output matches the group and whose current rating covers the total.
  5. Fuse at the battery, use short thick cable, check polarity with a meter, then connect the cheapest device first as a test.

Frequently asked questions

Can I run my laptop from a 12 V battery?+

Usually yes, with the right converter. Many laptops need 19–20 V, so you need a step-up converter with a documented adjustable or fixed output at that voltage, plus a matching plug. Check the laptop’s adapter label for the exact voltage and current.

Is it safe to power a router from a 13.8 V rail when it says 12 V?+

Often it is acceptable because many routers tolerate 12–13.8 V, but “often” is not a specification. If the label says 12 V with a tolerance, stay inside that tolerance. If it says 12 V only, use a converter whose output is documented at 12 V.

Do I still need the device’s own power brick?+

No — you are replacing it. Keep it for bench use, but the DC-DC converter takes its place. The converter must match the voltage and be able to supply at least the current the brick could.

Can one converter power several devices at once?+

Yes, if they all need the same voltage and the total current stays within the converter’s rating. Remember that devices sharing a supply also share its failure, so a separate small converter for critical equipment is sometimes worth it.

Will this work with a smart alternator or a lithium system?+

The converter only sees the voltage at its input. If your supply swings widely — a smart alternator can sit well below 13 V, a 48 V lithium pack can reach 58 V — choose a wide-input model with a documented range that covers the whole span.

Device consumption figures above are illustrative examples, not specifications of any particular product. Always read the label of the equipment you intend to power. Converter ratings quoted on product pages are supplier-stated and not independently tested by PVShop.