Buy an inverter that's too small and it shuts off; too big and you've overpaid for capacity your car socket can't deliver anyway. The fix is to work out the watts you need before you buy. This guide shows how to read a charger label, gives typical figures for common devices, and works through three real setups.
The one rule: add up the watts, add headroom
Every device lists its power on its charger or label. Add up everything you'll run at the same time, then add about 20–30% headroom so the inverter isn't running flat out. Pick an inverter rated at or above that total.
How to read a charger label
Many chargers print the wattage directly, such as "65W". If not, look for the output line and multiply volts by amps: a laptop charger marked 19.5V and 3.33A delivers about 65W. Ignore the input current line (for example "100–240V 1.5A"): that is the most it can draw in the worst case, not what it normally uses.
Quick reference: typical device draw
| Device | Typical draw (check your label) |
|---|---|
| Laptop charger | 45–100W |
| Phone / tablet charger | 10–30W |
| Camera / drone battery charger | 10–40W |
| Portable monitor | 10–25W |
| Router / modem | 10–20W |
| CPAP machine (no humidifier) | 30–60W, confirm with the maker |
Three worked examples
| Setup | Sum | With headroom | Verdict |
|---|---|---|---|
| Laptop (65W) + phone (20W) | 85W | About 110W | Easy on a 200W inverter |
| Laptop (90W) + camera charger (25W) + router (15W) | 130W | About 170W | Fits, but check your socket fuse |
| Two laptops (90W each) + phone (20W) | 200W | About 260W | Too much for a socket; charge one laptop at a time |
Continuous watts and surge watts
Inverters list two numbers. The continuous rating is what they can supply all the time; the surge rating is a short burst for devices that draw more as they switch on. Chargers and electronics barely surge. Motors and compressors, such as fridges and pumps, can draw several times their running power for a moment, which is why a household fridge will not start on a small inverter. Our page on running a fridge on a car inverter explains the numbers.
Your socket is the real ceiling
The 12V socket is usually protected by a 10 to 15 amp fuse, which is roughly 120 to 180 watts at the input. Inverters also lose some power in conversion, so expect a little less at the outlet. For more than that, an inverter has to be wired to the battery with proper cables and a fuse, with the engine running. Our cigarette lighter inverter guide shows how to find your car's fuse rating.
Use USB-C where you can
If a laptop or phone charges over USB-C, plugging it into a USB-C PD port skips the step of turning 12V into 230V and back again, which wastes less power. Use the AC outlet for chargers that need a normal plug.
What 200W will NOT run
High-heat appliances draw far more than 200W and need a much bigger inverter and a hard-wired supply:
- Electric kettle (1000W+)
- Induction cooktop (1200W+)
- Hair dryer (1000W+)
- Household fridge (startup surge far above 200W)
- Most power tools
Don't try to run these from a compact car inverter: size and socket limits make it impractical and unsafe.
Engine on or off
With the engine running, the alternator supplies the power. With the engine off, everything comes from the battery, and a laptop running for a few hours can leave too little to start the car. If you work from a parked car, start the engine now and then.
Pure sine wave for electronics
For laptops and sensitive chargers, choose pure sine wave. Modified sine wave can cause buzzing, extra heat and charging problems in some devices.
The simple takeaway
Add up your devices, add headroom, and check your socket. For laptops and everyday gear, a 200W pure sine wave inverter like EchoRepel's covers most real-world needs: clean power, sensible size, and safe for your electronics.