How to choose a good power supply? A guide for the average user

Here’s what you really should check before buying.

1. Start with the label – it’s about more than just voltage and current

You’ll find a sticker with the basic specifications on every power supply. Most buyers only look at the voltage (e.g. 12V) and current (e.g. 2A). This is necessary, but not sufficient.

What else to look out for:

Input voltage range. If you see “100–240 VAC”, the power supply will cope with fluctuations in the mains supply – for example, when someone in the block starts the lift or the air conditioning. A narrow range (e.g. 230 V only) means that the device may reset in the event of a momentary drop in voltage.

Energy Star Level VI logo. This is an energy efficiency certification introduced in 2016. A power supply bearing this logo is energy-efficient, generates little heat and is manufactured on a modern production line. The absence of the logo does not disqualify the product, but its presence is a good sign.

Protection class II (double square symbol). This indicates double insulation between the mains supply and the output. The power supply is safer to use and does not require earthing.

2. Efficiency and heating – this is where the biggest differences lie

This criterion is the best way to distinguish cheap power supplies from good ones, yet it is the one most often overlooked when making a purchase.

Every power supply loses some energy when converting electricity — this loss is converted into heat. In high-quality models, the loss is around 10%, meaning efficiency reaches 90%. In cheaper models, it can exceed 30–40%.

What does this mean in practice:

• A good power supply running at full load is barely warm.
• A cheap power supply gets noticeably hot – which shortens its lifespan and can pose a risk when left running unattended (e.g. 24 hours a day).
• The difference in electricity consumption over a year amounts to a few to a dozen or so zlotys – a small but real difference.

If you already have a power supply and want to assess its quality, touch the casing after it has been running for an hour. It should feel no more than slightly warm.

3. Safety — what you can’t see, but what matters

A power supply is a device that is connected to the mains 24 hours a day. That is why its safety is more important than with most other electronic devices.

Galvanic isolation – the transformer inside the power supply isolates the mains from the output. The quality of this isolation determines whether the power supply will survive a power surge during a storm without damaging itself or the connected device. 

Electronic safeguards – a good power supply will not fail in the event of a short circuit, overload or overheating. Cheap models often lack protection against mains surges (no varistor), which means that a single significant voltage spike can destroy it – along with the connected device.

Compliance with IEC/EN standards – these standards regulate electrical safety and prohibit the use of harmful substances in manufacturing (including lead). Certified power supplies undergo independent testing. No certificate = no guarantee that these tests have been carried out at all.

4. How to choose the right power rating – so you don’t overload the circuit or have to buy a new one in a month’s time

This is the most common question when choosing a replacement or a new power supply.

The formula is simple:

Power [W] = Voltage [V] × Current [A] Example: 12 V × 2 A = 24 W

The power supply must have a capacity of at least equal to the device’s power requirements. However, it is worth buying a model with a 25–35% margin – for two reasons:

1. Some devices (multi-port routers, IR cameras, printers) draw significantly more power momentarily when starting up than during normal operation.
2. A power supply that is constantly operating at full capacity wears out more quickly and gets hotter than necessary.

Practical example: For a router that draws 18 W, choose a 24 W power supply, not an 18 W one. The difference in price is minimal, but the difference in lifespan is significant.

5How to buy – warning signs and positive indicators

Especially when shopping online, it’s easy to come across a product that looks good in the photo but doesn’t live up to expectations in practice.

Warning signs:

• No catalogue card, or a card available only in English with a few entries
• A price well below the market average for similar specifications
• No information on certificates and standards
• A seller with no track record or no business address

Positive signs:

• Product data sheet in Polish with a full table of specifications
• A warranty longer than the standard two years
• A seller who has been operating in the market for several years, with contact details available
• Safety certificates displayed on the product or in the documentation

A practical tip: If you can’t find the product data sheet for a particular power supply, ask the retailer. Their response will tell you a lot about the quality of the product and the company.

Frequently Asked Questions

Is a more expensive power supply always better? Not always, but there is a correlation. Power supplies below a certain price point are practically unable to meet safety standards and feature high-quality components at the same time. In the £10–£20 price range for a mid-range consumer power supply, you’ll find products of solid quality.

Can I use a power supply with a higher current rating than required? Yes – it is safe to do so. The device draws only as much current as it needs. A higher current rating on the power supply simply means there is more power in reserve. However, you must not use a power supply with a lower current rating than required.

What should I do if I don’t know the specifications of the original power supply? Check the label on the device itself – the manufacturer often states the required voltage and current there. You can also look for the user manual online or contact the retailer.

Why is the power supply getting hotter than it used to? Increased heat under the same load may indicate that the capacitors inside are ageing. This is a sign that you should consider replacing it — before the power supply fails at the worst possible moment.

Will a power supply with a European plug work in Poland? Yes – the Polish mains supply is 230VAC/50Hz, which is the European standard. Power supplies with a type C or E/F plug will work without any problems.