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If an appliance that is rated for 220V is used with a 110v-to-220v transformer in a 110v country, or an appliance that is rated for 110V is used with a 220v-to-110v transformer in a 220v country, how much of the electricity is wasted? That is, how much more will it consume compared to the appliance used in its native current country?

If it matters which type of appliance, let's assume something like a humidifier, dehumidifier, air purifier type.

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  • Not as much as you think. Transformers these days are pretty efficient. – PNDA Feb 6 at 11:16
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    Is it really worth lugging [de]humidifers between countries? – user253751 Feb 6 at 11:25
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Something like a dehumidifier which has a fairly high load and a high start up current, will require a larger transformer. Cost and portability will be major considerations.

Smaller appliances with low current draws, will be easier to fit to a small transformer.

For reasonable quality transformers, figure a loss of about 3%.

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  • It may be even less. A good transformer is some 95-98% efficient and 110/220 setup is usually an autotransformer where half of the energy is used directly. – fraxinus Feb 6 at 21:37
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The bigger problem is the transformer is going to set you back about $100, so it spectacularly fails to make sense for a $40 humidifier.

Dollar for dollar, if you're coming to North America, you may be better off just adding a couple 240V circuits to your house, and running the appliances off 240V straight shot. North America has both 120V and 240V available.

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    Might have some 50Hz/60Hz concerns with some devices, but I don't think it's a big deal for most things. – JPhi1618 Feb 5 at 15:12
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    While NA has 240V, you only get that via +120 and -120, not +240 and neutral. This can be an issue in some cases. For example, you should confirm that ground and "neutral" are not connected (with a multimeter), or else you'll have a dead short to ground. – user3757614 Feb 5 at 17:40
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    This should not be a problem with certified european appliances. They are required to have separate ground and neutral, because live and neutral are interchangeable (i.e. either can be "live" and in practice this does happen often). – Juraj Feb 6 at 12:02
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    @JPhi1618 the compressor and fan in a dehumidifier will both run faster at 60Hz than 50Hz; it may run unreasonably hot and/or noisy at the wrong frequency. – Chris H Feb 6 at 14:35
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    @JPhi1618 What Juraj says; any EU appliance should be insulated for 240V both legs to neutral, because most European plug styles are non-polarized; plugging them in reverse polarity is supported by design. I can say that now that the UK is out... – Harper - Reinstate Monica Feb 6 at 18:34
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You're wasting about 2-3% of the transformer rating in inefficiency. If you're switching secondary voltage, then you're paying for the primary winding losses 24 hours a day even if there's no load on the transformer. The transformer is just one more item that can fail and leave you without your appliances.

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