Does a 25 watt lamp consume more or less when it is connected to 230 volts or 12 volts?

Answer

The lamp will consume more if it is connected to 230V.

The formula is as follows: P = U x I (power = current x voltage).

The 25W on the lamp is only valid at 230V and not at e.g. 12V

We can calculate the resistance of the lamp from those 25W and 230V and thus calculate the consumption at 12V.

P = U x I or 25W = I x 230V -> I = P/U or 25/230 = 0.109A = 109mA

If we know the current and voltage, then we can use Ohm’s law to determine the resistance.

U = I x R (voltage = current x resistance)

or R = U/I = 230/0.11 = 2116 ohms

At 12 V the current is then I = U/R = 5.67mA and the power P = U x I = 0.068 W

ATTENTION: this calculation does not take into account that the resistance of an incandescent lamp is lower at a lower current (temperature dependence).

In other words: in reality the current at 12V will be slightly higher and the power will also be slightly higher.

Your question does not specify which type of lamp is used: with an incandescent lamp the lamp will light up a lot less, an LED or energy-saving lamp has a lot of electronics internally that will probably not work at 12V.