No power in the house: power cut or a fault inside?

Look outside first. If street lights and your neighbours' homes are dark, it is a power cut on the network. Call 105, which is free from mobiles and landlines, or check the UK Power Networks power cut page, which also offers text updates. UK Power Networks also takes calls on 0800 31 63 105.

In Ealing and Hounslow the network is run by SSEN rather than UK Power Networks, according to the Mayor's answer to the London Assembly. 105 puts you through to whichever company covers your postcode, so you do not need to know which.

If someone at home relies on electrical medical equipment, ask about the Priority Services Register when you call. It is free and means the network operator knows to contact you during cuts.

If only your home is off, go to the consumer unit. Most London homes have one in a hall cupboard, under the stairs or by the front door of a flat. Look for any switch pointing down.

• Main switch down: someone may have switched it off, or a fault has tripped it on older boards.
• An RCD down: an RCD is a safety switch that protects a group of circuits. If it drops, every circuit it protects goes off, which is why half the house can lose power at once.
• One small switch down: that is a single circuit breaker (MCB) for one circuit.

Unplug everything on the affected circuits and try to reset once. If it trips again straight away, leave it off and book an electrician. Do not take the cover off the board.

57% of London homes are flats, according to the Valuation Office Agency. In a block, your meter may sit in a cupboard on the landing or in a ground floor intake room, and your consumer unit is inside the flat. If your flat is off but the communal lights are on, the fault is usually on your side: your consumer unit, your meter tails or your main fuse.

If the communal lights are off but your flat is fine, that is the landlord supply, which the freeholder or managing agent looks after. If the whole block is off, call 105. Prepayment and smart meters can also cut off when credit runs out, so check the display before anything else.

Sometimes the consumer unit looks normal, every switch is up, and the home is still dark while the neighbours have lights. That points to a problem between the street and your board. It could be the main fuse in the sealed cutout, the meter, or the meter tails, which are the thick cables from the meter to the consumer unit.

The cutout and main fuse belong to the network operator, and the meter belongs to your energy supplier. Neither should be opened by you or by an electrician. If an electrician tests at the meter tails and finds no supply coming in, the next call is to 105, and the network operator will send someone to replace the fuse or repair the service cable.

If the supply is there but the board is dead, the fault is on your side, often a failed main switch or a burnt connection on the tails. Signs of heat around the meter or the consumer unit, such as a smell or brown marks, need an electrician the same day. Our guide to telling a power cut from a fault in your home has a short checklist.