Backup power

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There are lots of ways of providing Backup power. This article introduces the various options, with their main features. Some of these have their own articles that go into the option in more detail.

What do you need?

The usual question that's posed when choosing is 'What do you want power for?'

  • lighting
  • ch/hw
  • tv
  • computer
  • freezer
  • cooking
  • etc

What will that cost, and is it worth it? Now, more realistically, what will do?

Most people choose no back up power, and are quite happy to sit in quiet for once. It can be a nice break in a hectic life, and quite refreshing.

Lighting options


Battery torches tend to turn out to have flat batteries. Rechargeable batteries are worse in this respect, as they have quite high self discharge rates.

Wind-up and shake torches don't need batteries, so are more likely to work when they haven't been used in ages. They don't run long on a charge, but can be useful to see to get another power source running.

LED torches give out less light and light of lower CRI, but run longer, are much less prone to bulb failure and are far more robust.

Gas lighting

Despite being historic technology, gas lighting has some advantages over electrical power. The main one is its long term reliability, which is hard to match using electrical kit. Gas lights can be expected to work for decades without attention or failure.

Gas cylinders don't discharge in storage, so power is always there ready to go. This contrasts with rechargeable batteries, which are prone to either

  • being flat when wanted if not left on trickle charge
  • having limited life if left on trickle

A small camping lantern will produce more light than a 60W incandesent light bulb for several hours at maximum brightness. If longer run time or less light is required they can be turned down to give long run times, 10 hours or more depending on cartridge size. Note that often maximum brightness is achieved at below maximum gas output, since at full the flame can be not ideally positioned.

The quality of the light is quite good, and much better than the low CRI and high CCT types of electric lighting, such as LEDs and cool white fluorescent tubes.

Gas lights produce a lot of heat due to low efficiency, and with care this can be used to cook or heat water. Even a small camping gas light will put out around 400w of heat, which is adequate to cook on.

The high heat output means it should be put somewhere it can't be knocked over in use. Hanging from the ceiling is often practical.

Putting foil round half the globe produces nearly twice the level of forward light.

Cost per volume of gas varies widely for different cartridge types. High gas cost lights can be uneconomic purchases, even if cheaper initially.


Tea light candles 2579-3.jpg

Candles can provide occasional lighting at trivial cost. £1 buys a lot of tea lights, but not a lot of light level.

Candles are associated with significant fire risk, so its only sensible to use some means to keep the candle safe in use. Wall sconces are effective, placing the candle high up out of reach, especially of young kids.

Always keeping candles and matches in the same place makes finding them in the dark easy. Finding things in an unfamilar location can be a hopeless task in darkness.

Emergency escape lights

Emergency light 0994-7.jpg

Non-maintained lights will light up automatically when mains power fails. (Maintained units are designed for a slightly different job.)

Run time typically 1-3 hours, typical cost £15-20.

Fluorescent units generally give more run time due to the much higher efficiency of fluorescent tubes.

Rechargeable torch

These are left permanently on trickle charge. Some can be set to come on automatically when power goes off. Available light output and run time vary.

Fluorescent torches generally give better run time, since the fluorescent tube is much more energy efficient than the filament bulb.

All share one defect, limited battery life, and will require re-celling after so many years. Using cells with higher capacity can give longer run time.

Touch light

Led push light 2491-3.jpg

These are a type of torch designed to be fixed in place, and usually using LEDs. They're an easy low cost way to get efficient battery lights, but light output level is poor, and the often used AAA batteries have little capacity.

Luminaire backup pack

Mains fluorescent lights are available with a battery backup pack built in which will light the tube when mains fails. Operation on battery is usually at much reduced power with these. While popular in commercial premises, this is not one of the cheaper options for domestic use, expect to pay anywhere in the region of £80 or so. They also aren't generally styled for domestic use, but this issue is easily sidestepped with trough installation.

12v battery & 12v lights

Fluorescent lights give several times the run time per light output than filament Lamps. A practical option where mains voltage isn't needed.

Its possible to feed 12v wiring to every room, and add a relay that switches a 12v lighting system on when mains fails. This may be an attractive option when carrying out work that permits running new low voltage wires, such as major redecoration, rewiring, installing a wired network, etc. See Low Voltage Wiring. Low power lights (a few watts max) can be run on cat5 wiring.

There are 3 types of lead acid battery. Deep discharge batteries and SLA are suitable for permanent installations. A battery borrowed from a car will do for occasional use. Lead acids require correct treatment to avoid early failure.

A Lead acid battery requires a few minutes of maintenance every couple of years to last well.


Low light output and high price, but long shelf life, completely safe to give to kids and keeps them amused.

Glow in the dark tabs can be added to door handles to help small children navigate the dark.

Paraffin lamp

Old fashioned pressurised paraffin & lamps (Coleman, Petromax etc) are much cheaper to run than gas canister lamps, but a little knowledge & time is needed to get them started. They burn pressurised boiling paraffin or petrol, and have a poor safety record.

Power options

Electricity can run most things, but offgrid electricity is the most expensive type of backup power. Hence its usually used for applications that other sources of power won't do.


Issues to consider:

  • how to detemine power needed
  • noise
  • maintenance
  • cost
  • service life (often very short)
  • 2 strokes and starting reliability
  • the Cult of the Listeroid
  • connecting to mains wiring


Uninterruptible Power Supplies provide a mains feed that's not interrupted in the event of a power cut. The switchover to internal battery is instant.

Large whole building units are available, but nearly all UPSes sold are plug-in items intended to run one or a few low power appliances, typically a computer system. Small cheap units may only manage minutes of backup time.

To avoid disappointment, the required capacity of UPS should be calculated before purchase, otherwise its likely to fail to deliver the run time wanted. A UPS unit also need to be of sufficient power output rating for the job.


A laptop is often a practical way to get computer time during power loss.

Where TV is wanted to sedate the kids, a laptop with a USB TV card can be cheaper than a UPS to run the main TV, and of rather more use.

Extra batteries can give more run time. As well as expensive Li-ion laptop batteries, laptops can be run off external lead acids with higher capacity and lower cost per hour, or even home made batteries.

Lead acid battery, charger, invertor

A good option for people that already have most or all of the parts. However the necessary bits and leads need to all be to hand when the power cut happens, and the battery must of course be charged and ready.

Home made battery

Home made batteries of some types can be made and stored dry, and can provide power at almost any voltage during a power cut. Water or other electrolyte is added to activate them. Because of this, shelf life is almost indefinate.

Construction cost of some types is minimal. Some are easily replenished after use. Very large batteries are quite constructable. Depending on the technology used, power output is anything from miniscule to plentiful.



Cold food is usually sufficient. But if power for cooking is needed, as well as the above options it can be provided by any of:

  • disposable charcoal BBQ tray
  • wood/coal fire
  • gas stove, either mains, bottled gas or camping canister type
  • paraffin stove
  • and maybe self heating instant meals

If considering these options, bear in mind that old fuel cookers such as paraffin types can sometimes produce significant CO, and will require good ventilation or use outdoors.

Some mains gas cookers cut the gas off when they get no electrical power, some don't.


Wired phones connected direct to the line should continue to work in a power cut. Not all companies keep as much reserve run time at the exchange as BT.

Mobile phones may stop working as most cell towers have no backup power source. A mobile with a flat battery will of course be dead - obvious, but sometimes overlooked.

Cordless phones stop working if the base unit doesn't have a built in rechargeable battery. Those that do should continue to work for a fair time if the battery is serviceable.


Dialuppers using a land line should have no problem for a while. Wireless mobile net access will suffer the same fate as mobile phones, and usually stop working.

Broadband services require power to the modem to work, otherwise net access stops. Any routers will also need power if you want them to work.


TV aerial systems that include an amplifier will play dead during a power cut.

Set top boxes and anything else the aerial signal runs through will also want power providing, or won't co-operate. The aerial lead can be plugged into the TV to just get the 5 main channels.


Desktop systems and CRT monitors are both power hungry. Cheap plug-in power backups won't run these for long, and in some cases won't run them at all. Laptops are more practical.

Desktop systems with a CRT monitor on a UPS suffer a particular issue: if the monitor is off when power is lost, switching it on tends to glitch the backup power, which often resets the computer or shuts down the UPS. This is due to the high initial current consumption of the degauss circuit in nearly all CRT sets. Even a 1kVA UPS can be caught out by this in some cases.

CRT monitors can be modded to sidestep this problem by putting the degauss circuit on a switch, and normally leaving it off, but few would bother for domestic use.

Hot water

Some gas fired hot water systems can work without electicity, most won't.

Fires with backboilers and gravity circulation work with no electricity.

Space heat

Some gas wall heaters work without mains, some don't.

Portable gas & paraffin heaters all work with no mains. Old paraffin heaters produce CO if the flame is adjusted wrong, or the wick needs trimming, so these can't be considered safe for everyone. Any more than the odd flicker of yellow in the flame indicates incomplete combustion, and thus CO output. Knocking these heaters over is a familiar cause of house fires. They also smell a bit, and the smell tends to permeate clothes.

Business use

  1. How much business is lost in one power cut?
  2. Roughly what does this add up to over say a 10 year life of a power backup system?
  3. What does a suitable UPS cost?

This quick calculation shows UPSes are a no-brainer for many businesses.

Bear in mind that power cuts are in practice due to on-premises faults as well as supply outages, and that getting an electrician generally takes many hours.

Connecting to house electrical circuits

Care is required when doing this since there are a number of non obvious complexities. The two primary requirements are to ensure that there is no possibility of back feeding the electrical supply (a linesman could be killed or injured if the right safeguards aren't in place), and that suitable earthing is provided for when the temporary supply is engaged.

If in doubt running an extension lead from power source to appliance is the simple option.

Suitable Supply

It is not usually realistic to provide a standby source of power that has the same load capacity as the normal mains feed, however by taking care which appliances are used and in what combinations it is possible to run a house from a relatively modest supply - say 10A. Realistically this means a diesel or petrol generator if the supply is required for any period of time.

Transfer of Power

In order to safely switch the supply from the mains to the standby source a proper standby transfer switch is required. This included the required interlocks to prevent the standby source from being able to back feed the mains supply. The simplest of these are manually operated. In the event of a power failure one would need to first disconnect any loads in the house that can't be supported on standby power, then activate the standby source, before finally throwing throwing the transfer switch to feed power into the house.

Provision of earthing

For most properties, the earth will be supplied to you by your electricity supplier. Unfortunately, one is not permitted to rely on this earth being present during a power interruption (since, depending on the cause of the power outage, it may be interrupted along with the power). This has a number of implications. Firstly in addition to providing standby power, one must also provide earthing locally - typically via an earth rod (see TT Earthing for details). Secondly, it is not usually practical to provide a local earth that is good enough to be used in the same way as an electricity supplier's earth. Hence you will need to use the same design as employed in houses with TT earthing. Obviously for properties that do not ordinarily benefit from a supplied earth this is far less of a problem since the consumer unit will already have the required levels of RCD protection in place. For many users however, one would need to implement their RCD protection with great care to ensure they retain adequate protection. This could greatly increase the cost and complexity of providing a fully integrated standby source for the house.

Jesus cord

One approach definitely not recommended but often seen is the Jesus cord or widowmaker cord. This is a lead with a 13A plug on each end, used to connect genny output to house circuits. It has some real safety problems:

  • exposed live pins on the plug
  • can electrocute people in the house
  • can electrocute a linesman working on what they thought was a dead circuit
  • can result in criminal prosecution
  • mains power backfeeds into the genny when it comes back on, usually out of sync. May pop a breaker or fry the genset.

Wiring Design

Its possible to choose what appliances you want to work in a power cut, and provide a separate circuit to run them, possibly with a UPS to keep the circuit alive for a while. Very few of us go that far, but some will wish to at design time.

A different style of socket can be used to avoid confusion.

When distributing 12v or 24v, its usual to power appliances from what would normally be the N and E pins rather than N and L. This removes most of the danger of plugging a low voltage appliance into a mains socket.

See Also