DIY Emergency Power Backup

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This is a project to allow you to automatically switch in a backup Mains power supply when the regular Mains goes and you may not be present when this happens and you need to keep your aquarium equipment going for a particular time.


A device to provide Mains voltage from a battery. This is called a Inverter as it changes the DC voltage (typically DC 12V from a Car or Leisure lead acid battery) to AC 240V (110V if you're in the USA).

They are easy to find as most car shops or electronic shops like Maplins (UK) or Radio Shack (USA) sell them.

They need to be connected to a 12V battery that is constantly under trickle charge ready to take over when the Mains fails.

The devices are sold rated to perform up to a certain amount of Power (watts). This needs to be calculated by the user before hand and for this project I'll take my 250 Litres (66 US G.) freshwater aquarium which has a Eheim Professional II 2026 external filter, a 150W Heater, two 38W fluorescent T8 tubes driven by a electronic ballast, a small aeration air pump and a Hydor Koralia Evolution 5200 circulation pump.

I went to the manufacturers web sites to discover the amount of power consumed by each of these devices and wrote up a league table.

  • Lighting - 100W
  • Heater - 150W
  • Eheim filter - 20W
  • Circulation pump - 5W
  • Air pump - 2W
  • Various Timers (total) - 1W

Grand total of 278W.

This isn't actually strictly accurate as the heater is barely on for more than a couple of minutes every hour in my home (even during winter). The lighting is only on for eight hours a day and the air pump is only put on at night. So the only 24/7 devices are the filter, circulation pump and the timers.

Trying to provide backup for this much power is expensive and only you can decide if you really need all your systems on during a Mains blackout period and you need to figure out how long will they be out for.

In the UK my power provider has to get the power back on within 6 hours. So I've used that figure for my worst case scenario.

Working out size of battery

To work out the capacity of the battery you require you use the electronic formula for power. P=VI. This is Power required per hour is Voltage multiplied by the Current. As we don't know the current, but we do know the voltage (12V) and the power, we reconfigure the formula to give us I=P/V (Current per hour required equals Power divided by the Voltage).

So in the above example, 278W divided by 12 gives us 23A (current) for one hour.

A typical new, fully charged Car battery will give 45Ah. That is 45A for one hour or 540W (12v x 45A) for one hour. So this typical battery would drive the equipment for only ~ 2 hours (45 / 23 = 1.95 hours). I'm going to need a bigger battery! A typical new leisure battery used in caravans is around 100Ah. So that would last ~4 hours (100 / 23 = 4.34 hours). Still not enough. The trick then is to link two of these batteries in parallel so I'll get more current. So two leisure batteries would give me 200Ah or ~ 8 hours. More than enough.

Still that isn't going to be cheap. So I decided I needed to look at ways to drop that power consumption. I realised that I could do without the lighting and the heater. I knew by previous testing that the temporary removal of the heater caused my large tank to drop 0.5°C (32.9°F) per half hour. So in 6 hours it would slowly drop from 25°C (77°F) to 19°C (66.2°F) in winter time (worse case). Nothing to worry my fish there.

I suggest you test this yourself. Disconnect your heater (switch off the central heating) and see how long your tank temperature would drop in one hour during the coldest time of winter in your home at night. Every tank will be different.

  • But obviously don't do this if you own temperature sensitive animals in your tank!

With the heaters and lighting removed from the equation, I'm left with a power consumption of 28W. This was more like it! I rounded it up to 30W (I like to build in a safety factor).

So I needed a battery capable of delivering 2.5A (30W / 12V) an hour, so a typical car battery of 45Ah would drive this amount of power for 18 hours (45 / 2.5). Nice.

Choosing an Inverter

An inverter is a electronic device that converts a DC 12V source into a AC 230V (110V if in the USA) source. They come rated as capable of delivering a certain amount of power output (watts). But I've seen cheap (or disreputable ones) quoting their peak output which the device can only handle for a few minutes. So watch out for that. You'll want one rated for continuous use. A typical device capable of handling ~200W won't cost you more than £30 ($40).

It is designed to be connected to either a motor vehicle battery or a cigarette lighter socket. I prefer one that has terminals on the back so I can wire my own battery to it for standby use. Also look for one that claims quiet operation. For the purpose of this article the device needs to be ready 24/7 for immediate use. A constant running ventilation fan may annoy you.

One further thing to look out for is that most inverters don't give out a proper Mains sinewave AC voltage. The industry calls these "modified sinewave". This rough output makes the inverters cheap but does put limits on the sort of Mains devices you can plug into it. Devices with large motors or fluorescent lighting may not operate correctly (they may stutter or flicker), cheap audio devices plugged in may hum for example.

You can purchase pure sinewave inverters. But these are typically more expensive.

I choose a Maplin 200W portable inverter ("modified sinewave") (part no. A10HZ) with its own 60W battery in it. Therefore on its own it is claimed capable of running 30W mains equipment for a maximum of two hours. Too short for my needs. But it is capable of taking an external battery via terminals, it came with its own charger and was on promotion at the time so it was only £30.

I attached it to my various Eheim external filters and Hydor pumps briefly to test it and listened up close to the filter as I switched the inverter on. The filters and pumps sounded no different and ran fine for an hour before I ended the test.

Choosing a battery

I needed for my use a sealed lead acid battery capable of delivering 30W for 6 hours. This works out at 180W consumed. I rounded this up to 200W (33W an hour) because as the battery ages its capacity to hold power will slowly degrade over time and I'll want it to last at least 3 years which is the average life of a car battery.

Look for a battery with a discharge rate of at least double your requirements. That is in my case capable of supplying ~17Ah (200W / 12V). This is quite a low value for a car battery as most are designed for much greater current discharge.

Maintenance free

I also wanted a sealed, no fuss battery with no topping up with distilled water every month. Also these can't leak if knocked over or fiddled with by a youngster.

If possible get a gel filled model as these will operate even if upside down or tilted.

Deep discharge

Next, I wanted (if possible) to get a deep discharge model. These are designed to be used until almost exhausted. Most lead-acid batteries are designed to be briefly used like just starting a vehicle and so never get seriously discharged. When any lead-acid battery is totally exhausted (ie goes below 10.0V~10.5V) it is dead. You can't recharge it and you'll need to buy a new one.

What type

I looked at motorbike batteries initially. But most seems to require a acid pack, topping up and were not rated high enough.

I soon found a web site (Battery Masters) selling all kinds of industrial batteries for small electric powered vehicles and soon spotted a battery just right for me. A Ultramax 12V 20Ah SLA battery for £26.

Choosing a Charger

I wanted to place this battery under constant floating charge so I needed to choose a smart or intelligent charger (see Battery Charging on Wikipedia). As I've chosen a 20Ah battery I need to charge it slowly at less than 1A.

That is at ~100th of it's maximum capacity and the charger must detect whenever the battery is fully charged and turn off (that's where the smart feature of the charger comes in). So decided on a Ansmann ALCS 2-24 A Charger for a mere £18. This is designed to float charge and to be left permanently connected to the battery even if the Mains powering the charger disappears. Perfect.

If you're more into DIY, you may wish to use a charge regulator device. This takes a 15-20V DC source of up to 1.5A that you may already have and is attached to the battery to keep it topped up. When the battery is full, the device switches off until needed again.

This module can be used with solar panels as well if you wish to investigate that route. Using a simple 2.4W or above 12V solar panel instead of a Mains DC supply would be an interesting way to keep the battery topped up.

Connecting them together

When connecting a large battery to the Inverter you must ensure the battery is stored in a upright level position unless it is a sealed lead acid type (SLA) in which case you can suit yourself. Connect the battery to the inverter with suitable thick car battery type leads capable of carrying more than the level of current when in full use.

I had calculated that my inverter should in theory use ~30W. But inverters are never 100% efficient. It'll always draw some power for itself. Mine was drawing about 0.5A extra at 30W load from the battery. Which meant it was using 6W just to power itself. This meant it was only 83% efficient. So total current being drawn from the battery was not 2.5A but 3.0A.

Therefore any wires you use to connect the inverter to the battery must take this into account and be of sufficient thickness to carry this full current. So be careful and get at least the minimum current rating thickness of wire for your needs as otherwise you could risk a fire if you use too thin a wire.

Make sure the wires are attached to the battery and to the inverter securely and cleanly. Inverters often have a car fuse fitted either on the body of the inverter or in the supplied lead. Check that this is of sufficient rating to carry your current with say an extra 25% for safely. In my case 3.5A is more than big enough. I checked the rating of the supplied car fuse in my inverter and it was 25A! Far too big. I wanted to be safe, so I replaced it with a 5A one, the smallest I could find.

Don't have coils of wire just in case you think you'll need to extend the wires at some future state. These coils can cause electrical interference. Keep them reasonably short and sweet.

Attach the Charger to the battery and observe it charging the battery.

Now the first time you attach your new battery/charger together, you'll have to charge it up before you can test it with your inverter. Your smart charger should tell you somehow when its fully charged and switched to trickle or float charged.

  • Remember to keep your battery in a well ventilated area. Charging SLA batteries give off hydrogen which can be toxic and highly inflammable in a closed area!

Once the battery is charged, switch off the charger and plug a decent sized load onto the inverter. I plugged in my 20W Eheim filter and listened carefully to ensure it sounded the same as when it was connected to the Mains. (If it doesn't then you'll need to buy yourself a pure sinewave inverter and use that instead.)

All was well as I tried all my various pumps and filters together in the inverter.

With the fully charged battery I allowed the setup to power my full 30W load continuously. Every hour I felt that the inverter wasn't too warm, the various wires weren't getting warm and the aquarium equipment was still working.

After 6 hours, the inverter was still powering my equipment and I was happy that it was all working.

  • Bear in mind new equipment is always at its most vulnerable at this stage. If it's going to fail, it'll likely be in the first month of use. So test and test again.

First Problem

The only thing about this set-up is that you need to be present when the power fails. Imagine it fails when you're out or in the middle of the night?

Ideally I wanted an automatic switch of some kind to kick in the inverter when the Mains fails. But so far I haven't found such a beast.

Use a UPS

Sure you can buy UPS (Uninterrupted Power Supplies) which are usually used by computers to keep the power going during a black out. But often I find these are designed to switch off after 20mins as they assume the computer has had time to power down safely. If you find one that doesn't do this, great.

These devices actually constantly charge their internal batteries as they're actually in use 24/7. So in my experience the UPS batteries usually fail after 3 years due to this constant use and during their lifespan their capacity gradually reduces. But if you can find room for them and they're not too expensive then sure go for it.

Remember UPS's usually quote their power rating in 'VA', this is the AC equivalent of Power in watts called Apparent Power. Basically if the UPS says it is capable of 500VA, this is ~0.7 x 500 or ~350W for one hour. But due to this decline in capacity over the years, I suggest seriously over estimate the amount of power you will require.

Auto-Mains Switcher

I decided to make my own automatic Mains switcher. This uses a simple DPDT (double pole, double throw) Mains power relay which is constantly energised by the Mains and using the double pole, it routes the power from the Mains plug to the aquarium equipment socket.

The inverter (which is connected to the battery and smart charger 24/7) output is plugged into the second plug.

Should the Mains fail, the relay will drop and swap the Mains voltage for the inverter power to the aquarium equipment.

See pictures.


  • Plastic food box
  • 2 metres of 3 core Mains cable rated to 6A
  • 3x 3 way Mains wire connector block rated to 5A.
  • 1x Relay socket with screws.
  • 1x 240VAC @ 5A DPDT relay
  • 2x neon lamps (1 red, 1 green)
  • 2x Mains plugs
  • 1x Mains socket
  • 2x 3A Mains fuse

Everything was a simple case of wiring it together with a screwdriver and drilling some holes for the cables inside the food box. I took out the 13A fuses (way over kill) from the plugs and fitted 3A ones. Even 3A is a little over the top (3 x 240V = ~700W) but it would be more than enough for my use. The wires and relay I used were rated at ~5A and I certainly wasn't going to use any where near that level of power. So the 3A fuses were a good safety factor.

I took only 2 hours to put it together and once I tested it and got it working I filled in the external holes with silicon sealant gel so they were water tight. Can't be too careful.

Please ensure that you know what you're doing with live Mains projects like this. You can seriously hurt yourself if you are not confident of what you're doing. Double check all wires, make sure the ends are making a fast and tight connection with the connection blocks or the relay socket and there is no fragments of wire exposed.

  • Note I used different coloured Mains cable to make it really easy to identify which of the two plugs I had to plug into the Mains and labelled them. But if I had accidentally swapped them over, no big deal. The pumps would come on, but the red neon wouldn't be lit.

With the Mains plug in place and live, the red neon lights and the relay clicks into life and the green neon lights also to indicate that the aquarium equipment is getting power.

When the Mains fails, the red neon goes out and the green neon should remain lit as the inverter takes over.

That's it.

Last pointers

To save some money I went to a cheap supermarket and bought myself a basic 1 Metre Mains 4 way extension lead and one with a socket/plug for hardly anything. I cut off the ends and discarded the 4 way end. This provided me with ready to use plugs.

The food box was a good bit cheaper that using a real plastic hobby box.

The end cost was about £20 for the auto-switcher. Half of that went on the relay, its socket and the two neons.

This auto switcher performs its duty very well. But it isn't instant. There is a tiny delay of about 0.5sec. So if you plug anything into the output socket be sure it automatically turns back on when every it is powered up. This shouldn't be an issue with the majority of aquarium equipment. But something to look out for it you wish to use it with other electrical equipment.

I also in the end opted for a 4PDT relay as one of the extra two poles could be used in theory to switch on an alarm when the Mains fails and activate a small 9V PP3 battery and a simple two tone or musical piezoelectric buzzer?

You may wish to fit a Mains Transient Suppressor (high voltage Suppressor) MOV component across the Live/Neutral of the Mains input to ensure no Mains power surges accidentally get fed into the aquarium equipment on switch on after a blackout. Can happen when engineers repair the fault, switch the Mains back on and fuses blow in your equipment due to the momentary excess surge.