which relay?

Hi guys..... I have something a little different I am trying to do. We have a gas hot water system which requires electricity to ignite the gas. If I knew that I would have told the builder just to put an electric hot water unit on the house instead. Anyhow, what I am trying to do is have a backup power system when there is a blackout. My idea is: As the hot water unit requires 240v I use and inverter hooked up to a 12v battery to supply the 240volts when the power goes out. I can work out what sort of relay I would need to be able to make it all work. Here is what I have: I have a 12v transformer that works on 240v house power. A 12v motor bike battery. A 150watt inverter I was planning on using the inverter full time, so in the event of a blackout a relay will switch over to a battery to run the inverter which is normally running from the transformer plugged into the house power. I hope I am not confusing you. My idea is: when all is running well, the house power is running the transformer which is supplying 12v to the inverter.   The inverter is supply the 240v the gas hot water unit requires. When there is a power blackout, then the relay recognises there is no power coming from the transformer any longer and switches over to the battery to now run the inverter. Can anyone please give me an idea on the wiring diagram and what type of relay I would need. Regards Paul

How about using a self-contained Uninterrupted Power Supply intended for a computer? Here in the States, our household voltage is 120V, but I would assume a 240V UPS is available in countries where 240V is the nominal household voltage. Or a small 120primary/240secondary transformer on the 120V UPS to step-up the output voltage.

Yeah, your best bet would be a U.P.S., converted for use with an AGM battery, or Gel Cell battery.   I used to have one that would power my entertainment system for 47 minutes. Then, I put a Gel Cell battery in it, and it would tun for 2 or more hours. With a relay, it won't switch fast enough to be uninterrupted. The way the U.P.S. works, is, by using "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment. Then, when the power goes out, your fully on the battery, there is no actual switching from 120V to backup, you're always running off the inverter. This is why they are always hot, and go through batteries. Think of it this way, and you can do it this way: Put a charger on a battery, and the inverter on the battery. Run whatever it is your running, off the inverter. Thus; you're running from the inverter all the time, however, if power goes out, the battery charger loses power, but you're still running from the battery with the inverter. No relays, no nothing. You'll need a smart charger to know when to power down when the battery is charged.

Domestic UPS should only be using sealed batteries (AGM, Gel, etc). The offline aka standby aka 3-way UPSs do have a "switching time" (sense time plus inverter startup time). True online UPS have no switching upon AC failure but do switch upon overload or transfer to mains (inverter failure etc). A switching break of 5mSecs was considered "no break" for modern loads (PCs etc) hence relays could be used for online UPS (instead of static switches) and most offline/standby UPS met the 5mS spec. It would be handy to know what power your HWS requires, and if it breaks down the AC to a lower voltage (24V or 12V DC etc).