Building a UPS, line-batt transfer

Happened to have enough parts laying around to build a UPS for the most part.

Now, whats the best way to transfer from line to battery power and back. AC isn't my specialty, so I'm hung up here. I don't want to simply run the battery chargers and inverters full out the whole time. I'd like to have them 'idle' when not in use. I could just use a relay that falls closed when line power drops, but I'm not sure that's going to be fast enough. I could use triacs to switch it 'instantly' but couldn't that pose a problem as well? A too fast jump from a true sine wave to a modified one could lead to surges?

Any suggestions?

The simple way implement a glitch-free "transfer" is to operate the AC load from the modified sine wave 100% of the time. The modified sine wave generator is connected to the battery 100% of the time and is supplied (if it is a lead acid battery) with a current-limited, constant-voltage charge voltage.

You are correct that a mechanical relay will take time to switch. For a small power relay (15 amps) this can be 10 ms or more. Triacs can switch very quickly but it will take some time for the modified sine wave circuit to come up. Can the load be allowed to loose power for a couple of AC cycles?

A few cycles probably won't matter. It'd have to power 5 computers, all which are loaded down with many extra hard drives and run hard 90% of the time. All the networking gear runs on 12 or 5VDC so that I can just run full time from the batteries with voltage regulators.

I figured keeping the inverters running would reduce the time it took things to come up. Then I thought about it and I'd be wasting electricity since I'm sure they still draw a good amount even with no load.

I'm thinking a constant 120 - 12 - 120 will probably work best. In the end reliability is the most important factor, but efficiency would be nice too.

An off-the-shelf UPS (or a few) is an easy and effective way to go. It takes care of everything and will run the PCs for several minutes if AC power is lost.

When the power goes out here its usually at least an hour, sometimes close to 3. Hence a large array of batteries will be needed. Under extreme circumstances that gives me plenty of time to wheel over the generator and recharge the batteries.