There are two ratings on a UPS. The first is the load it can handle. It sounds like neither of those two things has much of a load, not more than a amps a piece. The other is the duration of the battery charge.

You'll have to provide the specifics on exactly what the draw of your devices is and how long you envision being off the mains to answer your question.

One thing to be careful of. The output waveform of those computer UPSs is usually pretty horrid. While the computer (and your video system) may not care (they switcher power supplies just immediately run the incoming power into a diode anyway), you will need to see if your heater's electronics cares.

How long do you need the UPS to supply power for?

How much are you looking to spend on this? In addition to the caveats above about the lousy quality of the power cheap UPSs produce, you should know that the $30-50 mass market computer UPSes don't have much capacity. They assume that you aren't going to try to continue working on your computer during a power failure. They're mainly intended to power the system until you can save your work and shut down. So they can supply maybe 25-100 watt hours of energy, and then they poop out and shut down.

It's possible to hack these small UPSes for longer runtime (I've done it), but it's probably more hassle than most people would want.

Depending on how long a runtime you need, you may be able to use a fairly large computer UPS, or a small industrial UPS. Cost will be in the hundreds of dollars. For the really good industrial custom-configured units you can get well into the thousands, even for a 1000-2000va unit.

Don't forget that the initial runtime quoted for a UPS will decline as the battery ages. Batteries age even when they're not used. I'd recommend oversizing the UPS runtime by at least 100%.

If you need assurance that you can keep your gear going through multiple days of no mains power, then it might be worth considering a small grid-tied PV system with a backup battery. A reputable PV dealer can help you size such a system for your load and your site's typical insolation.

The other point to notice is that I've had more than a couple cheap UPSs actually fail so rather than providing more reliability, they provide less.

The real spectacular one was the one on our office telephone KSU. The phones didn't work so I drive in on a weekend to figure out why. Notice the power is off in the phone room, and flip the tripped breaker back on. Then the UPS at my feet throws a few sparks and the breaker trips again. Great. No more TrippLite UPSs for me.

Thanks for your responses. To clarify, I want the system to handle these two systems when there is a mains power failure, which usually happens once a year and lasts 24-48 hours. I'll do a bit more research as to the precise power requirements and see what I can find online. Any good online retailers that can be recommended? Thanks

48 hours is a very long time for a computer UPS.

Lets say your surveillance system draws 400w. That's 19.2 kwH or 160 Ah of battery (120v). Assuming 90% efficiency on the inverter and discharging to 80% of capacity, you need a battery bank of around 225 Ah. A quick look over at WestMarine and they had a 245 Ah battery for about $650 bucks. I think you'll need a fairly large inverter/charger for this. Not because of the load, but because of the charge rate of around 25 amps. The Xantrex Freedom 1800 will run you another $620. It's not a true sine wave invertor but hopefully you don't need it. If you do, then expect to pay more.

You might want to think about solar panels.

I don't think you're going to get enough battery at that price.

To give youself some wiggle room and not overdischarge the battery, especially as years go by and its capacity declines, you're going to need more like 40kWh of battery capacity.

Probably the cheapest way to get that capacity is commodity golf car batteries. Each is good for about 1.3kWh and costs $100. About 30 of them should do it.

I'd size your system for a battery voltage of 24v if you can, to keep battery currents reasonable. So maybe drop to 28 batteries, in 7 parallel strings of 4.

For fewer connections you could use L16 batteries (370ah at 20h, 2.2kWh each). You'd need 18 (make it 16, 4 strings of 4) at around $300 each.

A warning, lead is very volatile these days and these prices may be higher by now.

So you can do what you want to do, but definitely not with a simple computer UPS.

A small propane fueled standby generator can be had for about the same cost. A residential grade set is about $2k or a bit more, plus installation.