Supercap compared to a battery

I would say, anything that you would consider a coin cell battery for you can "start" to consider a supercap.

The other aspect is that in devices that need current in spurts (like a wireless transmitter), you can use a supercap to provide the short bursts of power needed and then slowly charge them back up from your battery. This method extends the life of your battery.

Supercaps can be worthwhile if you'll be doing energy harvesting. They're much easier to charge/discharge than batteries. If you have lots of incidental power coming in short bursts it makes sense to store it in a capacitor instead of a battery immediately. They don't match batteries for energy density yet (and they're certainly not as cheap joule for joule). If you plan on having this sit around a long time without any power coming in to charge it and expect it not to have lost any charge I don't think you'll be happy. You're better off with a high-resistance battery if it has to sit a long time and not source a large amount of current. The exact determination depends on what you're doing but I think you'll be happier with batteries.

Some ballpark figures I remember from a year or two ago:

• Batteries have 10 times the energy density of supercaps.

• Supercaps have 10 times the power density of batteries.

So for the same size you can store a lot more energy in batteries, but draw much more power from supercaps.

That's why some trams use supercaps and not lithium batteries.