Deficit

Deficit is created in general when more power is requested from a power interaction block than it has. It will search for another source of power (either a nexus, or another conduit with a known path to the nexus), and request it from there in order to serve the power up. Currently this only applies to conduits as they are the only block that is capable of providing power other than the nexus (but it has special rules and cannot build deficit).
By default, an Essence Conduit is capable of shedding 0.5 deficit per tick, or 10 deficit per second. All power being requested through the conduit creates an equal amount of deficit that is continually shed by the conduit at this rate. So if 10 essence is requested from a block, the conduit will build 10 deficit per second, but is able to shed 20, so it's fine. If more power is being pulled through the conduit than it is capable of shedding naturally, it will begin to build deficit. 
Each block that requests power requests it at a rate of n per second. Base is 10/second, certain blocks like the gateway are at 100/second by default, or the Mana Battery, which draws 1000/sec. Some blocks allow charge foci to be placed in them, which alters the rate at which they charge - this puts an equal strain on any conduits as well. The effects of charge foci in the blocks that allow them vary from block to block, but it's roughly a multiplicative 30% increase per focus on average, and they do stack (multiple slots will stack, not multiple foci in a single slot - only the first one matters).
Combining all endpoints' power draw together will allow you to calculate how much total draw per second, and therefore how much deficit per second will be generated.
It's also worth noting that applying a redstone signal to a conduit will immediately stop it from providing power, but still allowing it to shed deficit, which means it can catch up, or cool down. The more deficit a conduit has, the more the color will shift to red, and it will begin to smoke. So there are visual cues as well, in case you don't have sense energy handy.
==Managing Deficit==
Deficit can be managed in one of four ways. 
# The first, is by placing blocks in such a way that they use different conduit paths. Not always the most efficient, but a good workaround when resources are limited.
# The second, is by placing charge foci in the conduit itself. Charge foci in a conduit will decrease the amount of deficit built up. It's multiplicative as well - experiment with it to find the numbers. (mainly because I don't have the formulas in front of me ATM - this is all off the top of my head)
# The third, is by instead using mana foci in the conduit itself. While charge foci decrease the deficit built up, mana foci increase the rate at which the conduit is able to shed deficit each tick. They also stack multiplicatively. Again, experiment to find the best combination. Charge and mana foci can be combined in a conduit as well; use the most effective combination for your particular situation.
# The fourth is the most powerful, and is really intended for "core" lines, which may perhaps be the main channel out of a nexus, for example. This involves placing a Crystal Pillar right next to the conduit (or more than one, it just has to be within one block). Each crystal pillar provides an extreme boost to the amount of deficit shed per second; 400 to be exact (20 per tick) per pillar. So these are what you use when foci just aren't cutting it anymore - however, they are expensive to make, and are not intended to be commonly required for most simple setups. Something like a gateway central station however may require several of these. Technically, you could fit 26 crystal pillars around a single conduit with each adjacent block to the conduit being filled by a crystal pillar.
Currently, the only negative effect of deficit is that the block can explode. http://www.minecraftforum.net/topic/1486184-146-v532272-ars-magica-an-advanced-magic-mod-for-minecraft/page__st__1280#entry19834984
==References==