I know I can use force fields but those seem a bit out of reach in terms of energy.
Then setup an intelligence system rig that uses the energy as efficiently as possible such as it's set to only turning itself on when they are within range/base begins to take damage. Since you have Magnetic Engines, make a couple of max size steel Railcraft tanks and hook up your lava supply line(s) to it top valve to filler them up! Then you will need a butt load of those engines and hook them to up to the lava supply line from the "lava batteries". I know the controls may be a hassle to figure out at first but that force feild can be turned on as needed without requiring a full ridiculous energy amount. Let's take this for example
You have a Force Field setup that requires 45,000 RF per tick, you may be saying "yea so a lava bucket would only run that for 4 ticks or I would need a constant lava supply of 250mb per tick just to keep it working!!". That what you may think but I doubt that you need that much protection 24/7, all you need is have that energy supply
long enough to defend your base until they are
out of equipment/time to do so.
So let's say for instance for every 24 hours they will spend an average of an hour a day trying to destroy your base and they do it at about every 5 days. With this data we can plan out how much lava you would need to buffer for their moves. Here what we have accordingly to this example
Attacking every 120 hours cycles lasting 5 hours per cycle, so basically in 120 hours you need to have enough lava buffered (more if you can to better prepare for more damage) for at least 5 hours of constant power consumption of 45,000 RF per tick.
So let's do the math
1x5 (since a bucket of lava will run it for 4 ticks) which is 5 buckets per secound then..
5x60=300 so that is 300 buckets per minute then..
300x60 is 18,000 buckets per hour.. So to keep them at bay we will need..
18Kx5 is 90,000 lava buckets per attack cycle of 5 hours, now you will say.. (No way I have 100's of pumps at the nether to get that 18K lava per hour!!". Now remember in this instance the attack is every 120 hours so let's see how much lava you would actually need per hour to secure your base according to this data.
90,000/120= 750 buckets per hour, now let's see how much lava that is in per minute and per second to best effort to figure out the pump(s) you will need.
750/60= 12 and half buckets full every 60 seconds now already that doesn't sounds too bad right?
12500(converted to mb to get an accurate output)/60= ONLY 209 mb per 20 ticks (rounded up so that should makes up for the extra lava needed to power the pump(s))! A normal Buildcraft pump can pump 2 buckets per second (one full bucket per 10 ticks cycle) if supplied with 10Mj/t (one and half fully powered Magnetic engines) and that you can transfer that much lava. Slap a pair of liquid tesseracts and your good to go! Expect.. you will need to relocate the pump every time the nether lake that is pumping runs dry, so have a couple at the ready and you should be fine to relocate say like 3 daily instead of worrying about the one running dry so quickly.
So you see even a powerful contraption could be powered without necessarily "having" to have that amount of power at the ready day/night. The only drawback of this sort of plan is getting it's started because it sounds much better on "paper" than what's actually happens (like seriously who would actually attacks in defined cycles that I said here??). Then you keeping it up and running you can work on more powerful turn up and you can then be at the ready for longer hauls.