A major power drain on the beeb is the 16 DRAM chips. The HM4816-3 chips on the original design draw 88 mA each, or about 1.4 amps total.

The Mostek MK4516 fitted on my beeb use 25 mA each, or 0.4 amps total. These reduce the total power budget from 2.5 down to 1.5 amps.

That is as low as one can reasonably expect without hardware mods.

An easy mod, swapping the NMOS 6502 (160 mA) for a CMOS 6502 (3 mA at 2 MHz) would save 157 mA.

Swapping the many DRAM chips for a single 32K CMOS RAM  would probably save almost all of the DRAM current, bringing Icc down from 2.5 to 1.1 amps. 

The DRAM is soldered on my beeb, so not an easy mod.

The DRAM address multiplexing is designed around row/column for DRAM, so perhaps it is simpler to change the sixteen 16K x 1 bit chips to two 64k x 4 bit chips. Not as power-frugal as SRAM but two TMS4164 chips take 27 mA each, 54 mA for two, which is still a good saving.