1 letter:
.

2 letters:
ee, hp, kw, sa, th

3 letters:
4wd, age, asp, car, ebb, eld, eon, esp, fws, nrc, pto, saw, see, var, wit

4 letters:
aeon, bast, bore, cube, curb, czar, devi, draw, dumb, egtk, fall, foul, gain, grab, grid, hold, holy, juju, know, lakh, legs, line, load, melt, mojo, mute, outa, play, pull, real, rule, sirc, stay, vest, veto, volt, wane, watt, weak, wits, yoke

5 letters:
adorn, ambit, badge, beria, bossy, cabal, cable, check, court, crown, curse, drill, drive, eagle, elect, exert, faith, flora, flume, fomor, force, fusee, grasp, heavy, humor, junta, junto, kw-hr, large, light, midas, orbit, party, plane, plant, prime, proxy, pylon, raise, range, reach, reign, reins, royal, saved, scope, servo, shaft, spain, steam, think, vocal, wield, yacht

6 letters:
100000, abulia, abwatt, action, agency, allure, borgia, bridle, buffer, clothe, cogent, degree, depute, deputy, dredge, ebbing, espana, fasces, googol, greedy, hammer, hookup, humour, invest, jigsaw, kingly, master, memory, moloch, moment, muscle, office, pathos, peanut, people, planer, potent, router, ruling, sander, say-so, sekhet, shaker, speech, square, strong, tamely, throne, tyrant, unable, undera

7 letters:
aboulia, ambrose, arbiter, attract, brisant, chaldea, command, compass, control, crucial, decimal, deposed, diopter, draw in, driving, empower, faction, farrell, federal, freedom, fretsaw, gaddafi, gangsaw, give up, gondola, halcyon, harness, jackson, khadafy, kiloton, lepanto, linkage, mandate, marbles, massive, mastery, megaton, milldam, nominal, outstay, phallic, philter, philtre, plastic, powered, prevail, priapus, probate, pull in, qaddafi, qadhafi, refract, regnant, signory, silence, squeeze, stamper, stevens, subject, suharto, supreme, thorium, titania, titanic, unitary, upgrade, valence, valency, warlord

8 letters:
abdicate, abnegate, almighty, approver, ardently, blackout, bullocky, buzz saw, capacity, chainsaw, chaldaea, cogitate, converge, coronate, creative, crushing, decisive, delegate, demonize, denotive, disposal, dominate, domineer, dominion, dramatic, efficacy, ego trip, enabling, enormous, enthrone, evil eye, faustian, fiducial, fomorian, forklift, fracture, generate, gustavus, helpless, impotent, interest, invested, kilowatt, language, leverage, likewise, maintain, megawatt, mitchell, motivity, muscular, nautilus, nepotism, overload, polisher, politics, possible, powerful, prestige, quelling, reigning, religion, remember, renounce, republic, sceptred, shrunken, slippage, smoother, stifling, strength, totalism, turf war, unbloody

9 letters:
absorbent, attention, authorise, authority, authorize, ayatollah, big stick, breakdown, camarilla, captivity, cerebrate, chain saw, challenge, chernobyl, chokehold, dead axle, decidinga, democracy, dependent, detergent, detersive, digestive, dominance, dominated, elevation, empowered, endurance, ergometer, expiative, expiatory, eye of ra, fetichism, fetishism, free will, headstock, heat lamp, herculean, hopped-up, impellent, impotence, impotency, impulsive, inability, influence, ingenuity, kingmaker, live axle, loadstone, lodestone, magnitude, mandatory, milliwatt, monarchal, monarchic, naja haje, naked eye, nilpotent, operation, overdrive, paramount, plutocrat, political, powerless, power law, power saw, prevision, proconsul, proxy war, puissance, quadratic, recognise, recognize, repellant, repellent, retention, saber saw, satellite, sceptered, seigneury, seigniory, sovereign, supremacy, surrender, tolerance, translate, tyrannise, tyrannize, unfitness, unpowered, war power, watergate, watt-hour, wind farm, wind park, witticism, wittiness

10 letters:
absolutism, absorptive, alienation, allurement, apotropaic, ascendance, ascendancy, ascendence, ascendency, assumption, attractive, autocratic, brihaspati, cartwright, centralism, clamatores, cleverness, compliance, conductive, debt limit, denotative, detergence, detergency, devolution, discretion, dominating, domination, driveshaft, drive line, durability, entrenched, execration, full-scale, generative, googolplex, guomindang, horsepower, incapacity, involution, kuomintang, lieutenant, locomotion, louis viii, occupation, omnipotent, opposition, overriding, peremptory, persuasive, pilot lamp, plant life, power-dive, power cord, power dive, power grid, power pack, power play, power unit, preemptive, prescience, presidency, propulsive, receptacle, regenerate, relinquish, republican, resistance, scroll saw, submission, superhuman, talismanic, temptation, tongueless, tremendous, triphammer, trolleybus, vaticinate, vitalizing, waterpower, weak-kneed, wind power

11 letters:
abstractive, acknowledge, appointment, biquadratic, cherry bomb, clairvoyant, clericalism, combustible, connotative, containment, countercoup, determinant, drum sander, dynamometer, effectivity, efficacious, facultative, fluid drive, fusee drive, great falls, gustavus ii, idiotically, ineffective, ineffectual, influential, intra vires, isometropia, lastingness, legal power, life-giving, love-potion, low-powered, magisterial, meritocracy, mind reader, monarchical, old hickory, omnipotence, originative, penetrating, penetrative, pilot light, power drill, power meter, power pylon, predominant, predominate, punch press, quasi-royal, retentivity, royal court, scientology, servosystem, six-day war, six day war, st. ambrose, stock power, suppression, telekinesis, telepathist, ultra vires, volt-ampere

12 letters:
all-powerful, annihilative, atomic power, augmentative, calixtus iii, carpetbagger, circular saw, clairvoyance, commonwealth, common stock, condemnation, debt ceiling, deflationary, degeneration, determininga, disconnected, driving axle, effectuality, emasculation, enabling act, enduringness, excogitative, illegitimate, intoxication, jurisdiction, laying claim, love-philter, love-philtre, megaton bomb, motive power, police court, power-driven, power broker, power hammer, power hunger, power module, power outage, power system, power worker, preponderant, preservative, probate will, productivity, propitiatory, racing yacht, riding mower, ruling class, second power, second sight, significance, squeeze play, steam shovel, stevens' law, storm troops, stranglehold, supercharged, temptingness, throttlehold, transmission, unattractive, uneconomical, unscrupulous, word-painter, yellow peril

13 letters:
amplification, anisometropia, anisometropic, authorisation, authorization, bicycle chain, british crown, common shares, decentralised, decentralized, determinative, discretionary, effectiveness, effectualness, hydroelectric, imperceptibly, incapacitated, inefficacious, infrared lamp, ingeniousness, inquisitorial, insignificant, inventiveness, invested with, kilowatt hour, light circuit, nuclear power, parliamentary, powerlessness, power failure, power loading, power takeoff, preponderance, prime of life, psychokinesis, retentiveness, rocket firing, rotary engine, saint ambrose, spoils system, titanic oxide, traffic court, transposition, trolley coach

14 letters:
aggrandisement, aggrandizement, alfonso borgia, american party, andrew jackson, billy mitchell, centralisation, centralization, compressed air, convincingness, egyptian cobra, eileen farrell, electric drill, electric meter, electric range, eminence grise, exponentiation, extension cord, external drive, indicator lamp, ineffectuality, mental ability, military junta, motor-assisted, multiplicative, naval blockade, nuclear weapon, persuasiveness, power-assisted, power steering, preponderating, productiveness, representative, roman republic, rotating shaft, saber rattling, sabre rattling, sawing machine, servomechanism, shadow cabinet, smitty stevens, spike arrester, status seeking, thought-reader, titanium oxide, turbogenerator, water softener

15 letters:
efficaciousness, electric sander, enabling clause, high-and-mighty, ineffectiveness, ineffectualness, interestingness, kilovolt-ampere, ordinary shares, planing machine, pneumatic drill, political party, supernaturalism, surge protector, totalitarianism, wind generation

16 letters:
action mechanism, arab-israeli war, atomic number 90, balance of power, binomial theorem, classified stock, decentralisation, decentralization, decimal fraction, electrical cable, four-wheel drive, hundred thousand, hydroelectricity, kingdom of spain, lighting circuit, military control, mover and shaker, political system, religious belief, rocket launching, s. smith stevens, spike suppressor, stamping machine, strategic buyout, supernaturalness, surge suppressor, titanium dioxide, unproductiveness, volatile storage, william mitchell

17 letters:
adhesion contract, battle of lepanto, drive line system, edmund cartwright, executive council, gustavus adolphus, nuclear submarine, nuclear terrorism, reciprocating saw, spark transmitter, sturmarbeiteilung, supreme authority, synchronous motor, trackless trolley, transmission line, watergate scandal

18 letters:
compulsory process, electrodynamometer, executive clemency, flashlight battery, form of government, know-nothing party, legislative branch, lightning arrester, mental telepathist, muammar al-qaddafi, muammar el-qaddafi, national socialist, nuclear propulsion, order of magnitude, political relation, quadratic equation, russo-japanese war, stevens' power law

19 letters:
board of trade unit, electron microscope, nonvolatile storage, self-aggrandisement, self-aggrandizement, suborder clamatores, transmission system

20 letters:
combustible material, contract of adhesion, legal representative, non-volatile storage, nuclear-powered ship, power-station worker, wind energy facility

21 letters:
church of scientology, output-to-input ratio, stanley smith stevens

22 letters:
electrical engineering, exponential expression, heavier-than-air craft

23 letters:
constructive possession, extrasensory perception, power-assisted steering, rheumatic heart disease

24 letters:
executive vice president, lavrenti pavlovich beria, small-particle pollution

25 letters:
articles of confederation, nuclear-powered submarine

27 letters:
tupac katari guerrilla army

28 letters:
us fish and wildlife service

29 letters:
nuclear regulatory commission, war of the spanish succession

38 letters:
security intelligence review committee

39 letters:
united states fish and wildlife service

40 letters:
national socialist german workers' party