Demons are fallen angels. Are they as strong as angels are? Or are they weaker because they've been "kicked out" from heaven?
I think the question is wholly answered in these few verses :
7 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, 8And prevailed not; neither was their place found any more in heaven.
9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
10 And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.
11 And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.
12 Therefore rejoice, ye heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time.
Are demons strong as angels?
That will depend on the particular demon!
Some demons may be more superiorly stronger than some angels but not all. The fallen angels also called demons did not lose their nature or their connatural gifts at the moment of the turn away from God. Thus a demon from the order of Cherubims will be superior in strength than Angels from the order Dominions.
Since demons have not lost their connatural powers when they fell, one can not say with certitude which demon is stonger than which Angel. The differences between the angels is far greater than the differences between men and the various races of mankind. Each Angel being uniquely created by the Almighty, makes each Angel unique and endowed with it’s own individual powers. We have no way to determine which Angel is stronger than which demon and vice versa, except between the various choirs of angels.
Although it is now generally accepted that there are nine choirs of angels and each choir is of angel is different and ranked accordingly.
During the Middle Ages, many schemes were proposed, some drawing on and expanding on Pseudo-Dionysius, others suggesting completely different classifications. According to medieval Christian theologians, the angels are organized into several orders, or "Angelic Choirs”.
Pseudo-Dionysius (On the Celestial Hierarchy) and Thomas Aquinas (Summa Theologica) drew on passages from the New Testament, specifically Ephesians 1:21 and Colossians 1:16, to develop a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs. Although both authors drew on the New Testament, the Biblical canon is relatively silent on the subject, and these hierarchies are considered less definitive than biblical material.
Choirs in medieval theology
St. Thomas Aquinas in Summa Theologica (1225–1274):
Seraphim, Cherubim, and Thrones;
Dominations, Virtues, and Powers;
Principalities, Archangels, and Angels.
St. Thomas Aquinas' list is by far the most widely accepted list of angelic choirs and places the Seraphic order of angel as being at the highest level of angels followed by the Cherubim, and so on. According to St Thomas:
The angels that rebelled and became demons did not lose their nature or their connatural gifts. They cast away, by their sin, the grace in which they were created. They did not cast away the beatific vision, for they never had it. Now, if we think of angelic orders as orders of angels in glory, then, of course, there are no orders of bad angels. But if we consider angelic orders as order of angelic nature simply, there are orders among the demons.
Certainly, there is a precedence among bad angels; there is a subjection of some to others.
Demons of superior nature do not enlighten inferior demons; enlightenment here could only mean the manifestation of truth with reference to God, and the fallen angels have perversely and permanently turned away from God. But demons can speak to one another, that is, they can make known their thoughts to one another, that is, they can make known their thoughts to one another, for this ability belongs to the angelic nature which the demons retain.
The nearer creatures are to God the greater is their rule over other creatures. Therefore, the good angels rule and control the demons. - Orders Among the Fallen Angels
This stated it only makes sense that Satan was a Seraphim, for surely some of the Seraphim fell in great revolt against God. Although one can not say with certainty it is generally believed Satan was a Seraphim.
The thing that tips the balance in favour of the Good Angels is not only the fact that they were superior in number, but also that they cooperated with God’s grace. St. Paul’s words in Romans 5:20 are very telling here:
Now the law entered in, that sin might abound. And where sin abounded, grace did more abound.
What shall we then say to these things? If God be for us, who is against us? Romans 8:31
Irrespective of what you understand "strength" to mean, the demonic party would never have been "kicked out of heaven" had they been stronger than the angelic party. However, biblical language about such unseen, heavenly, spiritual events need not have a literal, physical outworking, as we envisage with mortals who have varying degrees of physical strength, especially if one side commands far higher numbers of warriors and more powerful weapons.
How about thinking in terms of a legal battle, where God's sovereign rights are challenged, Satan usurping the rule over this earth because he maintains that God has no right to it? And what if God decides the challenge must be lawfully settled, once and for all, so that the usurper will be exposed to all sentient beings as deserving of the most severe imprisonment and punishment? And suppose that settling such a cosmic legal case requires thousands of years?
Just switching your thinking to take on board far more massive issues than mere armies fighting with literal weapons could help you understand what "strength" can mean. Have you ever heard the sayings, "He doesn't have a leg to stand on", or, "There is no case to answer"? Have you thought about the strength of law? Have you taken on board God's requirement for utter justice?
When the Book of the Revelation speaks of Christ being whisked up to the throne of God, it says that then Michael and his angels battled with Satan and his angels, the latter being cast down, and confined to, this earth. They know their time is short before the last trump, the end of time, and entering into eternity in the lake of sulphuric fire as God's righteous judgment of them. All the language is symbolic, not literal. It's meant to strengthen God's people for the tribulations they have to suffer during their very short time on earth, before Jesus suddenly returns to start the Day of Resurrection and Judgment and take up his legal rights of rulership over this earth. Believers in Jesus have to be strong in faith. Strength can include physical strength, but in the Bible it's more likely to mean other things. As here:
"For [Christ] must reign, till he hath put all enemies under his feet. The last enemy that shall be destroyed is death... So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption: It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power... Death is swallowed up in victory. O death, where is thy sting? O grave, where is thy victory? The sting of death is sin; and the strength of sin is the law. But thanks be to God, which giveth us the victory through our Lord Jesus Christ." (1 Corinthians 15:45-46 & 42-43 & 54-57)
The question may ask about supposed physical strength of individual spirit angels and spirit demons, but nobody is going to get anywhere fast in grasping the enormity of "strength" in biblical language until they expand their vision, as does the Bible.