Downloading restarts every time after restarting Steam

This is not only dota-2 problem but it happens with every game. Each time a power cut occurs steam starts to download the game files again.

There are two ways to solve your issue -

1. Pause the download:- if you pause the download in progress before the power cut, it will surely resume from the stage you paused it.
2. Restore/backup:- steam also offers another way to your problem. Download from the internet or take a latest backup of dota-2 from your friend in a pen drive and just restore it.

@amit, as others have already noted, there is no "solution" per se for your problem. I suggest you try a simple workaround. As I've already stated here, you can paste the game files anywhere on your PC and when installing it on steam you just choose the folder they're in.

So, if you can get a copy of Dota 2 installation files (not the package, but the already installed game) you could install without downloading (it would still check all of your files, but it should be way faster).

If you don't know anyone with the game installed I suggest you try ciber cafes or ask online for someone to upload a torrent of it.

First of all, DoTA is only about 20GB.
Secondly, your download constantly restarts (well, backtracks; progress jumps) because when your computer power goes out (assuming it's the instant-powercut-and-everything-goes-black-and-the-system-suddenly-shuts-off type of power cut), Steam doesn't have a chance to properly finish finish downloading the file it is patching. Because of this, when the system restarts, Steam checks for the progress and will think the file it was patching is corrupted (not properly and fully downloaded). From there, it will restart downloading that file.
Usually, Steam downloads several files at once, which explains why there are big jumps.

Also, don't delete anything (not even use the "Uninstall" button in Steam) whilst something isn't downloading properly. Let Steam manage that; it automatically scans for any corrupted/incorrect files and delete/redownload/reaquire them.
Deleting/uninstalling will only mean it will have to redownload everything again. from scratch, which is the same as hindering your progress.

If you are constantly suffering power problems and cannot keep your computer on long enough to download all the files, you should consider not downloading DoTA as it contains lots of large files (and thus mean you'll be very likely to have a power cut before Steam manages to fully download a single file). Or instead, switch to battery power, if you have it.
Pausing the download and quitting before the power-cut (if you know when it happens) will also help as Steam will be able to leave off from the point that you paused it at.

I can't say for sure this will fix it, but I'd suggest:

1. Exit Steam.
2. Delete or rename Steam's ClientRegistry.blob file (usual location: C:\Program Files (x86)\Steam\ClientRegistry.blob)
3. Start Steam.

This forces Steam to recheck which games it considers to be installed.

after changing fire wall settings and user account control settings and my dota resuming .......... i found this suggestion somewhere else but thank u all guys for ur help .....http://prntscr.com/4thpx1 i have downloaded 82% now and after 3 power-cuts it still resuming....

The only option is to continue downloading your game without any breaks. If you have low KBPS consider replacing your internet