This is a free service and it is provided by
http://www.onlineleaf.com/
.Their standby engine is deliver a fully functional and simple way to help your website run requiring less energy to generate. It hides heavy animations, covers the window in dark colors (as these, in many cases are less energy consuming) and pauses heavily running background processes.

When your visitors are inactive, this engine launch a standby screen, with the text "Energy saving mode".

Login to your Blogger dashboard --> Design --> Edit HTML.

You don't need to click on "Expand Widget Templates".

Scroll down to where you see the </head> tag of your template.

Now copy below code and paste it just before the </head> tag.

<script language='javascript' src='http://www.onlineleaf.com/savetheenvironment.js' type='text/javascript'/>

This standby engine uses the jQuery Javascript library, so if you are using other Javascript libraries or code, add below code instead of above code :

<script language='javascript' src='http://www.onlineleaf.com/savetheenvironment.js' type='text/javascript'/>

<script>jQuery.noConflict();</script>

Time of inactivity

Also you can easily define
how long time your visitors have to be inactive
, for the engine to launch the standby screen, by adding ?time=X where X should be replaced with the number of seconds you would like to define the time interval. An example could be:

<script language="javascript" type="text/javascript" src="http://www.onlineleaf.com/savetheenvironment.js?time=120"></script>

... which will set the time of inactivity to 2 minutes (120 seconds).

This can be configured to display in any of the supported languages, if you add ?lang=code, where code is one of the language short codes below.

Supported languages

<script language="javascript" type="text/javascript" src="http://www.onlineleaf.com/savetheenvironment.js?lang=es"></script>

If you are using WordPress, just download their plugin, activate it and everything should work instantly.