diff --git a/setup/installer/windows/portable.c b/setup/installer/windows/portable.c
index 2a557e9174..05763bc303 100644
--- a/setup/installer/windows/portable.c
+++ b/setup/installer/windows/portable.c
@@ -95,6 +95,11 @@ void launch_calibre(LPCTSTR exe, LPCTSTR config_dir, LPCTSTR library_dir) {
         ExitProcess(1);
     }
 
+    if (! SetEnvironmentVariable(TEXT("CALIBRE_PORTABLE_BUILD"), exe)) {
+        show_last_error(TEXT("Failed to set environment variables"));
+        ExitProcess(1);
+    }
+
     dwFlags = CREATE_UNICODE_ENVIRONMENT | CREATE_NEW_PROCESS_GROUP;
     _sntprintf_s(cmdline, BUFSIZE, _TRUNCATE, TEXT(" \"--with-library=%s\""), library_dir);
 
diff --git a/src/calibre/constants.py b/src/calibre/constants.py
index f6b0a8d802..627f1b751a 100644
--- a/src/calibre/constants.py
+++ b/src/calibre/constants.py
@@ -32,6 +32,7 @@ isbsd = isfreebsd or isnetbsd
 islinux   = not(iswindows or isosx or isbsd)
 isfrozen  = hasattr(sys, 'frozen')
 isunix = isosx or islinux
+isportable = os.environ.get('CALIBRE_PORTABLE_BUILD', None) is not None
 
 try:
     preferred_encoding = locale.getpreferredencoding()