Actually there will be a lot of saved energy.
You're under the impression that the cellular data radio goes completely dormant when you're connected to and using wifi, which isn't necessarily true. The cellular radio still has to maintain a cellular data connection whilst on wifi, even if it isn't being used for data. It does this so when you jump off wifi you immediately have a cellular data connection. Sure it won't be
used for data while on wifi, but it has to be
maintained while on wifi and that drains battery. It drains even more battery if your cellular signal is weak, and/or if there are multiple network speeds available (3G/4G/LTE). It may find one signal stronger at times, switch to that, and then find another signal to be stronger, and then switch to that, etc etc... all these broken, refreshed, changed cellular connections are happening whether or not you're on wifi.
For the longest time when in my home I've turned wifi ON and cellular, bluetooth, and location OFF (as I have no need for those radios to be on while I'm at home). This DOES manifest in longer battery life.
I discovered BatteryDoctorPro through following @CydiaRepos. Glad I did. It's a phenomenal app, incredibly made. Also features a pull down menu with toggles, and built in "profiles." (tap on the top right corner of the status bar for the pull down menu which includes access to toggles and profiles. or tap the springboard app icon for the full package.) I could always "feel" the longer battery life, but BatteryDoctorPro actually gives you the numbers. Which is awesome. I've screencapped a few configurations to show the difference, according to BatteryDoctorPro, toggling things off makes.
Note that the Data toggle encompasses Edge, 3G/4G, and LTE. The 3G toggle has to be on to use anything other than Edge, however. (that's why the battery life drops so dramatically when the 3G toggle is on. Edge doesn't really use that much juice, but the others do).
(edit: don't pay attention to the Mode Setting, Outdoor, Indoor, Alarm stuff. I just used the Alarm profile to demonstrate the changes each of these radios makes to the expected battery life. For the record my Alarm profile only has Data on.)
With all radios off:
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With WLAN (wifi) on:
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With WLAN (wifi) and Data on (just Edge though because 3G is off):
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With WLAN (wifi) and Data and 3G on (Edge + 3G/4G/LTE):
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With WLAN (wifi) off and Data and 3G on (Edge + 3G/4G/LTE):
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With just Data on (just Edge):
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Check out the app. BatteryDoctorPro. It's absolutely stellar, the quality is through the roof:
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