<p><strong>The answer is both. The Steem Power (SP) of the main account and the amount of SP delegated both increase with the interest.</strong></p>
<p>When you delegate Steem Power to another user what you are actually delegating is the underlying VESTS (commonly referred to as MVests since the numbers are huge).</p>
<p>Interest is applied to all holdings of SP by a slow increase in the SP / MVests ratio.&nbsp;</p>
<p>So when you delegate SP the interest will accrue to your own holding of Steem Power but will also increase the amount of Steem Power delegated to other users. The underlying MVests (both your amount and the amount delegated) are unchanged.</p>
<p>You can see this by looking at the wallet page of a large account like freedom. The SP and delegated SP holdings increase a little each time you refresh.</p>
<p>However when the delegation is returned, the original amount of MVests is returned which will be equal to the amount of SP delegated plus the interest earned over the delegation period. So the delegated account will benefit from increasing SP over time while they have the delegation but will not retain the amount of interest accrued.</p>
<p>The amount of interest are small so generally it's not a consideration for most users.</p>
<p>[Note that this discussion is of the "interest" inherent in the Steem system, not any interest payments from users to whom you delegate the SP].</p>
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