The 1.5% feature of the notes declines significantly in value in a rising interest rate environment. If the conversion criteria are met this month, then next quarter a note holder will find more receptive potential buyers if they have the option to convert at anytime during the quarter. That increases the price they might be willing to pay. If the share price were to eventually enter a prolonged decline, it's conceivable that next quarter would be the only quarter in which the conversion option is available until 2018. For all anyone knows, in 2018 the shares could be worth $38, which is more than I paid seven months ago. In the meantime the liquidity for the notes could dry up. Premiums could evaporate. It's quite possible that the shares could peak next quarter and begin a decline that would motivate conversion and quick sale of the new shares, especially if the notes lose their added premium. The right to do that has value. Right now it is uncertain if note holders will have that right next quarter. If that right becomes guaranteed, then that should increase the value of the notes, at least during that quarter.
Actually most of the note premium is made up of the downside protection along with full upside potential. This is why the note carries a nice premium over plain conversion and over non-convertible issues.
The notes are senior debt, so they are paid off first before anything else. Common shareholders can be wiped out but the convertible note holders are holders of senior debt, and are obligated to receive their $1000 per note in 2018 unless Tesla has gone bankrupt (and even in that case the note holders are first in line to receive payment for liquidation of factory, etc).
When the stock was $92 (at the secondary offering) the note holders could have just bought common stock instead of the notes. But they paid an equivalent of $125/share ($1000 for a convertible note that could convert to 8 shares). This is a $33 (36%) premium they paid over just buying common stock.
The reason they were willing to pay the 36% premium over just buying common stock was largely because of the downside protection the convertible notes offered. Whether the stock was at $1 or $50 in 2018, they would still get their $1000 per note back from Tesla (or an equivalent of $125/share). So any downside under $125 is protected. They have zero loss if the stock is under $125 in 2018. This is of course Tesla remains solvent (but even if Tesla is insolvent note holders would be first in line still). So, it's more like if the stock is $1 or $124 in 2018, it doesn't matter the note holders will still get $125/share (or $1000 back).
This is great downside protection and something common stock can't provide.
It's kind of like buying a built-in put of sorts. This is the main reason the note holders paid a 36% premium over just buying common stock.
The other reason for the premium is that while the notes provide great downside protection under $125/share, the notes expose the note holder to the full upside potential of the stock above $125/share. In this manner, the note (purchased for $1000) acts like 8 common shares. If the common stock is $1000/share in 2018, the note holders realize the full gain from the stock rise (from $125 to $1000) when they convert their note to common shares. They'll have 8 common shares per $1000 note, so if each common stock share is worth $1000 in 2018 then their 8 commons shares will be worth $8000.
So, it's like buying stock at $125/share (when the common share price is $92/share) because your stock has full upside potential of stock but comes with almost full downside protection under $125/share. This is worth a lot to some people, thus the 36% premium paid.
The other benefit is that the note holder gets 1.5% annual interest. This is in addition to the full upside potential and almost full downside protection under $125/share.
Combine these three factors, and the notes are very attractive and it's no wonder that Tesla was oversubscribed in the secondary offering, and it's no wonder why people paid an equivalent of $125/share for these benefits while the common stock was trading at $92.
Now, fast forward to today, and the stock is around $160 or so.
The note still has the three main benefits: almost full downside protection under $125/share, full upside potential over $125/share, and 1.5% annual interest.
This is the reason why the notes fetch a premium in the open market.
If the note holder chooses to convert these notes early (ie., if conversion criteria is met) then the note holder is only realizing the conversion value (ie., value of 8 common shares for each $1000 note) and immediately loses the other inherent value factors of the note (especially downside protection).
Sleepy noted an example if you converted the note to stock (if it was possible today) you would get $130/share. This is the conversion value. However, on the open market the note is trading for $146/share. This is because the note has the added value I mentioned above.
Now, what happens when the early conversion criteria is met and the note holders can convert the notes early. Does this add any value to the note itself? I would say no.
The reason being is because by converting early you lose all the inherent downside risk protection that the convertible note provides. This downside risk protection is the main reason why the note is bought/sold with a premium in the open market. According to the example Sleepy brought up people are paying a 12% premium for note over common stock. The reason for this 12% premium is because the note holder is protected from almost all downside risk under $125/share. It's like having a built in put of sorts. This is worth a lot. By converting early, you're throwing away this value. It would be foolish to convert early. If you no longer want to be invested in Tesla as a note holder, it's much better to sell the note in the open market and get the fair market value for the note (which is conversion value plus note premium which includes downside protection under $125/share, along with some interest). In other words, you'll get 12% more for selling your note in the open market vs early conversion.
Curt, you mentioned how note premiums could evaporate and liquidity dry up if the stock price declines. Actually, threat of stock price decline only makes these notes more attractive because of the stellar downside protection they offer (ie., anything under $125/share is protected). These notes are very attractive and will always carry a premium over common stock because of the extra benefits they offer.
Again, if note holders want to cash out now they can do so by selling the stock in the open market and receive a 12% premium over conversion value.
Lastly, you mention that note premiums could increase if early conversion becomes an option but this is assuming that early conversion is desirable and has value. When early conversion fetches the note holder 12% less than the open market, early conversion has no real value. Thus, having an early conversion option effectively adds no real value to the note premium.