Does Ubuntu reduce battery life?

Without extra care, yes, although this can be mitigated/avoided, and the main cause is just wear and tear. The problem is not Linux per se, but the vendors' focus on Windows optimizations for battery life, and heavier power draw kills a battery faster.

The main things which degrade a rechargeable lithium-based battery are:

  • Heavy discharge
  • Full charge, particularly keeping full charge
  • Number of charge/discharge cycles
  • Heat
  • Storing battery fully charged/Discharge
  • Age - maximum capacity of a perfectly maintained battery still decreases with age.

Because Linux is heavier on the battery than Windows, the amount of heat is higher, causing faster wear. Similarly you likely have more charge/discharge cycles, because the battery is used more.

Some mitigations/trade-offs to improve battery -

  1. If your BIOS supports it, set maximum charge to less the 80% (or even 70%) - shorter work time, much longer battery life in net terms.
  2. Use powertop to reduce power draw.
  3. If you are a road warrior, try charging when you are not using laptop to reduce heat. Likewise a slower charger will do less damage.
    3a. It seems discharging from 80-40% once is less stressful than discharging 80-60% twice.
  4. Don't let your battery level fall below 20%
  5. Try keep the laptop comfortably cool. Leaving it on in a backpack causes heat buildup.
  6. Slow charging and/or using a larger battery causes less wear for a similar amount of usage, because it helps with the above.

(Where numbers are used above, they are indicative only.)

Good answer by davidgo, and that should be your starting point for understanding and minimizing the issue. But the wording of the question implies that you're thinking about this in a slightly wrong way, and I'll focus on that aspect.

You keep referring to Ubuntu "decreasing" battery life, and that's not really an appropriate way to look at it. Say one OS is better than another at optimizing battery life. If you use the better one as a baseline, the other OS won't be as good in that respect, so it kinda looks like battery life is "decreased" in relative terms.

But really, "batteries decrease battery life". When batteries are a source of power, any battery is a disposable item. Everything you do or don't do affects battery life a little. davidgo's answer lists the major factors.

If one OS uses a little more power than another, that might lead to recharging more often. So some factors can affect other factors. But if you get the daily run time you need even with the OS that uses more power, you might recharge at the same frequency and from a discharge level that, for practical purposes, is in the same range. So your usage pattern may be a bigger factor than the OS.

The point is that batteries have a finite life. There are things you can do to optimize and extend the life a little. To the extent it's practical to do those things, they can't hurt. But finite battery life is a characteristic of operating from batteries. In the scheme of things, a few months difference in battery life doesn't represent a big cost. It doesn't make sense to let it drive important decisions.

Let's assume one OS is not as good as the other at optimizing battery life so it costs you a little more over time for battery replacement. You own the computer because it is a tool to help you accomplish things important to you. People spend extra money to get a keyboard or mouse that they can use more efficiently, or a high-resolution monitor so they can see more of their work at once. The same applies to the OS.

You pick an OS because of what it allows you to do, and things like the user interface that make it easier for you to get your work done. You don't pick it because one will save you a few dollars every couple of years on battery replacement. For that matter, if you are comparing Windows and Ubuntu, Windows costs money and Ubuntu is free. So if Windows saves you a few dollars on battery replacement, you haven't saved money, you paid for that in advance.

Ubuntu doesn't "reduce" battery life. It might yield a slightly different life. But as they say, "that's life".