What is W′ Prime?: your anaerobic battery explained

What is W′?

W′ (pronounced “W‑prime”) is a way of describing your anaerobic “battery” on the bike. It’s the finite amount of hard work you can do above CP (critical power) before you have to back off or you simply crack. Think of CP as the ceiling you can ride under for a long time, and W′ as the extra energy you can burn when you go through the roof – closing a gap, hitting a climb, or sprinting out of a corner.

In practical terms, W′ is measured in kilojoules (kJ) and represents how much work you can do above CP. When you ride well below CP, you’re not touching this battery much. When you surge, attack, or grind up a steep ramp, you start draining it. Once it’s empty, you’re done with real intensity until it has time to recharge.

CP and W′: how they work together

CP tells you about your “all‑day” engine; W′ lives on top of that engine as your high‑octane reserve. Two riders can have the same CP but very different W′ values, and they’ll feel very different in races or hard group rides. One might float on steady climbs but struggle with repeated punches; the other might be able to kick hard again and again but fade on a long climb.

When you ride just above CP, you’re dipping lightly into W′ and can sustain that effort for a few minutes. Go much harder – like a full‑gas attack or sprint – and you burn through a big chunk very quickly. This is why you can’t sprint at 1,000 watts forever, and why repeated all‑out efforts start to feel impossible even if your average power for the ride doesn’t look outrageous.

Draining the battery: how W′ depletes

Every time you ride above CP, the “extra” power you produce comes from W′. The higher above CP you go and the longer you hold it, the faster you drain that battery. A few examples:

• A 15‑second sprint at 150% of CP might use only a small slice of W′, leaving you with plenty in reserve.
• A 2‑minute climb at 130% of CP could chew through a big portion, leaving you feeling heavy‑legged and less explosive for a while.
• A repeated pattern of 30/30s or 40/20s will gradually chip away at W′ until you suddenly feel like someone “pulled the plug”.

What matters is not only how often you go hard, but how hard above CP those efforts are. Short, violent surges cost more per second than controlled, slightly‑over‑threshold efforts.

Recharging W′: why easy riding matters

The good news is that W′ doesn’t stay empty forever. When you drop your power back below CP, your anaerobic battery starts to recharge. The lower you go and the longer you stay there, the more W′ you regain. This is why truly easy pedalling between efforts is so important in interval sessions or punchy races.

If you only back off to just under CP, W′ recharges slowly and never fully refills. That can be fine if the race demands steady, high power, but it makes repeated big kicks harder. If you sit up, spin easy, and let your breathing settle, you’ll restore more of that high‑intensity capacity and be ready to go again sooner.

Why W′ matters in real rides

Understanding W′ helps everyday cyclists make smarter decisions, not just theorise about physiology. It can:

• Explain why you can win one sprint but not three in a row.
• Help you pace short climbs so you don’t completely explode halfway up.
• Guide you on when to follow a move and when to let it go.

For example, on a rolling group ride, if you’ve already gone deep on a couple of hills, you’re riding with a half‑empty battery. When someone launches another attack, chasing at full gas might completely drain what’s left, leaving you unable to respond later. A slightly more measured chase – or even letting that one go – can keep enough W′ in the tank for when it really matters.

Training to improve your anaerobic battery

You can’t talk about W′ without talking about training. While genetics and muscle fibre type play a role, you can absolutely train how you use and refill this anaerobic battery. Common approaches include:

• Short, very hard efforts with full recovery (e.g. 30–60 second repeats) to improve how much power you can produce above CP.
• Repeated “over‑unders” that swing just above and below threshold, teaching your body to tolerate W′ depletion and refill it on the fly.
• Race‑like blocks of multiple short climbs or sprints where you manage effort across a series of surges instead of going all‑in on the first one.

The goal is not only to increase W′ itself, but to become more efficient at spending it and recharging it under real‑world conditions. That means learning how hard you can go and still recover enough to go again.

How Stride can help you use W′ smarter

Knowing W′ is useful; seeing how you actually spend it in your training is where it becomes powerful. This is where smart software comes in. By combining your CP/threshold with your ride data, a platform like Stride can estimate how much of your anaerobic battery you’re using during efforts and how quickly you’re refilling it between them.

For a time‑crunched rider, that means your intervals and race‑prep sessions can be built to stress W′ in a targeted way, without turning every ride into a random suffer‑fest. Stride’s structured, coach‑designed sessions can include repeated above‑threshold efforts with controlled recovery, while the platform tracks your overall load and fatigue so you don’t bury yourself with too much anaerobic work in one week.

Over time, Stride can show patterns: how you respond to certain types of efforts, when you tend to over‑spend your battery, and which sessions actually move the needle for your high‑intensity performance. You get to ride and race with a clearer picture of when to go all‑in, when to sit tight, and how to arrive at race day with both your big engine and your anaerobic battery ready to go.

Understanding W′ won’t magically make steep ramps any easier, but it will make them more predictable. And with the right training and smart guidance from tools like Stride, you can turn that finite anaerobic battery into one of your sharpest weapons on the road or gravel.We explain all the metrics in our docs that you can find here