I consider them to be random at best.

Stava power estimates are rubbish. Too many unknown variables in the equation... actual moving mass, road surfaces, weather. Doesn't work if you are riding in a group. Doesn't work if the wind is blowing.

I would expect that strava should be about to estimate long climb (5+min, 5%+ grade) average power OK if the weather conditions are neutral. There are several blog posts on this out there.

I just checked a few sites, you're right, the climbs can be okay, but mostly they are off by a wide margin.

Go to any popular segment page and look at the leaderboard. All the numbers with a lightning bolt are actual power meter figures. Do the strava figures line up with those? Not really. They will usually be within 100W. Unless you are talking about an all out sprint, you can guess your own power output much better than that.

"As long as they're consistent that's all that matters."

This has been researched in the past (second google hit for "Strava power accuracy"): http://stevebalescyclingblog.wordpress.com/...power-vs-real-power/

Based on some of the replies in this thread, I'm going to go with the individual is regurgitating what they've heard and not what Strava says vs. their own power meter. If you know that a computer algorithm is only able to calculate for one specific condition - specifically, mitigate your expectations - Strava isn't *that* bad. On my last ride in generally rolling terrain, with low wind, a longish steep climb (1mi at 10+%) and loads of flat fast riding (10ish miles of <±1%) on a 40 mile ride along the coast, my Quarq was ~10% lower than I would expect Strava to calculate for 1hr power (Strava, in my experience over estimates power on the short end, say up to maybe 30s power but that's to be expected based on calculations which input noisy data). To be fair, I should strip the power data from the Garmin file and have Strava calculate it, but that's what the link above is so much easier to read. Depending on nutrition, sleep (I have an 8 month old), or pacing, the ride could have brought my 1hr power 10% lower without much difficulty.

Bottom line, use common sense when deciding if Strava power is accurate - did you fight a huge headwind or catch a nice ride on a tailwind, have a very different weight than Strava thinks you do, are you looking at the power over a 0.1 mi segment where you mashed it, draft a car/rider, etc...? Of course Strava won't know those things, the only disappointment is people would expect Strava to know. Most likely, Strava calculates energy required to overcome gravity in hills, drag based solely on speed, and general friction which should work for most people - significantly changing those parameters will throw the calculation off (you can see this looking at Strava power between a road bike and mountain bike which have very different friction values). In my experience, if I'm riding hills or flats with no winds, it's reasonably close.

Alternatively, there are calculators online where you can input your own parameters, CdA, Crr, speed, windspeed, distance, time, etc... and it will calculate power (or speed/time/whatever depending on what you input); those calculators can be powerful as well but also are only as good as the inputs. Remember, computers will output exactly what you tell them - junk in, junk out. For being free, juxtaposed with the cost of a powermeter, non-power users (a group which I used to belong) should be so fortunate.

Or, to sum up:
Strava power is not accurate, and is useless.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

I'm being a pedant here, but do you agree that a better statement would be that strava power is of limited utility, instead of useless? If you wanted to structure your training around it, yes, that would be a terrible idea. But what if you're just wondering ~roughly how your output compares to that jerk who keeps taking your KOMs? Or, in my case, bumps me into the triple digits. If in general it's within 10% of actual power numbers, I think it's reasonable to make that sort of comparison, while knowing that it is by no means exact.

I really can't think of anything you can use it for.
It won't always be +/- 10%
It could be 50%
The jerk you are comparing too might have been drafting a bunch of other riders in a group ride.
Or on a recumbent
Or on gatorskins

You would get data at least as good, probably better, by just comparing *times* up the climbs, and/or then using that with known data on the rider in question to compute the power he did, if you know more than strava does.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

For a long ride that is not point to point I found it pretty accurate, usually within 5%. It gives a hint for shorter, harder, out and back-rides. Short point to point (ie flat KOM's) it can be VERY wrong. When there is downhills involved it can be just crazy.

For me I would only use it to figure out what kind of watts I can do for longer, easy rides. It is however not very interesting since the best use of the meter is pacing.

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Endurance coach | Physiotherapist (primary care) | Bikefitter | Swede

I agree with jackmott. Far better just to compare times over known routes, circuits and hills.
I wouldn't pay much attention to people's power numners on Strava even if they are from real power meters. Many likely deliberately calibrate their meters wrongly to inflate their power.

Which is hilarious when you really think about it, since it is more impressive when you can KOM with LESS watts. Lower watts would indicate a lighter and possibly more fit individual. High watts could just mean someone that is strong but fat.

Or have a very unaerodynamic set up.

I just set my weight in Strava to 600lbs, so that it says I did 1400 watts on a 5 minute climb.

After comparing my rides when I used a power meter to others on which I did not, I'd also still say Strava's estimates were useless. It wasn't even a rough comparison. It was a fantasy comparison, often over- or under-estimating the power by more than 100 watts. Anything I might have learned from that data would not have helped.

Exactly. Completely useless because there are so many factors Strava can't see. Here's an example:
http://www.strava.com/activities/175557514


55min ride, 10km, basically 0 elevation change or very little. 43W avg power. Of course, this sounds "good" if I was riding a road bike. Thing is, I wasn't. I was riding a quad bike with my wife and kids, a beastly thing that handled like a bulldozer where the kids were doing 0 pedaling and I was doing 60-75% of the work (my wife would argue she was doing 60-75% of the work ;) ). In any case, I was pushing much more than 43W average power.

I've seen that before. The odd thing is that high power and slow time tells me that something is very wrong with the set up of the bike.

This summer I did a 40k TT and my power meter was WAAAYYY off; it said my average power was huge at nearly 400w (...watch out pro tour). But if you compare that with my time, a pro rider could have probably hit my time on a hybrid bike!