Which single sided PM are you considering?

Personally, I’d keep saving and buy (in order of preference):
Quarq
Powertap wheel
(Insert preferred pedal meter here).

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Developing aero, fit and other fun stuff at Red is Faster

The 4iii precision is only 300 pounds but seems to have decent reviews

I don’t have personal experience of the 4iiii.

As you appear to be in the U.K., I suggest you talk to Bob at cycle power meters.com. He’s a nice bloke and will make sure you get the right gear.

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Developing aero, fit and other fun stuff at Red is Faster

The Power2Max NGeco is more expensive but not vastly so (€490/£430) providing it's compatible with your existing crankset and it will give you total power. Single sided estimates the total. It may be pretty accurate, it may not. If you already knew how your L/R balance was at different power levels, it would probably be fine. But you'd need to have already used a dual sided meter to know that.

The problem with single sided power meters is that people are not consistently symmetrical or asymmetrical.

One day you may be 50/50, the next 52/48, the next 47/53. And it's not even consistent during the day where at 200 watts you may be 50/50 and at 250 watts you could be 53/47 and then 49/51 at 200 watts again. (I chose those numbers because they represent myself)

You can spend $400 to $500 on a single sided meter or you can spend $490 on a dual sided Power2max or similar.

I've used a dual sided in the past (P2M). On my new bike, I wanted to use a dura ace crank, so for now I have the single stages crank arm. If I know from my P2M days that my balance was almost always 52/48 (52 on non-drive side), how do I now use that info to help with the single sided stages? If its reading 200, do I knock 2% off that number to be more accurate?

Thx

4%

100 watts (true power) would be ~52 watts from the left, ~48 from the right, but the left side doubled is 104 watts. 4/104 = 3.85% - the exact conversion would be to multiply displayed power by 0.9615 to get the "true" power.

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power2max gives total power but only pseudo l/r balance, not true balance. It is measuring the percentage of power delivered in each half of the pedal stroke, not the power delivered by each leg. To see that you need a meter that measures left/right power independently like a pedal or dual sided crank arm (pioneer, dual stages, dual 4iii). Using the percentages given by the power2max will most likely be inaccurate.

Cool, thx. Looks like I'll be aiming for a massive 190w for this coming Sunday, and then wait for the leg cramps....ugghh...

Right if I ignore the replies and go with the budget option of a single sided do I just need the same size as my chainset? e.g. SRAM Force 11 GXP Chainset No BB 172.5mm so 172.5mm? I know this is a stupid question but my knowledge of bikes extends to changing the tyre

Yes you definitely want the same length crank arm as your current chainset.
Also just make sure whatever you get is made for the same bottom bracket standard as your current chainset (ie. GXP vs BB30, etc).

Yes it should match your current crank in both type and length

One more option for you to consider is a single-sided pedal-based power meter that you can upgrade to dual at a later date. Best value out there right now seems to be the Favero Assioma. It's a 2 minute installation, with no worrying about crank and/or bottom bracket compatibility issues. Furthermore, it's more easily ported to a future bike, and slightly more "future-proof." There's been a reasonably long period of stability with road cranks, with 10 and 11 speed stuff being completely interchangeable. 12 speed road is here from Campy, and presumably just around the corner from Shimano and SRAM. When Shimano launches, I wouldn't be surprised if there's some change to the chain line or tooth profile or chain width that makes older cranks incompatible.

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Match the brand, length and model. The third concern is the width/distance between your feet (q-factor). You don't want one foot farther away from the centerline of the bike.

I tihnk the one sided pedal to eventually get a two sided might be the best bet

If cost were no object dual sided is the way to go. But, cost is always a factor!

A single sided PM + a coach will be 1000x better than the best most expensive SRM power meter and no coach so, if that is how your budget works, do it.

I have several friends who got Stages power meters back when the first came out because they were half the cost of the then available dual sided power meters. They all got way faster. Any power meter beats no power meter.

I start out nearly 50/50 and get progressively more right-biased as fatigue sets in. A single- (left-) sided PM would lead me down a bad path if used for pacing an IM.

Yeah, there's no real point to going single-sided when P2M and Quarq are available for the prices they are, or a used one of various sorts. I wouldn't even use a single-sided PM for free; bad data is worse than no data.

I wouldn't consider one-side powermeters these days. There are so good options out there at reasonable prices (for example mentioned power2max type S or NGeco) that are within 100€/USD of a single sided (for example stages) powermeter, it doesn't make sense these days in my opinion. When duals were 900€/$ and single sided 600€/$, ok maybe you could justify having potentially bad data.

Remember, the general consensus is I think that if you are 55/45 belance, you are still considered OK. So potentially you could be 10% off (as the a single sided powermeter doubles the output of the one leg), which could be huge deal. And as it was mentioned, depending on your day, you pre-training (not to be neglected for us triathletes), and your power output, if can change a few percent up and down on the balance easily.