I took advantage of a recent US trip to purchase a Surface Power Cover, at the Microsoft Store in Bellevue, near Seattle.

The concept is simple: you get an external battery integrated into a Surface keyboard cover. The keyboard is similar to the second version of the Type Cover, though curiously without backlighting other than a caps lock indicator. The keys are mechanical which for most people means you can type faster than on the alternative Touch cover, though it is less elegant when considered as a cover rather than as a keyboard.

The trackpad is the same on all three second edition covers, which is to say, not good. The problem is not the trackpad itself, but the mouse buttons, which are NOT mechanical keys (they were on the first edition Type Cove). Given that you need to press and hold a mouse key for some operations, having a physical click on the trackpad buttons is particularly useful and much missed. Another annoyance is that you cannot disable tap to click, which means some mis-clicks are inevitable, though on the flip side it is easier to tap to click than to use the fiddly mouse buttons.

Having said that it is the same, I have noticed that the trackpad on the Power Cover seems a bit smoother and better behaved than the one on the Type Cover 2. This could be sample variation, or that it is new, or that Microsoft has slightly tweaked the internal design.

As you would expect, the Power Cover is heavier and more substantial than the Type Cover, though I find you notice the weight more than the bulk. Even with the Power Cover, it is still smaller and neater than a laptop. The extra rigidity is a benefit in some scenarios, such as when the keyboard protrudes over the edge of a table. The fabric hinge, which is a weak point in the design of all the Surface covers, seems to be the same on the Power Cover and I fear this may cause problems as the device wears, since the extra weight will put more strain on this hinge.

As with the other keyboard covers, if you fold it back under the tablet, the keys are disabled. In this mode the Power Cover is purely an external battery.

I used the cover with the original Surface Pro (it is compatible with all the models other than the original Surface RT). I understand that a firmware update is needed for the power cover to work; if so, it installed seamlessly though I did need to restart after connecting the keyboard for the first time. Everything worked as expected. If you click the battery icon in the notification area you can see the status of both batteries and which is charging, if you are plugged in; generally one one charges at a time.

I boarded my flight and noticed that the Surface is smart enough to use the external battery first, and then the internal, presumably on the basis that you might want to remove the keyboard and use the Surface in pure tablet mode.

It is impossible to be precise about how much extra time you get from the Power Cover, since it depends how you use the machine. It is a big benefit on the original Surface Pro which has rather poor battery life; extended battery life is perhaps the biggest real-world difference between the Surface Pro and the Surface Pro 2. Subjectively I have doubled the battery life on my year-old Surface Pro, which for me makes the difference between running out of battery fairly often, and hardly ever.

The Power Cover costs $199, which is expensive considering that you can get an entire spare Android tablet or Amazon Kindle Fire for less; but put in the context of the equally over-priced Type Cover, which costs $129, you can argue that it is not that much extra to pay. Prices from third-party sites will likely be lower once availability improves.

If you need it, you need it; and this must be the best way to extend the battery life of a Surface tablet.

The Surface keyboard covers are not perfect, and I still sometimes see an annoying fault where the mouse pointer or keys stop responding and you have to jiggle the connection or tap the screen a few times to get it back (I am sure this is a driver issue rather than a poor physical connection). Still, I put up with a few irritations because the Surface gives me full Windows in a more convenient and portable form factor than a laptop, and there is more right than wrong with the overall design.

Summary:

• If you already have a keyboard and your Surface lasts as long as you need – forget it.
• If you have a Surface that runs out of power with annoying frequency (probably a Surface Pro 1), this is worth it despite the high price.
• If you don’t have a keyboard (for example, you are buying a new Surface) then this is worth the extra cost over the Type keyboard.