Over recent years the number of cordless (i.e. battery powered) tools available has grown enormously. In many cases mains power is but an extension lead away, and so you may not “need” a cordless tool. There are some items (notably drills and powered screwdrivers) for which the cordless tool is desirable and in class of its own - often in addition to a mains equivalent.
If in doubt as to whether to go cordless (for things other than drills) you are probably better sticking to mains.
There is also a huge difference between the best and the worst examples. The worst cordless tools are virtually useless. The best can be used as non- stop work horses.
The cordless jigsaw I was once given is a nice example. Cost was about £14 from a large DIY shop. In theory it will run on a full charge for 10 minutes, the reality is 30 seconds of full power, followed by ever decreasing speed and power for the next 5 minutes, with the remaining battery capacity being in effect unusable. Recharging then takes 14 hours. The one job I tried with it (a 9" cut in 1/2" ply) was completed.... just. Then that was it for the day!
The biggest influence on the quality and usability of a cordless tool are its batteries and the charger. It is simply not possible to purchase good quality rechargeable cells at very low cost. Many budget cordless tools are sold at a price that is less than the wholesale cost of a decent set of batteries, so something has to give! The quality of the batteries will affect how long it runs, and the power or torque available. The quality of the charger will affect how long the batteries take to charge, and more importantly, how many times you can recharge them and still get useful performance from the tool. Batteries will need replacement eventually. With a budget tool this will usually be a non economic exercise, whilst with a higher end tool it may well be more expensive than you anticipate.
A number you will see banded around with respect to batteries is the Amp Hour (AH) rating. This number tells you how much charge a battery can hold. The more charge it holds, the more work you can get out of it between recharges (but the longer it will take to charge). Battery quality tends to rise with capacity, so batteries with bigger capacities also tend to be better is other respects.
Higher capacity batteries need to be matched with better and faster chargers, otherwise you will be waiting longer for them to charge.
A good quality battery pack should take recharging many hundreds of times before it no longer holding enough charge. Some of the poorer ones may only last for a hundred or fewer charges. Good ones will hold their charge longer when left unused, whilst a poor ones will be flat within a week. With batteries it is very much a case that you get what you pay for.
The lowest capacity batteries are typically constructed from Nickel Cadmium cells with 1.2 or 1.3Ah capacity. Mid-range NiCd cells will usually have 1.8 to 2.2Ah capacity. Nickel Metal Hydride (NiMH) cells found in top-end tools will have capacities heading toward 3Ah. The latest tools will often come with Lithium Ion battery packs, with capacities of over 4Ah in some cases.
NiCd cells are being phased out (mainly due to the toxicity of cadmium which makes safe disposal difficult).
In the quest for more power, performance and speed from battery operated tools, there has been a slide upwards in battery voltage. This suits the marketers well since there is a nice “number” to use as a sales hook.
The bigger the number the better right? Err, no not always. The more volts, the more cells, the bigger and heavier the tool will be. If you want a nimble easy to use drill/driver this is not a “good thing”. Then we come down to quality of batteries again: a top end 14.4V drill will out perform an 18V or 24V budget tool for just this reason, while being smaller and lighter into the bargain.
Watts (W or kW) is the power consumption rating we are used to seeing on mains powered tools, but this quantity is not usually mentioned on cordless tools. This is because cordless tools are designed to be highly efficient so as to keep the electrical power consumption low to conserve battery life, whilst providing an adequate mechanical output power. This, however, makes for a small "Watts" figure that is not much use as a marketing hook! The most powerful top end professional cordless tools may consume some 400W, but most are less powerful and consume far less.
With all mains equipment, including power tools, the Watt figure stated is the input power, but the mechanical output power is rarely given. By the time you have accounted for all the heat and noise generated, the useful output power may be much less. Thus a well designed tool with good speed control and a well made gearbox may produce the same usable power at the sharp end as a less well made tool with twice the input power.
You can make a stab at estimating the actual power of your cordless tool by studying its performance. Say it runs for 15 minutes at full power, and has a battery capacity of 2Ah (Amp-hours). This tells you that the battery can delivery 2 amps for one hour, 4 amps for 30 minutes, or 8 amps for 15 minutes. We can therefore deduce from the time the battery lasts, and the capacity of the battery, that this tool is drawing about 8 amps from it. Multiplying the voltage of the battery by the current drawn gives the power consumption (Watts = Volts x Amps). So if this tool has an 18V battery we would get 18V x 8A = 144W.
Another influence on performance, is the quality of the motor and speed controller. A good speed controller will deliver lots of torque and control, even at low speeds. The poorer ones will only deliver torque at high speeds which is far less useful.