Why are large batteries described by their power? Reading about renewable energy sources, I came across the idea to use huge batteries as a temporary storage to be able to deal with the fluctuations that e.g. solar panels might produce.
The most important parameter for these batteries seemed to be their power in Watts (or MW) i.e. how much energy they can take up/release per unit time, not the amount of energy they can store. Why is that? Why is, in the context of renewable energies, power more important than storage capacity?
Thank you for your answers!
 A: The basic energy requirements of homes, industries etc are measured in watts. It may be sometimes seen how much energy each sector utilizes over a span of time, but generally we see how many watts are used in homes, industries then further in colonies, cities etc.
Atleast here in India (and I think it would be similar everywhere) we get our home supplies based on a base wattage that we chose; Since we also get very regular powercuts in most part of the cities nearly all homes are equipped by gensets, even these gensets are categorised by their wattage.
I even saw an old documentary yesterday about energy crisis and renewable energy sources, and while talking about any source they talked about energy generation in terms of power.
Although the total energy stored is an important aspect just think that you have a huge battery that can store large amount of energy but has very little power, then you can not use it in your home or city or anything for that matter. However if the power is equal or more than what you need, only then you can operate your appliances with ease may that be for a small or long time.
So in accordance with all above I find that measuring power is a significant method to judge a battery because power would be deciding whether it can be used or not. The total energy may define how long it can be used but power would define if we can use it at all!
A: First off, I would be cautious about interpreting the parameter that press releases focus on as being the "most important" - it's just as likely to be the one that a press officer happened to pick because it seemed like a Nice Big Number, or possibly the one that they understood. Think yourself lucky if the units actually made sense!
However, there is a difference in how it is useful to measure large batteries.
When we speak about small batteries - e.g. the ones in our mobile phones - we tend to concentrate on their capacity because that is what determines how useful they are for us. The manufacturer will have ensured that they power that they can deliver is sufficient, so what we sometimes get to choose is whether we want a slim low-capacity one or a thicker higher-capacity one. What we are choosing is how long the battery will last for.
However, the utility-scale batteries that can be used on the electricity grids serve a slightly different function: they're not usually about long-term storage of a large amount of energy, but about short-term smoothing - for instance, covering a few minutes lull in the wind in an area with a lot of wind turbines. For this purpose the capacity is, of course, important, but the rate of charge/discharge is equally important; you can't use a battery to smooth the output of a wind farm if the battery can't supply the equivalent of the wind farm's output. So both parameters are relevant and important at this level.
