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Martin Brown

On 11/10/2012 10:15, wrote:
I've seen batteries quoted here in watt hours and amp hours. I think of a battery as a store of energy and the SI unit energy is joules. Watt hours are merely a non-SI way of expressing joules.

Mostly you want a particular voltage battery so Ah is a good metric.

I know there are variables such as voltage variation and a duration of discharge and it's often best to use the unit that is less dependent on assumptions.

My impression is that phone and car batteries (for non-electric cars at least) are usually specified in amp hours.

And that makes sense at fixed voltage. You have to specify discharge
rate and derate the battery capacity if you want a high current out.

In the weird world of domestic pv:
Which unit is more common for battery spec?

Vague hand waving with extra smoke and mirrors.

Which unit is 'better' as a guide to battery capacity?

kWh avoids needing to know the battery voltage. It is the preferred unit
for mains electricity too which gives you a handle on it.
1 unit = 1kWh ~ 10p (very roughly for easy mental arithmetic)

That is about the capacity of two 40Ah car batteries (although for power
storage you need more expensive deep cycle lead acid).

Which unit would the sales people prefer in order to mislead you?

None or failing that horsepower ;-)

--
Regards,
Martin Brown