Consistent storage metric definitions

There is some discrepancy in how storage technology duration is determined. I think it would be helpful to have a consistent approach to the metrics we use for storage technologies, such as duration and ratings, particularly regarding if the efficiencies and soc bounds should be accounted for in the duration and rating calculation. See comments below from @elenya-grant and myself in #290.

Comments linked above

@jaredthomas68

I think this is incorrect. I think it should technically be discharge rate and include discharge efficiency. what we have is a nice back of the envelope calc, but we have more info here.

@elenya-grant

Could you provide what the equation would be and a supporting reference? Specifically on why including the discharge efficiency would make sense to include? I thought that losses (or efficiencies) are not taken into account for system sizing. System sizing is usually the nominal capacity.

I will change the output storage duration to be based on max discharge rate.

This paper is referenced in the ATB documentation for battery costs and :

Table 11 lists "Duration" of the battery as "Duration = rated energy / rated power"
but in Table 9 it does say that the battery is "300 kW rated dc power" and "1.2 MWh rated (usable) dc energy storage", and that it has a min SOC of 10% and max SOC of 90%, and is a 4-hour battery.

I read that as "it's rated at 1.2 MWh, where 1.2 MWh is the difference between 10% and 90% SOC".

So - I think this should be part of a larger conversation. If we say a battery has a max_capacity of 40 MWh, our models now assume that 40 MWh is the total capacity (not "usable" storage) from 0-100% SOC.

Start with the discussion linked from pull request #290 and trace the existing storage duration and rating calculations. Before implementation, establish the agreed equations and treatment of discharge rate, efficiency, and SOC bounds; done means storage metrics use one consistent, documented definition.