Current consumption - alternator fail

The Rotax 915iS installation manual states the following:

The specification of the battery must ensure that during each operating state and also during the transition between two operation states a sufficient supply of the avionics is guaranteed (e.g. during engine start). Depending on the applicable regulations avionics need to be supplied for at least 30 minutes with energy after a failure of the primary power supply in case they are required for safe operation of the aircraft. If the aircraft is regularly started at temperatures below - 5 °C (23 °F) (ambient- and oil/ coolant temperature are below this temperature) it is recommended to provide a connection for an external power supply and a possibility for adequate engine pre-heating.

The usage of a battery with lower capacity may have a negative impact on the starting behavior of the engine. Additional electrical loads, which are in some cases required by law, may affect the battery performance during the starting process,(e.g. ACL (Anti Collision Light), Navigation Light, Avionics).

https://www.rotax-owner.com/pdf/IM_915iA_Series_Ed0_Rev3.pdf

The specific section is in the attachment that has more information that you need to know. 

Also, check out the Rotax-Owner classified  section - https://www.rotax-owner.com/en/classifieds/advert/422-lithium-battery-for-rotax

Someone listed a Lithium Battery

Thank you Rotax-Owner, I have seen the information in the installation manual. My question is, what is the current consumption of a running 915iS for the 915 and ECU?

It's not stated in the manuals; however, when we tested our 915iS engine, it drew 14amps. This was at full power (5800RPM) and with both fuel pumps operating. 

Wire size, the distance the fuel pumps are from the engine, the location of the fuel pumps, fuel line size, battery location, location of ECU will all play a factor regarding the load. 

You have to test your installation to determine the load as it may be different.

If you need a hard number for YOUR aircraft, Configure the electric load as if you have a dual Gen Failure.
This means NAV Lights, Strobes, Heaters, Fans, and anything else Not absolutely essential to flight are turned off.
One COM radio and optionally, the transponder might be the only thing ON.
If this is an IFR-rated aircraft, turn ON whatever is needed to maintain IFR Flight also.

Unplug BOTH GEN cables from the Fusebox and start it up.
Measure the battery drain.
If you see 14 Amps, that is YOUR Target.
In this case, I would select a battery with AT LEAST a 14 Ah capacity.  (15-20 Ah)
This should give you 30 minutes of reserve.
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A battery rated at 14 Ah assumes the current will be drawn at a rate of c/20  (14/20= 0.7 Amps) over a 20 hour period down to 10.5 volts.
NOT 14 Amps for an hour at 12 volts.
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If you are just looking for a quick answer, a 16Ah battery would be a reasonable choice.
And there are more pressing things to worry about than a double Gen Failure.

In the event of a SINGLE Gen failure, remember that the BACKUP Power Switch works both ways!