So your 845cca battery specification shows that it is 122 amp-hours of capacity. This is about 1500 watt-hours at 12 volts - https://learnmetrics.com/amp-hours-to-watt-hours/

So 1500 watt-hours divided by 300 watts is 5 hours. It can supply 300 watts for 5 hours ideally.

Use this $13 watt meter to measure your actual watt load and you can size everything very accurately.

kuman KW47-US Electricity Usage... https://www.amazon.com/dp/B07DPJ3RGB