How do I derate battery capacity for temperature?

Available capacity falls as temperature falls. AS/NZS 4509.2 provides derating curves for lead-acid at C₁₀₀ and C₂₀ rates from −10 °C to +40 °C, with correction factors running roughly 60% to 105%. Size to the minimum expected electrolyte temperature, not the annual average. Separately, and in the other direction, sustained high temperature does not reduce available capacity much but shortens life sharply, see B1.