StandbyCalc

StandbyCalc is the fastest way to calculate standby battery requirements for UK fire and intruder alarm systems. Built for engineers, by engineers. Enter your quiescent and alarm currents, select your system grade or category, and get an instant answer — including which standard battery size you need. SUPPORTS: - Intruder alarms — BS EN 50131 / BS 9263, Grades 1-4 - Fire alarms — BS 5839-1:2025, Categories L, M, and P - Standalone PSUs — custom standby and alarm periods FREE FEATURES: - All battery calculations - All standards and grades - Save unlimited calculations - Standard battery size recommendations PRO UPGRADE (one-time purchase): - iCloud backup across devices - PDF reports with full formula working - Add your company logo and branding - Share reports via email, AirDrop, or Files - Professional compliance documentation Compliant with NSI and SSAIB requirements. Calculations per BS 9263 Annex C and BS 5839-1:2025 Annexe E. No account required. No subscription. Just open and calculate.