Why Vendor UPS Runtime Charts Are Often Misleading
When shopping for an Uninterruptible Power Supply (UPS) from manufacturers like APC, CyberPower, or Eaton, runtime estimates quoted on retail boxes typically assume small resistive loads like a single desktop monitor. In reality, homelab server racks present inductive power factor loads that stress inverter circuits differently.
The Physics of Battery Autonomy
To accurately calculate how long a battery bank will keep your virtualization hypervisors and storage pools operational, three physical factors must be accounted for:
- Total Stored Energy (Watt-hours Wh): Calculated as DC Voltage multiplied by Amp-Hour capacity (
Wh = V * Ah). - Inverter Conversion Efficiency: DC-to-AC inverters dissipate 10% to 20% of stored power as heat. High-grade pure sine wave inverters operate around 85% to 92% efficiency.
- Usable Depth of Discharge (DoD): Traditional Sealed Lead-Acid (SLA) and AGM batteries should never be discharged past 50% capacity if you want them to survive more than 200 charge cycles. Conversely, modern Lithium Iron Phosphate (LiFePO4) chemistries safely permit 85% to 90% DoD.
To estimate your exact battery autonomy under active wattage draw, check out the Homelab UPS Runtime Estimator.
Peukert's Law and High C-Rate Penalties
Peukert's Law demonstrates that battery capacity decreases drastically as the discharge current increases. If you pull 30 Amps from a 12V 50Ah lead-acid battery (a 0.6C discharge rate), the effective capacity is cut nearly in half compared to a slow 20-hour discharge.