Battery Backup Runtime
Enter bank voltage, Ah, depth of discharge, inverter efficiency and load to get runtime in hours, plus the Ah needed for a target runtime.
Step-by-step
- Enter valid values to begin.
Runtime = V × Ah × strings × DoD × ηinv × kT × EOL / Pload, with the load assumed constant. Chemistry sets only the notes and the C-rate warning; enter the DoD you actually want, because the chemistry presets are typical values, not ratings. The temperature factor applies to lead-acid only and is a typical table: 1.00 at 25 °C and above, 0.92 at 15 °C, 0.80 at 5 °C, 0.70 at 0 °C, linearly interpolated and held at 0.70 below 0 °C; use the datasheet curve for the exact cell. Rated Ah is normally quoted at a slow discharge (often the 20 h rate), so heavy loads deliver less than this simple model shows; see the Peukert page. Inverter standby losses, surge loads and charger behaviour are not modelled. Check the manufacturer data and local code before you rely on the result.
Battery Backup Runtime: How Long Will the Load Stay Up?
Core Engineering Principles
Runtime is energy bookkeeping. The bank holds volts times amp-hours in watt-hours, but you may use only part of it, the depth of discharge. Lead-acid is normally held near 50% to protect cycle life, while LiFePO4 routinely gives 80–90%. The inverter then takes its cut: at 90% efficiency every 1,500 W of load pulls about 1,667 W from the DC bus. Divide delivered energy by load and you have hours; standby losses come on top.
Then come the things the nameplate hides. Rated Ah is measured at a slow discharge, often the 20 h rate, so a heavy load cuts what you actually receive; that is the Peukert effect, covered on our Battery Peukert Capacity page, and this tool does not apply it. Cold costs capacity: lead-acid at 5 °C gives about 80% of its 25 °C figure (typical values). Age costs more, since stationary practice treats a cell as end of life near 80% of rated capacity. Above roughly 0.5 C on lead-acid, voltage sags early and the inverter trips on low DC.
T = Eusable × ηinv / Pload • IDC = P / (η × V) • C-rate = IDC / Ah
Ahreq = P × T / (V × DoD × η × EOL × kT)
NEC & Standard References
IEEE 485 is the recommended practice for sizing lead-acid batteries for stationary applications, including the design margin, temperature and aging factors. IEEE 1188 covers maintenance, testing and replacement of VRLA batteries. NEC Article 480 covers stationary storage batteries, and NEC Article 706 with UL 1973 and UL 9540 applies to energy storage systems. This tool estimates runtime only. Confirm the adopted edition and local authority having jurisdiction (AHJ) requirements.1. Rated energy = 48 × 200 = 9,600 Wh = 9.6 kWh.
2. Usable = 9.6 × 0.50 = 4.80 kWh, of which the inverter delivers 4.80 × 0.90 = 4.32 kWh.
3. Runtime = 4,320 / 1,500 = 2.88 h, or 2 h 53 min.
4. DC current = 1,500 / (0.90 × 48) = 34.7 A; C-rate = 34.7 / 200 = 0.174 C, well under the 0.5 C lead-acid limit.
5. For 4 h: Ah = 1,500 × 4 / (48 × 0.50 × 0.90) = 277.8 Ah, so a second 200 Ah string (400 Ah) is the practical answer.
- Nameplate Ah overstates. The 20 h rating is not what you get at a 2 h discharge, so treat the result as optimistic for heavy loads.
- Inverter surge. Motors and compressors can draw several times running watts at start; size for the surge and the DC cable and fuse with it.
- Cold derating. A battery room at 5 °C delivers roughly a fifth less.
- Aging. Enter 80% in the end-of-life field to see what the bank will do at replacement time, not on commissioning day.
- High C-rate. Above about 0.5 C lead-acid voltage sags and heating rises; add strings or capacity rather than trusting the arithmetic.