The battery must support the protected loads from utility failure until the generator is accepted and actually supplies those loads—not merely until its engine starts. A hybrid outage bridging timeline separates the battery-only period, generator start sequence, AC acceptance, and eventual shutdown so that each interval has a defined beginning and end.
Bring the protected-load demand in kW, the available battery energy in kWh, and the timing settings for the installed equipment. The table below identifies which intervals consume battery energy and where to obtain their durations. It does not assign universal startup, warm-up, or transfer times.
Battery-to-Generator Timeline
The control examples below come from Victron Energy’s GX — Generator auto start/stop online manual, which does not display a publication or revision date; accessed October 19, 2026. They describe supported GX and inverter/charger configurations, not every hybrid system. The documented warm-up and cool-down feature requires VE.Bus inverter/charger firmware 502 or later.
| Stage | What supplies the protected loads? | Timing input and endpoint | Source and version basis |
|---|---|---|---|
| Utility fails; battery operation begins | The battery through the inverter, once backup operation is established. | Record the installed inverter’s interruption or transfer specification separately from battery runtime. Do not assume that battery backup means interruption-free power. | Installed inverter manual and firmware revision required; no universal transfer value is assigned here. |
| Battery-first operation before a start request | Battery and inverter. | Determine when the configured start condition is reached, then include any persistence delay before the start request. A state-of-charge trigger does not produce a fixed elapsed time after an outage. | Victron GX online manual, §1.5.2, undated; accessed October 19, 2026. |
| Start request issued; generator prepares to supply power | Battery and inverter until generator input is accepted. | Establish the complete interval from start request to accepted AC: starting, any retries, warm-up, and source qualification as implemented by the installed controls. Do not automatically add timers that overlap. | Victron GX online manual, §1.4.3, undated; accessed October 19, 2026; generator-specific start and retry details require its own manual. |
| Generator AC is accepted and connected | Generator supplies the connected loads; charging or battery assistance depends on the configuration. | End the initial battery-only bridge when generator power actually reaches the protected loads—not when the start command is issued or the engine begins running. In the documented Victron arrangement, the Multi/Quattro waits for the configured warm-up period before closing its transfer switch. | Victron GX online manual, §1.4.3, undated; accessed October 19, 2026. |
| Generator remains online | Generator, with any configured battery charging or assistance. | Record the minimum-run setting and active stop conditions. Minimum run time is not an additional startup delay and does not by itself define the total generating period. | Victron GX online manual, §§1.4 and 1.5, undated; accessed October 19, 2026. |
| Generator disconnects, cools down, and stops | Depends on the available source and control arrangement. In the documented Victron cool-down sequence, the inverter runs the loads from the battery after generator AC is disconnected. | Account for battery energy during cool-down if utility power is not supplying the loads. Record any separate generator-stop delay; do not assume that a stop request produces immediate shutdown. | Victron GX online manual, §1.4.3, undated; accessed October 19, 2026. |
For an installation-specific timeline, replace each open timing input with the applicable equipment setting or documented performance value. Record the model, manual revision, firmware where relevant, and operating conditions alongside it. If startup under cold conditions or a failed-start retry remains uncharacterized, that part of the bridge remains uncertain.
Calculate the Initial Battery Bridge
Define the initial bridge using two events:
[
t_{\text{bridge}}=t_{\text{pickup}}-t_{\text{outage}}
]
where:
- (t_{\text{bridge}}) is the battery-support interval, in seconds.
- (t_{\text{outage}}) is the time utility power becomes unavailable.
- (t_{\text{pickup}}) is the time accepted generator power actually supplies the protected loads.
Use event timestamps or non-overlapping intervals. In particular, a controller’s “warm-up” timer may begin at the start request rather than after the engine reaches running speed. Victron documents its warm-up interval as beginning with the generator start signal, so adding a separate full cranking interval to that timer could double-count time.
For a changing load, estimate the battery-side energy consumed during the bridge as:
[
E_{\text{bridge,DC}}
=
\sum_i \frac{P_i,\Delta t_i}{3600,\eta_i}
]
where:
- (E_{\text{bridge,DC}}) is energy withdrawn from the battery, in kWh.
- (P_i) is the protected AC load during interval (i), in kW.
- (\Delta t_i) is that interval’s duration, in seconds.
- (\eta_i) is the assumed battery-to-AC conversion efficiency, expressed as a decimal.
This estimate excludes auxiliary consumption not represented by the load and efficiency assumptions. Compare it with battery energy available from the actual starting state of charge down to the permitted operating limit—not with nominal nameplate capacity. If available energy is already specified as AC-delivered kWh, do not apply inverter efficiency a second time.
Worked Example: A 10-Minute Battery-First Period
Assume the following illustrative inputs, not manufacturer defaults or code requirements:
- Utility fails at 00:00.
- The protected load remains at 2.0 kW for 600 seconds.
- A generator start request is issued at 10:00.
- The load rises to 3.0 kW at that point and remains there until generator pickup.
- The complete start-request-to-pickup interval is assumed to be 45 seconds, including all overlapping start, warm-up, qualification, and connection actions.
- Battery-to-AC efficiency is assumed constant at 90%; auxiliary consumption is omitted.
Generator pickup occurs at 10:45, giving a total initial bridge of 645 seconds.
The AC energy supplied during that interval is:
[
E_{\text{bridge,AC}}
=
\frac{(2.0\times600)+(3.0\times45)}{3600}
=
0.3708\ \text{kWh}
]
The estimated battery-side energy withdrawal is:
[
E_{\text{bridge,DC}}
=
\frac{0.3708}{0.90}
\approx
0.412\ \text{kWh}
]
The result is approximately 0.412 kWh withdrawn from the battery before generator pickup. It is not a recommended battery nameplate capacity: it excludes additional time for unsuccessful starts, later battery-supported cool-down, and any separately required reserve.
The inverter and battery must also support the assumed 3.0 kW load and any actual starting surge. Sufficient kWh alone does not establish sufficient output power, acceptable transfer interruption, or successful generator load acceptance.
Check the Timing Requirement Before Delaying Startup
A battery-first waiting period is a control choice, not a universal permission to delay emergency power.
Cummins’ 10-Second Time to Readiness, Bulletin 5544190, revision September 2024, explains that NFPA 110 Type 10 timing includes sensing, starting, and transfer delays through restoration of acceptable power at the load side of the transfer switch. It is not simply a ten-second engine-cranking allowance. This is manufacturer guidance, not verification of the currently applicable standard’s full text.
Where emergency or legally required standby loads are involved, the specifying professional must establish the locally adopted requirements and the relevant transfer points before selecting a battery-first delay. Do not assume that an upstream generator-and-transfer arrangement satisfies its required sequence merely because a downstream battery keeps selected equipment operating.
For the installed system, the remaining checks are the inverter’s kW/kVA and surge ratings, battery discharge limits, generator capacity for loads plus charging, and AC-input acceptance settings. Transfer equipment, source isolation, neutral and grounding arrangements, and local installation requirements require the equipment instructions and a qualified professional’s assessment. The timeline establishes the bridge interval; it does not establish final equipment selection or installation compliance.