Quick answer: Ordinary grid-connected solar will not normally keep your business powered during an outage—even in sunshine. Backup requires a system specifically designed to disconnect safely from the utility and supply selected loads independently. Solar, batteries, compatible inverters and islanding controls can provide that capability, but installing solar or storage alone does not establish it. (energy.gov)
Your first step: Write down which business functions must remain available, their electrical requirements and the outage duration you want to cover. Ask bidders to demonstrate that their proposed system meets those requirements.
This guide uses U.S. sources. Utility requirements, adopted codes and equipment approvals must be checked for the project’s actual location.
1. Why grid-connected solar normally shuts down
An ordinary grid-connected photovoltaic system is not an independent power supply. Many solar systems detect a loss of utility power and disconnect to prevent an uncontrolled electrical “island.” Without proper isolation and controls, local generation could energize equipment that workers expect to be disconnected or supply unstable power to connected loads. (energy.gov)
That protective shutdown is different from an equipment failure. A roof full of functioning panels can still leave the building without usable electricity.
Intentional islanding means operating a defined electrical system independently after safely separating it from the utility. DOE’s commercial-campus microgrid example shows the solar array initially shutting down, followed by opening the utility connection before islanded operation. (energy.gov)
The procurement question is therefore not “Does this building have solar?” It is:
“Which circuits can this installation supply when the utility is unavailable, and under what conditions?”
2. What a solar backup system needs
A backup design must provide several functions, whether through integrated equipment or separate components:
- Utility isolation: An approved transfer or isolation arrangement prevents unintended backfeed.
- An independent electrical reference: A suitable inverter or another generating source establishes the island’s electrical supply.
- Energy availability: Batteries, operating solar and possibly other generation supply the loads.
- Load control: Selected circuits remain connected while unnecessary loads are excluded or shed.
- Coordinated controls: Equipment manages the transition, generation, charging and eventual return to utility service.
DOE distinguishes ordinary grid-following inverters, which rely on an external grid signal, from grid-forming inverters that can create that signal themselves. Its resilience research also identifies transfer equipment, critical-load panels, controls and engineering as additional islanding requirements and costs. (energy.gov)
For an existing solar installation, ask the designer to verify compatibility rather than assume a battery retrofit will work. Specifically request confirmation that the existing PV can operate inside the proposed island and recharge storage during an outage.
Ask what happens when the battery reaches its shutdown threshold. Can the system restart without utility power? Which source starts it? Obtain a documented operating sequence, not a general assurance of “backup-ready.”
3. Start with business functions and a critical-load inventory
Define the minimum operation worth protecting: preserving stock, maintaining communications, completing a controlled shutdown or continuing a limited service.
For each proposed critical load, record:
| Worksheet field | What to establish |
|---|---|
| Business function | What interruption would affect |
| Equipment and circuit | Exactly what must receive backup |
| Running power | Measured or otherwise verified kW |
| Starting requirement | Inrush current or short-duration power requirement |
| Operating pattern | Continuous, cycling or scheduled |
| Required duration | Hours needed during the outage |
| Interruption tolerance | Whether a brief transfer interruption is acceptable |
| Priority | Must remain on, may be shed, or may wait |
Have facilities staff validate the equipment list and finance or operations staff approve the priorities. DOE’s resilience analysis identifies critical-load magnitude, stored energy at outage onset and available solar as important determinants of backup duration. (energy.gov)
Look for dependencies. A backed-up workstation is of limited use if its network equipment is excluded. Ask whether communications also depend on services outside the building.
Do not assume this inventory establishes compliance for legally required emergency loads. Have the responsible professional identify those obligations separately.
4. Check battery power and energy—not just its advertised size
Battery specifications describe two different constraints:
- Power, in kilowatts (kW): How much equipment the system can supply at once.
- Energy, in kilowatt-hours (kWh): How much electricity it can deliver over time.
A useful first approximation is:
Runtime = available usable energy ÷ average backed-up load
However, storage loses energy during conversion and delivery. Nameplate capacity is therefore not automatically the energy available to your circuits. DOE explicitly distinguishes storage power capacity from energy capacity and notes that storage is not completely efficient. (energy.gov)
Check the complete battery-and-inverter combination for continuous output, short-duration overload, battery discharge limits, supported voltage and phase configuration, and temperature derating.
For example—not a product recommendation—Sol-Ark’s 60K-3P-480V Revision 10 datasheet lists 60 kW maximum continuous real power, 90 kVA peak apparent power for ten seconds off-grid, and a 15-millisecond adjustable backup transfer time. It also specifies derating above 45°C and requires active battery-management-system communication for lithium batteries. These are distinct operating specifications, not one interchangeable “capacity” rating. (sol-ark.com)
Ask the engineer to assess motor starting and load compatibility. Do not treat kVA as interchangeable with kW or assume a fast transfer specification guarantees every device will ride through it.
5. Worked example: a hypothetical four-hour critical-load worksheet
The following figures are illustrative assumptions, not measured business loads, equipment recommendations or a performance forecast.
Assume a U.S. business wants four hours of limited operation without crediting any solar generation.
| Hypothetical load | Running power when on | Assumed operating fraction | Average power | Four-hour energy |
|---|---|---|---|---|
| Network and communications | 1 kW | 100% | 1 kW | 4 kWh |
| Monitoring and controls | 0.5 kW | 100% | 0.5 kW | 2 kWh |
| Selected lighting | 1.5 kW | 100% | 1.5 kW | 6 kWh |
| Refrigeration | 6 kW | 50% | 3 kW | 12 kWh |
| Essential ventilation | 4 kW | 100% | 4 kW | 16 kWh |
| Selected process equipment | 8 kW | 50% | 4 kW | 16 kWh |
| Total | 21 kW if all are on | — | 14 kW | 56 kWh |
This produces two different screening requirements:
- Energy: 56 kWh delivered to the loads over four hours.
- Power: Up to 21 kW running simultaneously, before considering starting surges.
Now assume a hypothetical battery has:
- 100 kWh nameplate energy;
- 90% state of charge when the outage begins;
- a 10% minimum operating threshold;
- 90% delivery efficiency.
Here, state of charge means the assumed fraction of stored energy remaining.
Available delivered energy would be:
100 × (0.90 − 0.10) × 0.90 = 72 kWh
At a constant 14 kW average load:
72 ÷ 14 ≈ 5.1 hours
That exceeds the four-hour energy target in this simplified calculation. It does not demonstrate sufficient starting power, equipment compatibility or real-world runtime.
If the outage instead starts at 40% charge:
100 × (0.40 − 0.10) × 0.90 = 27 kWh
27 ÷ 14 ≈ 1.9 hours
The same assumed battery now misses the target. These calculations illustrate why initial charge and load assumptions must be explicit.
Replace every worksheet assumption with site evidence. Request modeling that includes system consumption, aging, temperature effects and realistic equipment operating patterns. Count conversion losses only once if a supplier already specifies usable AC-delivered energy.
6. Separate demand management from outage backup
Bill savings and resilience can use the same equipment, but they are different design objectives. DOE’s resilience study explains that valuing avoided outage losses can change optimal solar and battery sizing, while islanding introduces additional costs. (energy.gov)
Scenario A: demand management. Suppose the hypothetical battery above delivers 20 kW for two hours during normal grid-connected operation. It supplies 40 kWh. If an outage immediately follows, with no intervening recharge, the previously calculated 72 kWh falls to 32 kWh:
32 ÷ 14 ≈ 2.3 hours of simplified backup
This is an energy illustration, not a utility-bill savings estimate.
Scenario B: backup-first operation. Keep an agreed reserve available for outages and restrict routine dispatch accordingly. Under the earlier assumptions, preserving the full 72 kWh would support approximately 5.1 hours at the assumed average load.
Ask bidders to model both strategies using your interval consumption data and actual tariff. Require explicit assumptions for charging, demand-measurement intervals and reserve settings.
Also request outage cases starting at night, after routine discharge and during low-solar weather. DOE notes that PV output varies with time of day, season, clouds and other site conditions. Average annual generation is not a substitute for an outage-specific energy balance. (energy.gov)
7. Verify utility approval and applicable safety standards
Obtain the utility’s written requirements for the proposed configuration, including modifications to an existing interconnection agreement and permission to operate.
PG&E provides one location-specific example. Its April 2020 backup guide tells customers to confirm whether their equipment and interconnection agreement permit islanding, and specifies UL 1008 certification for manual and automatic transfer switches. Its current contractor guidance also states that customers may not turn on an interconnected system without permission to operate. Treat the older guide as a planning reference, not a substitute for current project approval. (pge.com)
For a U.S. battery proposal, ask the designer and approving authorities to identify the applicable editions and requirements, including:
- UL 1741: Inverters, converters, controllers and DER interconnection equipment.
- UL 9540: Energy storage system and equipment safety.
- UL 9540A: Testing of thermal-runaway fire propagation.
- NFPA 855: Installation of stationary energy storage systems.
- Applicable electrical, building and fire codes. (ul.com)
Do not accept “UL 9540A tested” as equivalent to complete-system UL 9540 certification. They address different evaluations. Request documentation for the exact proposed equipment combination and installation arrangement. (ul.com)
Have qualified professionals resolve battery location, access, separation and fire-protection requirements. Never bypass anti-islanding protections to obtain backup.
8. Make acceptance testing and operating responsibilities contractual
Before signing, request a written acceptance plan. It should identify who will verify:
- Isolation from the utility during backup operation.
- The circuits actually receiving backup.
- Transfer behavior with representative loads.
- Starting of the largest approved equipment.
- Load shedding at agreed thresholds.
- Solar operation and battery charging while islanded.
- Low-charge shutdown and restart behavior.
- Safe return to utility service.
Specify the starting charge, load schedule, solar assumptions and success criteria for each test. Arrange testing through qualified personnel with the necessary approvals; do not create an outage by operating electrical equipment yourself.
The operating plan should assign responsibility for reserve settings, alarms, inspections, maintenance, software changes and staff training. PG&E’s safety guidance emphasizes following manufacturer operating and maintenance instructions and checking readiness before an emergency. (pge.com)
Common mistakes are buying storage without verified islanding capability, sizing from average loads while ignoring simultaneous demand, and assuming normal bill-saving dispatch leaves enough outage reserve.
The purchase specification should promise a defined capability—not merely “solar with batteries.” State which loads must operate, for how long, under which starting conditions, and how the supplier will demonstrate that capability.