In short. A rental battery energy storage system solves short-duration power problems differently from a generator: it delivers power instantly, absorbs load steps, covers short outages and lets generators run less or at better load. It must be specified by both power (kW or MW) and energy (kWh or MWh), because a battery with enough energy but too little inverter power, or the reverse, fails the application. Availability is confirmed per project with specialist supply partners.
Where temporary storage helps
- Peak support, where a site’s demand briefly exceeds its grid connection or generator capacity.
- Generator hybridisation, where a battery carries light overnight load so the diesel set can stop.
- Short-duration backup for loads that need power for minutes rather than hours.
- Temporary grid constraints, where a sanctioned load cannot be increased in time for a project.
- Commissioning support, where a stable source is needed to test equipment without running a generator for long periods.
The first question is always which behaviour you are trying to change: generator run hours, low-load running, short peaks, number of starts, transient response, or short-duration continuity. Each points to a different power-to-energy ratio.
Power and energy are separate requirements
Power in kW determines how much the BESS can deliver or absorb at any instant, limited by the power conversion system. Energy in kWh determines how long it can sustain that power. “We need a 1 MWh battery” does not say whether the site needs 100 kW for ten hours or 1 MW for one hour, and the two systems are completely different in inverter size and cost.
Installed energy (kWh) = load (kW) x duration (h) / (usable depth of discharge x round-trip efficiency to the load)
Load is the real power the battery must supply, duration is how long it must supply it, usable depth of discharge is the fraction of nameplate energy the system allows you to use while keeping its reserve, and efficiency accounts for inverter and battery losses. As an illustration (illustrative), supporting 200 kW for 2 hours with 85 percent usable depth and 92 percent efficiency needs about 200 x 2 / (0.85 x 0.92), roughly 511 kWh installed, with a power conversion system of at least 200 kW plus margin for transients. That establishes only the minimum steady requirement; motor starts, auxiliary loads, temperature derating and the state-of-charge reserve kept for emergencies all add to it. The battery energy storage systems page explains chemistry and duration classes.
Typical rental use cases and ratios
| Application | Power need | Energy need | Typical ratio of kWh to kW |
|---|---|---|---|
| Absorbing motor starts on a small generator | High | Low | About 0.5 to 1 |
| Peak shaving above a grid limit | Moderate | Moderate | About 1 to 2 |
| Overnight load so the generator can stop | Low | High | About 4 to 8 |
| Short outage cover for a process | Matches process load | Minutes to an hour | About 0.25 to 1 |
Batteries with generators
Most temporary BESS deployments in India sit alongside a diesel set rather than replacing it. The battery handles light load and step changes, while the generator runs at an efficient load to carry the bulk and recharge the battery. This reduces low-load running, which causes wet stacking and carbon build-up, and reduces fuel per kWh. A hybrid study needs more than peak demand: minimum, average and peak load, transient events, an hourly profile, and which loads are critical. The hybrid temporary power page covers the control strategy, and the generator plus BESS screening guide gives a method for checking whether the combination pays off.
Charging is a load too
When the generator recharges the battery, the charging current adds to the site load. A charge rate set too high can push the generator into overload exactly when the site load is also rising. The controller must limit charging to the generator’s spare capacity.
Safety does not reduce because the system is temporary
Lithium-based containerised systems need the manufacturer’s separation distances from buildings and other equipment, working thermal management for Hyderabad summer temperatures above 40 degrees Celsius, DC and AC protection, fire detection and a suppression approach appropriate to the chemistry, and an emergency plan that local responders understand. Placement must keep the container out of waterlogging during the monsoon. Cooling auxiliaries draw power continuously, which counts against the energy available to the load.
Project inputs
A temporary BESS is configured from the load profile and duration, the expected cycling per day, the connection voltage, the state-of-charge reserve to be held for emergencies, the environment and the charging source, whether grid, generator or solar. Without those inputs, any quoted container size is a guess.
Related photographs and videos
Common mistakes
- Specifying only kWh. The battery has plenty of energy but its inverter cannot deliver the peak, and the load trips.
- Specifying only kW. The battery handles the step perfectly but empties within minutes of an outage.
- Letting the generator recharge the battery at full rate. Combined charging and site load overloads the set.
- Placing the container in full sun with no allowance for cooling energy. Output is derated and usable energy falls.
What to send us
Send the load profile, ideally hourly data or a logged week, the outcome you want from storage, the required power and duration if known, the connection voltage, the existing generator or grid source, and the site location for the container. Use the application engineering enquiry so the requirement can be screened with our storage partners.
Questions engineers ask
Can a rental BESS replace a diesel generator entirely?
Only for short durations or where it can be recharged from the grid or solar. For long outages, the energy required makes a battery-only system large and expensive, so the usual arrangement is a battery supported by a generator.
How fast does a BESS respond to a load change?
Grid-forming systems respond within a few cycles, far faster than a diesel engine governor. This is why a battery is effective at absorbing motor starts and load steps that would otherwise dip a generator’s frequency and voltage.
What is a sensible state-of-charge reserve?
It depends on the purpose. Where the battery also provides outage cover, keep enough energy to carry critical loads until a generator starts and stabilises, typically several minutes of the critical load. For pure peak shaving the reserve can be smaller.
Does a temporary BESS need approvals?
Grid-connected operation may need DISCOM agreement, and fire safety arrangements may need local approval depending on size and location. Confirm requirements early, since approvals can take longer than delivery.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.