A battery should not be sized from the solar capacity or monthly electricity bill alone. Backup design is a separate exercise based on critical loads, peak power, starting current, desired hours, acceptable depth of discharge, conversion losses, and how frequently outages occur.
Create a critical-load schedule. Multiply each load's watts by the hours required, add realistic losses and reserve, and verify that the inverter and battery can deliver both the total energy and the highest simultaneous power.
Energy and power are different
Battery energy is usually expressed in kilowatt-hours. It describes how much energy is stored. Power, expressed in kilowatts, describes how much can be delivered at one moment. A battery may store enough energy for several hours but still be unable to start a large motor or support too many appliances simultaneously.
Build a critical-load worksheet
| Load | Quantity | Running watts | Required hours | Starting surge? |
|---|---|---|---|---|
| Lights | Customer input | From rating | Customer input | Usually low |
| Fans | Customer input | From rating | Customer input | Check type |
| Refrigerator | Customer input | Measure or estimate | Duty-cycle based | Yes |
| Pump or motor | Customer input | Nameplate and measurement | Operating schedule | Often significant |
Do not automatically include every air conditioner, water heater, induction cooker, and pump. Separate essential, preferred, and non-backup loads. This keeps the system realistic and allows clear operating instructions.
Usable energy matters more than nameplate energy
The usable portion depends on the battery chemistry, manufacturer limits, battery management system, temperature, discharge rate, age, and reserve settings. The proposal should state nominal and usable energy rather than presenting only ampere-hours without voltage and configuration.
Lead-acid and lithium-based options
Lead-acid batteries may have a lower initial cost and familiar service ecosystem, but generally require more space, ventilation, maintenance or operating discipline depending on type, and shallower cycling for useful life. Lithium-based systems can offer higher usable depth, compact size, integrated management, and strong cycle performance, but compatibility, enclosure safety, certified controls, warranty, and replacement support are essential.
Compatibility checks
- Battery voltage and communication protocol match the inverter.
- Charge and discharge current remain within both devices' limits.
- The battery management system can communicate alarms and limits.
- Expansion rules are defined; old and new batteries may not be safely mixed.
- Backup output circuits are separated and labelled correctly.
- Generator interaction, if any, is explicitly designed.
Installation and safety
Follow the manufacturer's temperature, ventilation, clearance, enclosure, fire, cable, fuse, isolation, and mounting instructions. Keep batteries away from standing water, direct heat, untrained access, and combustible clutter. Provide a clear shutdown procedure and emergency contact.
Warranty questions
- Is warranty stated in years, cycles, energy throughput, or a combination?
- What operating temperature and depth-of-discharge conditions apply?
- Does registration or internet connectivity affect coverage?
- Who diagnoses the battery and who pays transport or labour?
- What capacity retention is promised, and how is it tested?
- What happens if the exact model is unavailable later?
Define backup before buying a battery
Your Energy can help list critical loads and connect the backup requirement to a compatible hybrid or off-grid design.
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