Path-Q Solar
How large should a battery be if I only want to cover essential appliances?
Answer
Size the battery from the essential loads' simultaneous power demand and expected runtime, then add allowance for conversion losses and a reserve. Battery energy capacity determines how long the loads run; inverter power and surge capability determine whether they can start and run together.
Key points
- List the appliances that must operate during an outage and their running watts, starting watts, and expected hours of use.
- Calculate approximate energy as watts multiplied by hours, then account for usable capacity and system losses.
- Check simultaneous load and motor-starting requirements separately from daily energy.
- A smaller battery can be adequate if loads are scheduled or managed rather than run together.
What it depends on
- Critical appliances and their running and starting power
- Desired backup duration and outage pattern
- Battery usable capacity, reserve setting, and round-trip losses
- Inverter continuous and surge ratings
Risks and trade-offs
- Sizing only by total watt-hours can cause failures when pumps, compressors, or other motors start.
- Oversizing increases cost and may leave capacity underused, while undersizing reduces runtime or causes overloads.
- Essential-load definitions may change during an outage, especially for heating, cooling, communications, and medical equipment.
Information you may need
- Appliance nameplates or measured power
- Required runtime and simultaneous-use assumptions
- Battery usable capacity and inverter power specifications
Your next step
What to do next
Create a critical-load schedule showing each appliance's running watts, startup watts, hours, and priority, then size both battery capacity and inverter output from it.
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