Off-Grid Battery Bank Sizing

Use measured daily energy

Create separate seasonal energy budgets. Refrigeration, water pumping, communications, lighting, workshop loads, and heating-system controls may change throughout the year.

For each load, multiply average watts by hours. Add conversion loss and reserve after totaling energy. Do not use inverter nameplate power as daily energy.

Autonomy is a risk decision

More battery capacity reduces generator starts and tolerates poor weather, but costs more and takes longer to recharge. Choose autonomy from local weather, generator reliability, fuel access, critical loads, and willingness to curtail demand.

Model at least a low-solar scenario. Average annual sun hours can hide the winter period that determines whether the system works.

• Worst practical seasonal production
• Critical versus deferrable loads
• Generator availability and fuel storage
• Maximum battery charge current
• Array and controller capacity
• Acceptable low-state-of-charge events

Check recharge time

A large bank paired with a small array can remain partly discharged through successive poor days. Estimate net charging after daytime loads, controller loss, battery charge limits, and seasonal conditions.

Generator charging should be powerful enough to be useful but remain within inverter-charger, generator, and battery limits. Frequent low-load generator operation can also be inefficient.

Design power and protection

Inverter continuous power, motor surge, battery BMS current, conductors, and fault protection must be checked separately from energy capacity. Pumps and workshop tools often determine surge.

Permanent off-grid buildings can still be subject to electrical, building, and fire requirements. Use qualified design and installation where applicable.