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Size the battery from essential energy, not the inverter label

Start with the loads that must run during an outage. A 5 kW inverter does not mean you need 5 kWh of battery, and a 5 kWh battery does not necessarily…

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Start with the loads that must run during an outage. A 5 kW inverter does not mean you need 5 kWh of battery, and a 5 kWh battery does not necessarily deliver 5 kWh of usable AC energy. Usable capacity is reduced by permitted depth of discharge, conversion losses, temperature and battery ageing.

What applies to this exact problem

Start with the loads that must run during an outage. A 5 kW inverter does not mean you need 5 kWh of battery, and a 5 kWh battery does not necessarily deliver 5 kWh of usable AC energy. Usable capacity is reduced by permitted depth of discharge, conversion losses, temperature and battery ageing.

Build the load-hours worksheet

LoadWattsQuantityHoursWh
FansW eachnhW × n × h
LightsW eachnhW × n × h
RefrigeratorUse measured/estimated duty cycle1hAverage W × h
Router/laptopWnhTotal

Add the watt-hours, then divide by expected inverter/system efficiency and by the usable fraction of nominal battery capacity. Finally add an ageing/reserve margin. For example, if essential loads need 3,000 Wh and you assume 90% conversion efficiency and 80% usable depth of discharge, the theoretical nominal requirement is roughly 3,000 ÷ 0.9 ÷ 0.8 = 4,167 Wh before ageing reserve.

Power and energy are different constraints

The battery must supply enough energy for the required hours, while the inverter and battery must also support the instantaneous power and surge of loads such as refrigerators, pumps or motors. A battery can have enough kWh but still fail if the system cannot deliver the startup surge.

Do not size from one perfect-day solar assumption

If the battery is meant for outage resilience, decide what happens during cloudy weather and multi-hour grid failures. Separate “daily solar self-consumption” from “emergency reserve”. A system optimised to discharge deeply every evening may have little backup left when the grid fails at night.

Check the safety and warranty envelope

Use equipment and installation practices consistent with manufacturer specifications and applicable Indian electrical/safety requirements. Consult current guidance from MNRE, the Central Electricity Authority and relevant BIS standards. Battery chemistry, ventilation, protection, earthing and installation location are safety decisions, not only efficiency choices.

Decision rule: size from essential Wh, surge W, usable depth of discharge, efficiency and ageing reserve. If a vendor cannot show all five assumptions, the quoted battery size is not yet auditable.

Check these first

  • Load: Watts.
  • Fans: W each.
  • Lights: W each.

Fix it in this order

  1. Load: Watts.
  2. Fans: W each.
  3. Lights: W each.
  4. Refrigerator: Use measured/estimated duty cycle.
  5. Router/laptop: W.
  6. Build the load-hours worksheet Load Watts Quantity Hours Wh Fans W each n h W × n × h Lights W each n h W × n × h Refrigerator Use measured/estimated duty cycle 1 h Average W × h Router/laptop W n h Total Add the watt-hours, then divide by expected inverter/system efficiency and by the usable fraction of nominal battery capacity. Finally add an ageing/reserve margin. For example, if essential loads need 3,000 Wh and you assume 90% conversion efficiency and 80% usable depth of discharge, the theoretical nominal requirement is roughly 3,000 ÷ 0.9 ÷ 0.8 = 4,167 Wh before ageing reserve. Power and energy are different constraints: The battery must supply enough energy for the required hours, while the inverter and battery must also support the instantaneous power and surge of loads such as refrigerators, pumps or motors. A battery can have enough kWh but still fail if the system cannot deliver the startup surge.
  7. Do not size from one perfect-day solar assumption: If the battery is meant for outage resilience, decide what happens during cloudy weather and multi-hour grid failures. Separate “daily solar self-consumption” from “emergency reserve”. A system optimised to discharge deeply every evening may have little backup left when the grid fails at night.

Evidence to keep

  • Signed quote and scope — keep it with the evidence for “Size the battery from essential energy, not the inverter label”.
  • Module/inverter serials and warranty — keep it with the evidence for “Size the battery from essential energy, not the inverter label”.
  • DISCOM/portal application IDs — keep it with the evidence for “Size the battery from essential energy, not the inverter label”.
  • Generation data, photos, and installer tickets — keep it with the evidence for “Size the battery from essential energy, not the inverter label”.

Do not make it harder

  • Paying the final amount before checking agreed milestones For “Size the battery from essential energy, not the inverter label”, that can hide whether the underlying issue is actually resolved.
  • Assuming low generation is automatically a bad panel For “Size the battery from essential energy, not the inverter label”, that can hide whether the underlying issue is actually resolved.
  • Treating installer completion as DISCOM/subsidy completion For “Size the battery from essential energy, not the inverter label”, that can hide whether the underlying issue is actually resolved.

How you know it is fixed

  • The official record and your real-world result agree for “Size the battery from essential energy, not the inverter label”.
  • You have enough written evidence to prove the issue is finished if it returns later for “Size the battery from essential energy, not the inverter label”.

If this still isn't resolved

  1. Installer/vendor grievance State the unresolved issue explicitly: “Size the battery from essential energy, not the inverter label”.
  2. DISCOM/portal grievance route State the unresolved issue explicitly: “Size the battery from essential energy, not the inverter label”.
  3. Consumer forum or other official remedy where appropriate State the unresolved issue explicitly: “Size the battery from essential energy, not the inverter label”.

Sources for this path

Use these references to confirm provider-, model-, policy-, or jurisdiction-specific details before an irreversible step.

Need the complete context?

This page solves one branch. The parent guide covers the full decision, edge cases, alternatives, and related checks.

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