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Learn/Energy foundations

LESSON 2 OF 4 · ABOUT 8 MINUTES

Read the battery’s energy boundary.

Follow nominal energy through usable share, reserve and conversion without applying a loss twice.

A clearly stated energy boundary5,000 WhNominal4,500 Wh90% usable share3,600 WhAfter 20% reserve3,240 Wh AC90% conversion
Illustrative shares only, not a chemistry or product claim. If a source already measures usable or AC-output energy, skip reductions already included.

Ask what the capacity figure includes.

Nominal energy, documented usable energy and energy already delivered at an AC outlet are different bases. Read the exact documentation and test conditions. A kWh capacity also does not establish the battery or inverter’s kW output limit.

Apply each allowance once.

For an illustrative 5,000 Wh nominal battery: 90% usable share gives 4,500 Wh. Holding 20% of that usable window in reserve leaves 3,600 Wh before conversion. At an assumed 90% inverter efficiency, that supplies 3,240 Wh of AC energy. These percentages are exercise inputs, not defaults for a chemistry or product.

If a source already gives usable battery-side energy, skip the nominal-to-usable reduction. If a figure already measures delivered AC energy, do not subtract an inverter loss a second time. Temperature, standby loads and source test conditions can also matter.

Backup is a system function.

A battery on a product list does not establish outage operation. Controls, inverter capability, circuits and the intended installation must support the use. The runtime tool can calculate energy arithmetic while leaving output capability and compatibility unresolved.

Keep these distinctions.

  • Identify nominal, usable and delivered-output figures.
  • Keep battery-side and AC-output boundaries visible.
  • Capacity is not an output rating or backup approval.

TRY THE REASONING

Practice with visible assumptions.

Answers stay on this page. Check all three exercises to record this lesson as practiced; there is no attempt limit or access gate. Worked answers are available even without JavaScript.

Practice 1

Assume 2,000 Wh is documented usable battery-side energy. With these allowances, what is the constant AC-load runtime?

Show the worked answer

Starting values: 2,000 × (1 − 20%) = 1,600 Wh before conversion. Draw = 100 W ÷ 90% = 111.111… W at the battery. Runtime = 14.4 hours. Do not apply a second usable-share reduction.

Practice 2

A test reports 1,800 Wh delivered at an AC outlet. What should an AC-only runtime estimate do?

Show the worked answer

The measured output already crosses the AC boundary. Match the load to that boundary and keep the original test conditions visible. Correct choice: Use that AC-output energy without a second inverter reduction.

Practice 3

A listing gives 10 kWh of energy but no power rating. Can it be assumed to run a 6 kW load?

Show the worked answer

kWh and kW have different dimensions. A mathematical energy duration does not prove output capability. Correct choice: No; power/output limits and the complete system are unresolved.

Try the runtime calculatorRead battery specifications

Sources & learning scope

Source concepts reviewed 2026-11-07. Exercise values are explicitly illustrative. This path explains concepts and arithmetic; it is not an installation design or professional certification.