Battery & energy
Home Backup Power Calculator
Home backup energy starts with each critical load's running watts, daily hours, quantity, and the chosen outage duration. Inverter planning separately compares simultaneous running demand with the largest single added startup demand. The method follows the electrical relationships and NIST unit references documented on the Electrical Formulas and Sources pages.
Plan battery, inverter, solar, and generator capacity from editable critical loads, outage duration, efficiencies, reserve, and optional recharge.
Calculated result
Home backup power plan
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
Equipment planning
Compatible options to compare
The specification is calculated from the result above. Products that meet every displayed requirement appear as matches; reference products are clearly marked with compatibility limits.
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Calculate above to generate a parts requirement.
Cable, fuse, breaker, disconnect, mounting, grounding, and code requirements need separate equipment and installation review.
What this calculator returns
Compare the battery-only, solar-assisted, generator, and hybrid scenarios, then verify actual appliance demand and every equipment rating. The result is an outage energy budget, not an electrical installation design.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Daily load Wh = Σ(running W × quantity × hours/day). Required usable battery Wh = outage load Wh ÷ inverter efficiency ÷ (1 − reserve). Nominal Wh = usable Wh ÷ usable depth. Peak W = running total + largest single added startup demand.
- Eload
- Critical-load energy from running watts, quantity, daily hours, and outage duration.
- Prun
- Sum of the selected loads' simultaneous running watts.
- Psurge
- Running total plus the largest single added startup demand.
- ηinv
- User-entered inverter efficiency applied to delivered AC load energy.
- DoD
- User-entered usable share of nominal battery capacity.
- reserve
- User-entered battery energy held outside the planned outage budget.
Solar and generator support are explicit daily energy inputs. Inverter headroom, usable depth, efficiency, and reserve are visible planning assumptions.
Worked example
| Input | Value |
|---|---|
| Critical loads | See the example load list in the calculator |
| Desired outage duration | 48 hours |
| Battery system voltage | 48 V |
| Optional solar recharge | 0 kWh/day |
The editable refrigerator, router, lights, laptop, and blower example uses 5.72 kWh/day. For 48 hours, 90% inverter efficiency, 10% reserve, and 80% usable depth, the nominal battery estimate is about 17.65 kWh before any solar or generator support.
Reference table
| Planning quantity | Derived from | Use in the result |
|---|---|---|
| Daily load energy | Running W × quantity × hours/day | Battery and recharge energy budget |
| Continuous demand | Sum of connected running watts | Inverter continuous rating with entered headroom |
| Single-start peak | Running total + largest added startup demand | Minimum inverter surge comparison |
| Nominal battery | Outage energy, efficiency, reserve, and usable depth | Nameplate storage planning |
Frequently asked questions
How do I estimate appliance watts for an outage?
Prefer a meter reading or exact manufacturer documentation. The editable examples only help structure a first pass and must not be treated as appliance specifications.
Why is nominal battery capacity larger than outage energy?
The calculator accounts for the entered inverter loss, reserve, and usable-depth limit, so the nameplate battery must exceed energy delivered to the loads.
Does this size a transfer switch or home wiring?
No. It intentionally stops at energy and equipment-output requirements; installation requirements need an appropriate professional.
Assumptions and limitations
- Editable load examples are planning defaults, not exact appliance consumption; use measured or manufacturer values.
- Assumes the single largest additional startup load occurs while other selected loads are running.
- No breaker, conductor, transfer switch, service-capacity, fuel-system, ventilation, or code-compliance calculation.
- Solar input is user-entered daily recharge, not a production forecast.
- Do not use this planning estimate as the sole basis for medical or life-safety backup.
Method and sources
Review the electrical formulas, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and provenance. Content release .
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