Solar
Inverter Size Calculator
Inverter planning starts with the sum of running watts and the largest added startup demand. The continuous estimate applies the entered reserve to the running total.
Aggregate running and surge loads, then apply your own headroom and efficiency assumptions.
Calculated result
Inverter planning totals
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
Equipment planning
Components that meet these requirements
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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.
Data provenance: NLR PVWatts v8 grid awaiting its first private-key refresh; the visible peak-sun-hours fallback is active. EIA residential state averages updated 2026-08-13. ZIP centroids use the 2025 Census Gazetteer. No ZIP or calculator input is sent to those sources. Read the solar data provenance and limitations.
What this calculator returns
Compare continuous and surge requirements with the inverter manufacturer's ratings and time limits.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Running W = Σ(W × qty); peak W = running total + largest added startup demand; planned continuous W = running × (1 + headroom).
- V
- Nominal battery or bank voltage.
- Ah
- Charge capacity in amp-hours.
- Wh
- Nominal or usable stored energy in watt-hours.
- W
- Average load power in watts.
- η
- Entered conversion efficiency.
Starting multipliers and headroom are user-entered planning assumptions.
Worked example
| Input | Value |
|---|---|
| Loads | See the example load list in the calculator |
A 120 W laptop plus an 800 W pump with a 3× startup multiplier produces 920 W running and a 2,520 W single-start peak.
Reference table
| Appliance | Planning watts | Use note |
|---|---|---|
| LED lamp | 10 W | Nominal example |
| Laptop computer | 60–120 W | Typical planning range |
| Refrigerator while running | 100–250 W | Cycling load; startup can be higher |
| Microwave oven | 900–1,500 W | Input varies by model |
| Portable space heater | 1,500 W | Common high setting |
| Electric kettle | 1,200–1,800 W | Varies by market and model |
Frequently asked questions
Why is surge power higher than running power?
Some loads draw extra power while starting, so the tool adds the largest entered startup demand to the other running loads.
Does inverter efficiency change the AC size?
The efficiency input estimates DC-side demand. Continuous and surge AC ratings still need to meet the load totals.
Assumptions and limitations
- No authoritative appliance presets.
- No conductor, breaker, fuse, or protection sizing.
- Assumes one highest-surge load starts at a time.
Method and sources
Review the solar data sources, calculation methodology, and electrical formulas for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .