PowerLab

Solar system sizing

Solar Load Calculator

Estimate your total daily appliance electrical energy consumption (Wh/day and kWh/day) and connected running watts to plan off-grid solar arrays and battery storage.

Quick estimate

Build your solar load profile

Calculations run in your browser•No sign-up required•Instant client-side model

Presets are editable planning estimates, not universal appliance specifications. Add Custom Appliance when you know a different load.

RefrigeratorCycling estimate: 35%
Row details

Hours/day is the scheduled operating window. Duty cycle is the fraction of that window the appliance draws its listed watts. Do not enter an already-derated runtime and apply a duty cycle, as that double-counts the reduction (e.g. Refrigerator: 150 W × 24 h × 35% = 1,260 Wh/day).

Wi-Fi Router
Row details

Hours/day is the scheduled operating window. Duty cycle is the fraction of that window the appliance draws its listed watts. Do not enter an already-derated runtime and apply a duty cycle, as that double-counts the reduction (e.g. Refrigerator: 150 W × 24 h × 35% = 1,260 Wh/day).

LED bulbs
Row details

Hours/day is the scheduled operating window. Duty cycle is the fraction of that window the appliance draws its listed watts. Do not enter an already-derated runtime and apply a duty cycle, as that double-counts the reduction (e.g. Refrigerator: 150 W × 24 h × 35% = 1,260 Wh/day).

LED TV
Row details

Hours/day is the scheduled operating window. Duty cycle is the fraction of that window the appliance draws its listed watts. Do not enter an already-derated runtime and apply a duty cycle, as that double-counts the reduction (e.g. Refrigerator: 150 W × 24 h × 35% = 1,260 Wh/day).

How to Calculate Your Solar Load

  1. Select or Add Appliances: Choose common household appliances from the pre-populated catalog or enter custom wattages.
  2. Set Daily Run Hours & Quantity: Enter how many hours each device operates per day across its scheduled operating window.
  3. Check Duty Cycles: Thermostatically cycled appliances (refrigerators, freezers) default to realistic 30%–40% duty cycles.
  4. Filter Essential vs Total Load: Designate critical blackout loads to evaluate essential-only backup requirements.

Solar System Load Profiles & Inverter Planning Guide

Illustrative daily watt-hour energy consumption profiles and continuous inverter power ranges for common off-grid and backup solar scenarios:

Typical solar load profiles and illustrative continuous inverter power ranges
Application ScenarioRepresentative Appliance MixDaily Energy (Wh/day)Illustrative Continuous Inverter Range
Overland / RV Setup12V Fridge, LED lights, fan, phone/camera charging~600 – 1,000 Wh / day1,000 W – 1,500 W
Off-Grid Tiny House / CabinEnergy-star fridge, Starlink, TV, laptops, lighting~2,500 – 4,500 Wh / day2,000 W – 3,000 W
Critical Home BackupFridge, deep freezer, well pump, router, security~5,000 – 8,000 Wh / day4,000 W – 6,000 W
Whole-Home Off-GridAll appliances, mini-split heat pump, induction cooktop~15,000 – 25,000 Wh / day8,000 W – 12,000 W

Inverter ranges are illustrative estimates. Final inverter sizing requires evaluating motor/compressor starting surge currents, power factor, and continuous power ratings.

Daily Solar Load & Energy Demand Formulas

Calculates cumulative daily energy requirement (Wh/day) and connected running wattage (Watts) across all household AC/DC appliances.

DailyWh=∑ (Wattsi × Quantityi × Hoursi × DutyCycle, i) | ConnectedRunning, Watts = ∑ (Wattsi × Quantityi)

Variable Definitions

Watts_iAppliance Running Wattage(W)
Nominal electrical power drawn by appliance i.
Quantity_iUnit Count(count)
Number of identical active appliances.
Hours_iOperating Window(hours/day)
Scheduled duration per day during which the appliance operates.
Duty_Cycle_iDuty Cycle Factor(fraction)
Fraction of the operating window during which the appliance actively draws rated power (e.g. 0.35 for refrigeration, 1.00 for lighting).
Connected_Running_WattsConnected Running Watts(W)
Sum of listed running wattages if all selected appliances operate simultaneously (not a measured peak or surge calculation).

Calculation Notes

  • Daily energy (Wh/day or kWh/day) is the baseline energy input for sizing solar arrays and battery banks.
  • Connected running watts indicates simultaneous operating load; actual inverter sizing requires evaluating motor/compressor startup surge, continuous power ratings, power factor, and safety margins.
  • Do not enter an already-derated runtime into Hours/day when a duty cycle is applied, to avoid double-counting the reduction.

Technical References & Model Basis

PowerLab implements a deterministic appliance load aggregation model for pre-engineering solar planning. The following technical references provide context for array sizing and electrical load principles:

IEEE Std 1562

IEEE Guide for Array and Battery Sizing in Stand-Alone Photovoltaic (PV) Systems, establishing the methodology of daily load profile summation as the foundation for system sizing.

NFPA 70 / NEC Article 220 Context

National Electrical Code standards for branch-circuit and feeder load calculations, providing engineering background on continuous versus non-continuous load classifications.

Frequently Asked Questions (FAQ)

How do you calculate total solar electrical load?
Multiply each appliance's running wattage by its unit quantity, daily operating window (hours/day), and duty cycle (the fraction of time actively drawing power). Sum all appliances to find total daily energy in Watt-hours (Wh) or kilowatt-hours (kWh). This daily energy figure serves as the baseline input for downstream solar panel and battery bank sizing; complete system sizing also requires solar resource (peak sun hours), conversion losses, battery DoD, and autonomy margins.
What is the difference between daily energy load (Wh) and connected running watts (W)?
Daily energy load (Watt-hours or kWh) measures total energy consumed over 24 hours and serves as the baseline input for solar array and battery storage sizing. Connected running watts (Watts) is the sum of listed running wattages if all selected appliances operate simultaneously. It does not measure actual peak demand, does not account for motor or compressor startup surge, and does not determine final inverter capacity on its own.
Why do cycling appliances like refrigerators use duty cycles?
A refrigerator rated at 150 Watts does not draw 150W continuously for 24 hours. Once the interior reaches set temperature, the compressor cycles off. In typical room temperatures, a refrigerator has an active duty cycle of roughly 30% to 40% (~7 to 10 hours of active compressor run time per 24-hour window, yielding 150 W × 24 h × 35% = 1,260 Wh/day). Preset duty cycles are editable planning estimates, as actual energy consumption depends on ambient temperature, age, thermostat settings, and door openings. Plug-in meter measurements should be used whenever available.
What is an essential versus non-essential load in solar planning?
Essential loads are specific appliances you designate to keep powered during grid outages or cloudy periods (e.g., refrigeration, medical devices, Wi-Fi router, LED lighting, water well pump). Non-essential loads (such as discretionary air conditioning, clothes dryers, or dishwashers) can be turned off to reduce the required battery storage and solar array capacity.
How does connected running watts relate to inverter sizing?
Connected running watts indicates total simultaneous running power if all appliances run at once. Actual inverter selection also depends on startup inrush surge currents (e.g., inductive motor/compressor starting loads that can draw 3× to 6× rated watts for several seconds), power factor, continuous output derating at elevated ambient temperatures, and manufacturer surge specifications.