Solar Panel Sizing Tool

Size panels from daily energy use + peak sun hours + losses. Optional battery autonomy estimate and panel count.

How to Use

  1. Enter your daily energy use (kWh/day or Wh/day).
  2. Set Peak Sun Hours for your location (typical 2–6).
  3. Adjust system losses/derate (wiring, controller, temperature, dirt).
  4. Choose a panel wattage to estimate panel count.
  5. (Optional) Enable Battery Planning for autonomy and usable battery energy.
Sizing Summary
Live readout updates from your inputs. Use “Show Work” for formulas and steps.
Array (W)
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Panels
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Daily (Wh)
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Usable Batt (Wh)
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Confidence: —
Inputs & Settings
Enter usage + sun hours. Adjust losses and panel wattage to estimate array size and count.
Tip: Sum appliance watt-hours or use an energy monitor.
Typical: 2–6 depending on season/location.
Includes temperature, wiring, controller, dirt. Common: 15–35%.
Used only for panel count estimate.
Quick Adjust
4.5 h
25%

Tip: Loss/derate is a planning factor. You can add margin below for off-grid/autonomy planning.

Battery Planning (optional)
How many days you want to run without sun.
Common: 12V, 24V, 48V.
Lead-acid often lower; LiFePO₄ often higher.
Set to 100% for pure DC loads.
Battery results are estimates. Real sizing depends on discharge rate, temperature, and battery chemistry specs.
Show Work (step-by-step)
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Work is shown using Wh/day, hours/day, and watts (W). Battery steps use Wh and optional Ah at your system voltage.

Reference (Sizing Formulas)

Core idea: Your array needs to produce your daily energy within the available sun window, accounting for losses.

  • Daily energy (Wh/day): Wh = kWh × 1000
  • Loss factor: lossFactor = 1 − (loss% / 100)
  • Adjusted energy target: Wh_target = Wh_daily / lossFactor
  • Array watts required: W_array = Wh_target / PSH
  • Panel count: panels = ceil(W_array / W_panel)
  • Battery usable energy (optional): Wh_batt_usable = Wh_daily × autonomyDays
  • Battery nameplate energy: Wh_batt = Wh_batt_usable / (DoD × efficiency)
  • Battery Ah estimate: Ah ≈ Wh_batt / systemV
This tool is planning-grade sizing. Hardware limits (controller current, wiring, battery C-rate) should be checked in your system design.

FAQ

What are “Peak Sun Hours”?

Peak Sun Hours (PSH) is a daily average of solar energy expressed as equivalent hours at 1,000 W/m². It’s a common sizing input.

What loss/derate should I use?

Many systems plan around 15–35% total losses. Higher loss values are more conservative (hot climates, long wiring runs, less-than-ideal tilt).

Why does the tool suggest “margin”?

Margin helps cover cloudy days, seasonal dips, battery aging, and real-world inefficiencies that aren’t captured by a single loss percentage.

Does this replace a full solar design?

No—this sizes energy and rough hardware counts. You still need to validate charge controller limits, wire gauge/voltage drop, fusing, and battery chemistry specs.

Tool Info

Last updated:

Updates may include UI improvements, additional sizing modes, and calculation edge-case handling.