Solar PV System Sizing Calculator
Size a solar PV system from your daily energy usage, peak sun hours and panel wattage: required array capacity, number of panels, recommended inverter size, and — for off-grid systems — battery bank capacity from days of autonomy and depth of discharge. Runs entirely in your browser.
Input
Pull this from a utility bill (kWh ÷ days in the billing period).
Average daily equivalent hours of full sun for your location — check a solar insolation map (typically 3–6 h/day).
Combined losses from inverter conversion, wiring, soiling, temperature and shading. 77% is the NREL PVWatts default.
Array DC capacity ÷ inverter continuous AC rating. 1.2 is a common design target; above ~1.3 expect noticeable inverter clipping.
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
Work out how big a solar PV system you actually need. Enter your average daily energy usage, the peak sun hours for your location and the panel wattage you're considering, and get back the required array capacity, how many panels that takes, a recommended inverter size, and — for off-grid systems — a battery bank size. Everything runs in your browser: no uploads, no sign-up.
How it works
The core formula every solar sizing guide starts with:
System size (kW) = Daily usage (kWh) ÷ (Peak sun hours × Derate factor)
The derate factor bundles the losses a real system never avoids — inverter conversion, wiring resistance, soiling, temperature and shading. 77% is the default PVWatts (NREL) assumption for a well-designed grid-tied system; drop it lower if your site is shaded or the panels run hot.
Once the array's kW requirement is known, the panel count rounds up to a whole number at your chosen wattage, and the inverter size follows from the actual (rounded-up) array capacity divided by a DC:AC ratio — most real installs deliberately undersize the inverter relative to the DC array (a ratio around 1.2) since panels rarely reach their rated output at once.
How to use it
- Average daily energy usage — pull this straight off a utility bill: total kWh for the billing period ÷ number of days.
- Peak sun hours — the average daily equivalent hours of full sun for your location, not daylight hours. Check a solar insolation map; most of the US falls between 3 and 6.
- Solar panel wattage — the STC-rated wattage of the panel model you're pricing (300–450W covers most residential panels today).
- System derate factor — leave it at 77% unless you have a reason to adjust it (heavy shading, a hot climate, an older inverter).
- DC:AC ratio — how much you're willing to undersize the inverter relative to the array. 1.2 is a common target; push past 1.3 and you'll start losing meaningful production to clipping on sunny days.
- System type — switch to Off-grid to also size a battery bank from days of autonomy and depth of discharge.
Why is the inverter smaller than the array?
Because panels almost never hit their nameplate (STC) rating at once — that requires perfect sun angle, no clouds and cool panel temperature simultaneously. Sizing the inverter a bit below the array's DC capacity (a DC:AC ratio above 1.0) captures more of the array's real-world output per dollar of inverter, at the cost of clipping the rare peak. Most designers land between 1.1 and 1.3.
How is the battery bank sized for an off-grid system?
Usable energy needed = daily usage × days of autonomy. That gets divided by the depth of discharge to get the nominal (not just usable) capacity, since discharging a battery past its rated DoD shortens its life. A 50% DoD is the conservative assumption for lead-acid; lithium (LiFePO4) banks can be sized against 80–90% DoD instead, shrinking the required capacity substantially for the same usable energy.
Is this a permit-ready design?
No — treat it as a planning-stage estimate. A real installation needs an actual irradiance and shading study for your roof, a code-compliant electrical design (NEC Article 690 in the US), and a review by a licensed installer before anything gets purchased or mounted.
Related tools
Already know your panel count and want to estimate the energy output and savings instead of sizing from scratch? The Solar Panel Output Calculator runs the calculation the other direction — production, cost savings and CO₂ offset from a given system. For sizing a backup power supply more generally, see the Battery Life & Runtime Calculator.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/solar-pv-system-sizing-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"dailyUsage": "30",
"peakSunHours": "4.5",
"panelWattage": "400",
"systemEfficiency": "77",
"dcAcRatio": "1.2",
"mode": "grid-tied"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `solar-pv-system-sizing-calculator` tool (Solar PV System Sizing Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.