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Solar Panel Array Size Calculator — Off-Grid & Grid-Tie

Find out how many kW of solar panels you need to cover your daily energy use — for an off-grid system, or to size a grid-tie array against your bill.

Array size needed2.78 kW

How to measure your inputs

  • Daily load: add up each appliance's watt-hours per day (watts × hours used), or take your utility bill's monthly kWh and divide by ~30.
  • Peak sun hours: not daylight hours — the equivalent hours of full-intensity sun your location gets. Check Swales' Solar Potential map for a specific site, or a PVGIS/NREL lookup for your region; typically 3-6 depending on latitude and season.

Methodology

Standard PV sizing formula: array size (kW) = daily load (kWh) ÷ (peak sun hours × system efficiency). System efficiency bundles inverter loss, wiring loss, and temperature derating — 75-85% is the commonly cited real-world range, exposed here as a preset rather than hidden as a fixed assumption.

How solar array sizing actually works

kW vs. kWh — the mix-up that breaks most DIY sizing attempts

kW (kilowatts) is a rate — how fast your system produces or your appliances draw power at any instant. kWh (kilowatt-hours) is an amount — total energy used or produced over time. Your daily load needs to be in kWh (energy used over a day) for this formula to work; array size comes out in kW (the panel capacity needed to produce that much energy given your sun hours).

Off-grid sizing vs. grid-tie sizing

Off-grid systems need to be sized to fully cover daily load, usually with a safety margin and battery storage for nights/cloudy days, since there’s no grid to fall back on. Grid-tie systems can be sized to offset a target share of your bill rather than 100% of load, since the grid covers any shortfall — the same formula applies either way, just with a different target daily load figure.

Why oversizing slightly is usually the safer call

Panel output degrades a small amount every year (typically 0.5-0.8% annually), and real-world conditions (dust, partial shading, temperature) rarely hit the ideal case a spec sheet assumes. Sizing to the "typical" efficiency preset rather than "optimistic," or adding a 10-15% margin on top of the calculated result, is common practice to avoid a system that falls short within a few years of installation.

Frequently asked questions

How many solar panels do I need for an average house?

It depends entirely on your actual daily energy use and local sun hours — there’s no single "average house" number that applies everywhere. Use your utility bill’s monthly kWh divided by ~30 as your daily load, and your region’s peak sun hours (check Swales’ Solar Potential map), then divide the resulting array kW by your chosen panel’s wattage to get a panel count.

What's the difference between kW and kWh?

kW measures power — the rate of energy flow at a given moment (like a car’s speedometer). kWh measures energy — the total amount used or produced over time (like the car’s odometer). A 5kW array running at full output for 4 hours produces 20kWh of energy.

Does cloudy weather change how I should size my array?

Yes, indirectly — your region’s peak sun hours figure already accounts for typical cloud cover averaged over the year, so using a location-specific figure (rather than a clear-sky assumption) already builds in realistic weather. For regions with strong seasonal variation, size against your worst realistic month rather than the annual average if year-round reliability matters.

Should I intentionally oversize my solar array?

A modest margin (10-15%) is common practice to offset gradual panel degradation and real-world losses that don’t show up in spec-sheet numbers. Significant oversizing beyond that mostly just adds cost without proportional benefit, unless you’re specifically planning for future load growth.