Solar Panel System Size

How Solar Panel System Sizing Works

A solar system’s size is how much electricity your home uses, divided by how much sunlight your system can capture and convert, adjusted for real-world losses. Enter your average monthly electricity usage and your location’s peak sun hours per day, and this calculator estimates the system size (in kilowatts) needed to offset your usage, along with roughly how many panels that takes.

This is the number to work out before talking to an installer — knowing roughly what system size and panel count you’re aiming for means you can sanity-check a quote instead of just trusting whatever number shows up on it, and it’s the natural first step whether you’re planning a new install, comparing quotes from different installers, or just curious how big a system your usage would actually require.

“Peak sun hours” isn’t the same as daylight hours — it’s the equivalent number of hours of maximum-strength sunlight your location gets, since solar irradiance varies throughout the day and by season. This varies significantly by region (a sunny desert climate gets meaningfully more peak sun hours than a cloudy northern one), so this calculator asks you to enter your own — check your utility, a solar installer’s quote, or the U.S. Department of Energy’s rooftop solar potential resources for your specific area’s typical figure.

If you don’t already know your monthly electricity usage, the Electricity Cost Calculator can help you work it out, and once you have a system size in mind, the Solar Panel Savings Calculator estimates the payback period once you get a quote.

Key Factors to Consider

The four adjustable inputs above are simplifications of real-world factors that genuinely change how big a system you need — worth understanding even though this calculator treats each one as a single number you supply:

• Panel wattage. Modern residential panels commonly range from about 350W to 450W each — a higher-wattage panel means fewer panels for the same system size, which can matter if roof space is limited. There’s no universally “correct” wattage to enter; use the figure from a quote you’ve received, or a reasonable mid-range estimate if you’re still shopping.
• Roof orientation and tilt. A south-facing roof (in the Northern Hemisphere) tilted close to your latitude typically captures the most sunlight over a year; east- or west-facing roofs still work but produce somewhat less. The U.S. Department of Energy notes panels can generally face up to 45° east or west of true south without a large drop in output — but a roof that’s shaded for a meaningful part of the day, or oriented well outside that range, will need a larger system (or more peak sun hours entered) to hit the same target.
• Shading. Even partial shading from a nearby tree or chimney on part of an array can reduce a system’s real-world output disproportionately more than the shaded area alone would suggest, since a single shaded panel can bottleneck an entire string. This calculator has no way to know about your roof’s specific shading — if you know your roof is partially shaded, consider entering a higher system-losses percentage than the default to compensate.
• Regional and seasonal variation in peak sun hours. The same home in Arizona and the same home in Washington State need very different system sizes for the same usage, purely because of how much peak sun each location gets on average. Peak sun hours also vary by season (higher in summer, lower in winter) — the figure you enter should represent a realistic annual average for your location, not just a sunny summer day.
• Net metering and your offset target. If your utility credits excess solar production against what you draw from the grid at other times (net metering), a 100% offset target is a reasonable starting point. If your utility doesn’t offer net metering, or offers it at a reduced rate, you may want to size for a lower offset percentage rather than overbuilding a system whose excess production isn’t worth as much.
• Battery storage. This calculator sizes a system to offset your usage, not to guarantee power during an outage. Adding battery storage doesn’t change the system-size math here, but it’s a separate decision worth factoring into your overall budget and installer conversation if backup power matters to you.

Interpreting Your Results

The kilowatt figure and panel count are a starting estimate, not a purchase order — a few things change between this number and an actual installed system:

• Installers round to real hardware. A calculated 7.5 kW system with 400W panels needs 18.75 panels — no installer sells three-quarters of a panel, so a real quote will round to a whole number (19 panels here, matching this calculator’s own rounding) or adjust the panel count/ wattage combination to fit your specific roof.
• DC vs. AC nameplate sizing. Solar systems are commonly described by their DC (panel) capacity, which is what this calculator estimates — the AC output your inverter actually delivers to your home is typically somewhat lower due to inverter losses (already folded into the “system losses” input above). Don’t be surprised if an installer’s AC-rated number looks a little different from this calculator’s kW figure; ask which one they’re quoting if it’s unclear.
• Check the panel count against your actual roof. This calculator estimates how many panels you need electrically — it doesn’t know whether that many panels physically fit your roof. Use the Solar Roof Area Calculator to check whether your available roof space can accommodate the panel count this calculator suggests.

Common Mistakes

• Confusing peak sun hours with daylight hours. A location can have 14 hours of daylight in summer but still average only 5-6 peak sun hours, since peak sun hours measure sunlight intensity, not just how long the sun is up. Using daylight hours here will significantly undersize the resulting system.
• Ignoring system losses entirely. Setting system losses to 0% assumes a perfectly efficient system with no wiring resistance, inverter loss, or dust — unrealistic for any real installation, and it will undersize the system needed to actually hit your offset target.
• Sizing only for today’s usage. If you’re planning to add an EV, a heat pump, or otherwise expect your electricity usage to grow, sizing strictly to your current monthly usage means you’ll likely need a second, more expensive system expansion later rather than accounting for that growth now.
• Assuming 100% offset is always the right target. A 100% offset makes sense if net metering credits excess production well; without generous net metering, a smaller system sized to your minimum year-round usage can be the more cost-effective choice — see the Key Factors section above.

The Formula

$$\vE{\text{Daily Usage}} = \frac{\vA{\text{Monthly Usage}}}{30}$$ $$\vF{\text{Required Daily Production}} = \vE{\text{Daily Usage}} \times (\vB{\text{Offset Target}} \div 100)$$ $$\vG{\text{System Size (kW)}} = \frac{\vF{\text{Required Daily Production}}}{\vC{\text{Peak Sun Hours}} \times (1 - \vD{\text{System Losses}} \div 100)}$$ $$\text{Panels Needed} = \frac{\vG{\text{System Size (W)}}}{\text{panel wattage}}$$

Rounded up.

System losses account for inverter inefficiency, wiring, dust, and other real-world factors that reduce a system’s output below its theoretical maximum — commonly estimated in the 15-25% range, adjustable above.

Worked Example

900 kWh/month usage, 5 peak sun hours/day, a 100% offset target, 20% system losses, and 400W panels:

1. Daily Usage: $\vA{900} \div 30 = \vE{30} \text{ kWh}$.
2. Required Daily Production: $\vE{30} \times 100\% = \vF{30} \text{ kWh}$.
3. System Size: $\vF{30} \div \vC{5} \div 0.8 = \vG{7.5} \text{ kW}$.
4. Panels Needed: $\vG{7,500}\text{W} \div 400\text{W} = 18.75$ → 19 panels (rounded up).
5. Estimated annual production: about 10,950 kWh.

Useful to Know

Solar panels alone won’t power your home during a blackout. Grid-tied solar systems (the type this calculator estimates) are required by code to include “anti-islanding” protection — when the grid goes down, your inverter automatically shuts off, even if the sun is shining and your panels could otherwise produce power. This isn’t a flaw; it’s a safety requirement: without it, your system could backfeed electricity into “dead” power lines that utility crews are actively working on to restore service, putting them at serious risk. If you want power during an outage, you need battery storage (or a separate backup generator) paired with your solar system — the system size this calculator estimates doesn’t include or guarantee that capability on its own.