01What this calculator estimates
This solar panel sizing calculator turns your household electricity use into the four numbers that actually matter when planning a rooftop system: the system size in kilowatts (kW), the number of panels, the roof area they occupy, and the expected annual generation in kWh. Instead of a bare wattage figure, it draws your array as a grid of panel tiles so you can see the physical scale of the install at a glance.
The method is the same peak-sun-hours approach used by NREL’s widely trusted PVWatts model and by mainstream retail estimators. It is documented in the U.S. Department of Energy’s Homeowner’s Guide to Going Solar and its Solar Energy Technologies Office resources. For related home-energy tools, see our energy efficiency upgrade calculator and embodied carbon calculator.
02Typical inputs and ranges
The result is only as good as the inputs. The table below shows sensible ranges. Peak sun hours is the single most location-dependent figure — it is the equivalent hours of full 1,000 W/m² sun per day, not the number of daylight hours, and the U.S. Energy Information Administration’s solar explainer is a good primer on how the resource varies.
03What changes your result
Sizing is a starting point, not a guarantee. Several real-world factors move the final numbers up or down:
- Location and sun hours. The same array produces far more in a sunny region than a cloudy one. Use a local peak-sun-hours figure rather than a national average.
- Roof orientation and tilt. South-facing (in the Northern Hemisphere) at a tilt near your latitude is optimal; east/west roofs generate less and may need more panels.
- Shading. Trees, chimneys and neighbouring buildings cut output disproportionately. Heavy shading is captured by a lower derate.
- Panel efficiency and degradation. Higher-wattage panels need fewer tiles and less roof; output also declines by roughly 0.5% a year over the panel’s life.
- Net metering and tariffs. Whether you are credited for exported surplus determines how valuable a 100% offset really is — policies differ widely by utility.
- Enter your annual electricity use in kWh (from your bills). If you only know a monthly figure, put it in the monthly box and leave annual blank — it is multiplied by 12.
- Set the desired offset percentage (100% aims to match your full yearly use).
- Enter your local peak sun hours (default 4.0), your panel wattage (default 400 W) and a derate/performance ratio (default 0.8).
- Press Calculate to see the system size, panel count drawn as a roof grid, roof area and expected annual generation.
- Adjust the offset, sun hours or panel wattage to see how the array and roof area change.
Wiring the system? Our conduit fill calculator helps size the containment for DC and AC runs.
This is a planning-stage estimate for comparison and awareness, not a bankable system design. It assumes a single representative sun-hour and derate value for the whole year and does not account for:
- Monthly and seasonal variation in sun hours and consumption
- Roof-specific orientation, tilt, shading and available unshaded area
- Inverter clipping, string design, panel degradation over time
- Local net-metering rules, tariffs, incentives and installation costs
01The sizing formula
Solar sizing rests on one idea: convert the energy you want to generate into a system size using your local solar resource and a loss factor, then translate that size into panels and roof area.
Where:
- annual use= your yearly electricity consumption in kWh (monthly × 12).
- offset%= share of use you want solar to cover (e.g. 100%).
- peak sun hours= equivalent hours of full 1 kW/m² sun per day, region dependent.
- derate= performance ratio after all losses, typically 0.75–0.85.
- panel W= rated wattage of a single panel (e.g. 400 W).
02Worked example
A home uses 10,000 kWh/yr and wants a 100% offset. The site gets 4.0 peak sun hours, the installer proposes 400 W panels, and a 0.8 derate is assumed. Here is the calculation end to end:
So this home needs about an 8.8 kW system — 22 panels across roughly 42 m² of roof — generating around 10,300 kWh/yr, a little over its 10,000 kWh use. Because panel count rounds up, the installed size and output land just above the target, which is normal. If the roof only faced east–west or was partly shaded, you would lower the derate (say to 0.72), which would push the required size and panel count higher. Cross-check any design against NREL’s PVWatts and the loss factors described in the DOE’s solar PV technology basics.