Solar System Size Calculator
Sizing note: This calculator converts energy use into a first-pass array size. Roof space, shade, utility export rules, future loads, and permitting can change the best final size.
Use annual or monthly usage, target offset, panel wattage, and either a PVWatts 1 kW production value or the built-in rough fallback.
Inputs
Result
How to use this calculator
Use this tool when you have annual or monthly electricity use and want a rough solar system size before comparing designs or installer proposals.
What the result means
The result estimates DC array size and panel count for a target annual energy offset.
What the result does not settle
It does not prove the array will fit on the roof, qualify for interconnection, or produce the same value under every utility rate.
Inputs that change the answer most
- Annual electricity use in kWh
- Target offset percentage
- Annual production from 1 kW of solar at the site
- Panel wattage and system losses
- Future EV, heat pump, or battery-charging loads
Readable method
Target kWh = annual use × target offset. Required kW DC = target kWh ÷ annual kWh from 1 kW DC. Panel count = required kW × 1,000 ÷ panel watts, rounded up.
Before you act
Check a full year of utility bills, local solar-resource data, shade, roof layout, utility export rules, and installer design assumptions before buying equipment.
How this is calculated
Target annual production = annual kWh × offset. Required DC kW = target annual kWh ÷ modeled annual kWh per 1 kW DC. Panel count = ceiling(required kW × 1000 ÷ panel watts).
Turning a System Size into a Real Design
The calculated solar size estimates annual energy offset. A buildable design also needs roof or ground-mount area, shade analysis, panel orientation, inverter limits, setbacks, structural review, utility interconnection, and local permitting. Two homes with the same annual kWh use can require different array sizes if one has winter shading or a west-facing roof.
Refine the Estimate
- Use twelve months of utility bills rather than a single high or low month.
- Model future loads such as EV charging, heat pumps, induction cooking, or battery charging.
- Account for local export rules; a larger array is not always better if excess production is credited poorly.
- Compare production by season, not only annual totals, if winter loads matter.
Pair this with the payback calculator to test several array sizes. The best system is often the one that balances usable production, roof constraints, incentives, and rate design.
Assumptions and formula
Use these inputs as planning assumptions, not as a final design, tax filing, permit package, or equipment approval.
- annual electricity use
- target offset
- annual production per kW
- panel wattage
- future loads and system losses
Formula
Target kWh = annual use × target offset. Required kW DC = target kWh ÷ annual kWh per kW. Panel count = required kW × 1,000 ÷ panel watts, rounded up.
Solar System Sizing Guide
A solar system size estimate answers how much PV capacity may be needed to offset energy use. It does not automatically answer how much can fit, how much the utility will allow, or what size makes the most economic sense. A good design balances consumption, roof or land area, production profile, interconnection rules, export value, and future loads.
Start with usage data
Use twelve months of utility bills where possible. Monthly totals reveal seasonal heating, cooling, EV charging, pool pumps, irrigation, or occupancy changes that a single bill can hide. If future electrification is planned, model those loads separately instead of inflating current use with a guess.
Right-sizing considerations
- Roof area, setbacks, vents, shade, and structural capacity
- Utility export limits or net-billing rules
- Panel orientation and seasonal production
- Battery charging needs and backup goals
- Inverter clipping and DC-to-AC ratio
Frequently Asked Questions
Should I size solar to 100% of annual use?
Sometimes, but not always. If export credits are low or fixed charges are high, a smaller system focused on self-consumption may be better.
Should I include future EV or heat-pump loads?
Yes, if the purchase is likely. Model them as separate scenarios so the array is not oversized for loads that may never arrive.
Sources
Source notes
Use these as starting points when the page affects a purchase, design, tax, utility, or safety decision.