Solar System Sizing Explained

One of the most common questions homeowners ask after receiving a solar proposal is:

“Why is my system this size?”

It’s a fair question — and an important one. A properly sized solar system isn’t random, and it’s not about covering as much roof as possible.

System sizing is the result of balancing energy usage, roof constraints, safety requirements, and long-term performance.

Here’s how that sizing actually works.

1) Your Energy Use Sets the Starting Point

Every solar design begins with how much electricity your home uses, measured in kilowatt-hours (kWh).

Installers typically review the last 12 months of utility bills to understand:

• total annual electricity usage

• seasonal swings (summer cooling vs. winter heating)

• whether usage is trending up or down
  
  

Solar systems are sized to match realistic, historical usage, not guesses about future behavior. This ensures the system performs consistently throughout the year, not just during peak months.

2) Panel Count × Panel Wattage = System Size (kW)

Solar system size is measured in kilowatts (kW), which represents the system’s maximum power capacity.

Here’s the simple math:

Panel wattage × number of panels = system size (in watts)

Watts ÷ 1,000 = system size (in kilowatts)

Example:

• 20 panels × 400 watts = 8,000 watts

• 8,000 ÷ 1,000 = 8.0 kW system
  
  

This number tells you how large the system is — not how much energy it produces over time.

kW vs. kWh (Important Distinction)

• kW (kilowatts) = system size / capacity

• kWh (kilowatt-hours) = energy produced or consumed over time
  
  

An easy way to think about it:

• kW is the size of the engine

• kWh is how far you drive

  
  

A larger kW system can produce more kWh — but actual production depends on sunlight, shading, and layout.

3) Sunlight and Roof Direction Shape Production

Not all roofs produce solar energy equally.

Designers evaluate:

• roof orientation (south, east, west)

• roof pitch and tilt

• seasonal sun exposure

• shading from trees, chimneys, or nearby buildings
  
  

A smaller system on a well-positioned roof can sometimes outperform a larger system placed on shaded or poorly oriented sections. That’s why good designs prioritize performance, not just panel count.

4) Roof Setbacks: The Constraint Most Homeowners Don’t See

One of the biggest — and least understood — factors in system sizing is roof setbacks.

Setbacks are required clearances where panels cannot be installed. They exist to meet:

• building codes

• fire safety access requirements

• inspection standards
  
  

Common setback considerations include:

• distance from roof edges

• walkable pathways for firefighters

• clearance around ridges, hips, and valleys

• spacing around vents, skylights, chimneys, and dormers
  
  

Once setbacks and obstructions are applied, the usable solar area is often smaller than homeowners expect. Two homes with similar roof sizes can end up with very different system capacities because of these rules.

5) Roof Layout and Shape Matter as Much as Size

Beyond square footage, roof geometry plays a major role:

• multiple roof planes may split panel placement

• dormers and complex shapes reduce usable space

• some areas may be skipped entirely due to shading or access rules
  
  

Designers often focus on the most productive roof sections, even if that means leaving other areas unused.

6) Bigger Isn’t Always Better

A larger system doesn’t automatically mean better value.

If a system is oversized:

• you may pay for energy you can’t fully use

• panels may sit in less productive areas

• overall efficiency and cost per kWh can suffer
  
  

Well-designed systems aim for balanced production, not maximum panel count.

7) Why Systems Aren’t Always Sized to Offset 100%

Not every solar system is designed to cover 100% of a home’s electricity usage — and that’s often intentional.

Common reasons include:

• limited usable roof space

• shading on certain roof planes

• diminishing returns on lower-performing areas

• homeowner plans (future renovations, EVs, or additions)
  
  

In many cases, a slightly smaller, more efficient system delivers better long-term value than a larger system stretched across weaker roof areas.

8) Why Two Proposals for the Same Home Can Look Different

It’s normal to see different system sizes from different installers.

That usually comes down to:

• different shading assumptions

• different setback interpretations

• different panel wattages

• different design goals (maximum offset vs. efficiency)
  
  

This is why transparent modeling and clear explanations matter when comparing proposals.

The Bottom Line

Your solar system size is determined by:

• how much energy your home actually uses

• how much sunlight your roof reliably receives

• how much usable roof space remains after safety setbacks

• the efficiency of the equipment being used
  
  

A good solar design doesn’t maximize panels — it maximizes performance and long-term value.

If you’re reviewing a solar proposal and want to understand why it was sized the way it was — or whether a different approach could improve efficiency or pricing — VirginiaSolar.org can help break it down in plain language before you decide what to do next.

Want a Second Set of Eyes on Your System Size?

If you’re reviewing a solar proposal and want to understand why it was sized the way it was — or whether a different approach could improve efficiency or pricing — VirginiaSolar.org can help walk through the assumptions behind the design before you move forward.