How to Size a Solar System for Your Home

Sizing a solar system correctly is the difference between a system that pays for itself in 6 years and one that never breaks even. The goal is to match your system's annual production to your annual electricity consumption — or at least offset the most expensive tiers of your utility bill.

Start with your electric bill. Look for your total kilowatt-hour (kWh) usage over the last 12 months. Most utilities show this on a bar chart on your bill. If you only have a monthly bill, multiply by 12 for a rough annual total. In the United States, the average home uses about 10,500 kWh per year, but this varies dramatically by home size, climate, and appliance efficiency.

Next, consider your location's solar resource. A kilowatt of solar panels in Arizona will produce roughly 1,600 kWh per year. The same kilowatt in Seattle might only produce 1,000 kWh. This is why our calculator uses state-specific sun hours — it accounts for the actual solar irradiance your panels will receive.

Roof orientation matters. South-facing roofs in the Northern Hemisphere capture the most sun over the course of a day. East- and west-facing roofs are still viable but typically produce 10-15% less annually. Flat roofs require tilt racks to avoid severe production losses. If your roof has shading from trees, chimneys, or neighboring buildings, that must be factored into your sizing estimate.

Factors Affecting Solar Production

Solar panel output is not just about the wattage printed on the spec sheet. Real-world production is affected by a long list of variables:

• Temperature: Panels actually lose efficiency as they heat up. A hot summer day can reduce output by 10-20% compared to the panel's Standard Test Conditions (STC) rating.
• Inverter losses: Even the best inverters convert DC to AC with 97-98% efficiency. String inverters with mismatched panels or partial shading can suffer additional losses.
• Soiling: Dust, pollen, bird droppings, and snow can reduce output. In most climates, rainfall is sufficient to keep panels clean, but dry climates may need occasional washing.
• Wiring losses: Voltage drop across long wire runs reduces the energy that reaches your inverter.
• Degradation: Quality panels degrade at about 0.5% per year. Over 25 years, a 400W panel may produce closer to 360W.
• Tilt angle: The optimal tilt is roughly equal to your latitude. Steeper tilts favor winter production; flatter tilts favor summer.

Our calculator applies a 0.77 derating factor to account for these real-world losses. This is the industry-standard "performance ratio" used by NREL and most professional PV modeling software.

Understanding Your Electric Bill

Your electric bill is the single most important input for solar sizing. Most US utilities charge tiered or time-of-use rates, meaning the more you use, the more you pay per kWh. Solar is most valuable when it offsets your highest-tier usage.

Look for your "average cost per kWh" on your bill. This is your total bill divided by total kWh used. It is not the same as the "baseline rate" your utility advertises. In California, for example, baseline rates might be $0.20/kWh, but heavy users often pay $0.40-0.50/kWh on upper tiers. A solar system that eliminates those top-tier charges delivers outsized savings.

Net metering is the policy that lets you sell excess solar energy back to the grid. Under full net metering, every kWh you export is credited at the full retail rate. Under net billing or avoided-cost rates, you may only receive a wholesale credit of $0.03-0.05/kWh. Always check your local utility's current net metering rules before sizing your system. In many jurisdictions, it is advantageous to size your system to 100% of your usage while net metering is still available.

The 30% Federal Solar Tax Credit (ITC)

The Investment Tax Credit (ITC) is a dollar-for-dollar reduction in federal income tax equal to 30% of your total solar system cost. For a $25,000 system, the ITC is worth $7,500. This credit can be carried forward to future tax years if you cannot use it all in year one.

To qualify, the system must be installed at a US residence or business that you own. You must have sufficient tax liability to absorb the credit. The IRS requires you to retain receipts and manufacturer documentation. The credit applies to equipment and installation costs, including racking, wiring, inverters, and energy storage devices installed in the same tax year as the solar array.

The 30% rate was extended through 2032 by the Inflation Reduction Act. In 2033, it steps down to 26%, and in 2034 to 22%. There is no better time to go solar from a tax perspective than right now.

Why Buy Solar Equipment from Portlandia Electric Supply?

Portlandia Electric Supply is a wholesale distributor of solar and electrical equipment serving homeowners, contractors, and installers nationwide. We operate at the same tier as the distributors that supply your local installer — which means you pay what they pay, not the marked-up retail price.

• Wholesale pricing: We cut out the installer middleman and pass the savings directly to you. Our panel, inverter, and battery prices are typically 30-40% below national installer quotes.
• Fast shipping: We stock inventory in multiple US warehouses. Most orders ship within 24-48 hours and arrive in 3-5 business days.
• Expert support: Our technical team includes NABCEP-certified professionals who can help with system design, interconnection paperwork, and troubleshooting.
• Tier-1 warranties: Every panel and inverter we sell carries the full manufacturer warranty. We are an authorized distributor, so warranty claims go directly to the factory with our support.
• Complete kits: From 5kW starter kits to 10kW whole-home systems and off-grid cabin packages, we have pre-engineered kits with everything you need.

Getting Started

Use the calculator above to get a rough system size and cost estimate. Then open a Pro Account for bulk pricing, or explore financing options for projects over $10,000. If you need help designing your specific array, contact our team with your roof dimensions, photos, and last 12 months of electric bills.