Introduction
Solar sizing generates the same questions over and over — about panel counts, costs, roof requirements, and what the numbers on a quote actually mean. This FAQ answers them directly, with the math where it helps and pointers to deeper guides where it doesn’t.
For quick estimates while you read, the Solar Panel Calculator turns your monthly kWh and sun hours into system size, panel count, roof area, and rough cost. The complete guide walks through the full sizing process with a worked example.
Sizing Questions
How many solar panels do I need?
It depends on your usage and sun hours. The formula: monthly kWh ÷ 30 ÷ peak sun hours ÷ 0.8 = system kW; then (kW × 1,000) ÷ panel wattage, rounded up. A 900 kWh/month home at 5 peak sun hours needs about 7.5 kW → 19 panels at 400 W each. See the formula guide for the full derivation.
What is a peak sun hour?
One hour of sunlight at 1,000 W/m² intensity — a standardized measure of solar resource. Most US locations average 3.5–6.5 peak sun hours per day annually. It’s the highest-leverage input in sizing: fewer sun hours means proportionally more panels.
What size system does the average home need?
The average US home uses ~900 kWh/month, which typically means a 6–10 kW system — roughly 15–25 residential panels — depending on local sun hours.
Should I size for 100% of my usage?
Often, but not always. Under strong net metering, 100% offset maximizes savings. Under weak export credits, sizing to daytime self-consumption can pay back faster. Check your utility’s current rules before deciding — the cost guide explains the trade-off.
How much roof space do I need?
Each residential panel covers about 1.8 m² (≈19.4 ft²). A 19-panel system needs ≈34 m² (≈366 ft²) of panel surface, plus 15–25% more for setbacks, walkways, and racking. Measure your usable roof — not gross roof — as shown in the estimating guide.
Cost and Savings Questions
How much does a residential solar system cost?
The typical US planning range is $2.50–$3.50 per installed watt before incentives — so a 7.5 kW system runs roughly $18,750–$26,250 gross. Regional labor rates, roof complexity, and equipment choices move quotes within (and sometimes outside) that band.
What incentives are available?
Federal, state, and utility incentives can reduce net cost substantially, but they change frequently and vary by location and tax situation. Check current federal tax credit rules, state rebates, and your utility’s programs before budgeting — and confirm eligibility with a tax professional.
How long is the payback period?
Divide net system cost by annual electricity savings. A 7.5 kW system generating ~9,000 kWh/year at $0.17/kWh offsets roughly $1,530/year — about a 15-year simple payback at gross cost, commonly 7–12 years after incentives in favorable regions. High-rate areas pay back faster.
Do solar panels increase home value?
Studies of US home sales have generally found owned (not leased) solar systems associated with modest resale premiums, varying by market. Leased systems can complicate sales since the buyer must assume the lease. Treat resale value as a bonus, not the core of the investment case.
Equipment Questions
What’s the difference between monocrystalline and polycrystalline panels?
Monocrystalline panels (19–22% efficiency) are the current residential standard — more power per square meter at a moderate price premium. Polycrystalline (15–17%) is being phased out of most markets. Thin-film (10–13%) suits large ground arrays where space is cheap. The material guide compares all three plus inverters and racking.
String inverter or microinverters?
String inverters are cheaper upfront with one central unit; microinverters cost more but give panel-level monitoring and better performance under partial shading. For complex or partially shaded roofs, microinverters (or power optimizers) are usually worth it.
How long do solar panels last?
Panels carry 25-year performance warranties (typically guaranteeing ~80–90% of rated output at year 25) and often keep producing well beyond that. Inverters last 10–25 years depending on type and may need one replacement during the system’s life — budget for it.
Do I need batteries?
Not for a standard grid-tied system — the grid acts as your “battery” under net metering. Batteries add backup power during outages and can improve economics under time-of-use rates, but they add significant cost and are a separate sizing exercise.
Roof and Installation Questions
Which way should panels face?
In the Northern Hemisphere, south-facing at a tilt near your latitude maximizes annual output. East/west roofs still work but typically need 10–20% more capacity. Flat roofs use tilted racking.
My roof is old. Can I still install solar?
If the roof has less than 10–15 years of life left, replace it first — removing and reinstalling an array for a re-roof costs thousands. Coordinate both projects if replacement is due soon.
Do I need a permit?
Yes — virtually every jurisdiction requires electrical and building permits for rooftop solar, plus utility interconnection approval and inspections. Your installer normally handles this. The code compliance guide outlines the approval chain.
Can I install solar panels myself?
Physically possible, but the electrical work, structural attachments, permitting, and utility interconnection carry real safety and code risks — and DIY systems may not qualify for some incentives. The DIY vs pro comparison breaks down the trade-offs honestly.
What maintenance do panels need?
Very little: occasional cleaning in dusty areas, keeping vegetation from shading the array, and monitoring output for faults. Most systems need no scheduled servicing beyond inverter replacement once in their lifetime.
Related Guides
- Complete Guide — the full solar sizing overview with worked example.
- Cost Guide — per-watt pricing, savings, and payback math.
- Formula Explained — the math behind the estimates, step by step.
- Estimating Guide — reading your bill and measuring your roof.
- DIY vs Pro — honest trade-offs of self-installation.
- Code Compliance — permits, NEC Article 690, and inspections.
Get Your Number
Questions answered, math in hand — now make it specific to your home. Enter your monthly kWh and sun hours into the Solar Panel Calculator and you’ll have system size, panel count, roof area, and a cost baseline to carry into installer conversations.
Disclaimer: Calculator results are planning estimates based on the inputs you enter. Actual system size, panel count, output, and cost depend on roof conditions, shading, orientation, equipment choices, and local rates. Verify sizing with a licensed solar installer or electrician — and check requirements with your local code authority — before signing contracts or starting work.
