Sizing a solar PV system starts with one number: your daily energy consumption in kilowatt-hours. Everything else — panel count, inverter size, roof space, battery bank — flows from that single figure. Get it wrong and you either overspend on capacity you do not need, or undersize and end up supplementing with grid power or a generator year-round.
The process is straightforward but has several places where shortcuts lead to expensive mistakes. This guide walks through each step: auditing your loads, accounting for solar resource at your location, selecting panel wattage, calculating system losses, choosing between grid-tied and off-grid architectures, evaluating roof space, and understanding the permit and interconnection process.
Understanding Your Energy Needs
Pull 12 months of electric bills and record the kWh consumed each month. Annual total divided by 365 gives your average daily consumption. A typical US household uses 30 kWh per day. A well-insulated rural shop might use 15 kWh. An off-grid cabin with propane appliances and LED lighting might need only 5 kWh.
Do not use just one month. Seasonal variation matters. If you heat with a heat pump, winter consumption may be double summer. If you run heavy air conditioning, the opposite. A solar system sized to your annual average will overproduce in summer and underproduce in winter. For grid-tied systems, net metering handles the imbalance. For off-grid, you need to size for the worst month or accept running a generator in winter.
If you are building new or adding loads (EV charger, shop equipment, heat pump), estimate those loads separately. An EV driven 12,000 miles per year at 3.5 miles per kWh adds about 9.4 kWh per day. A 3 HP air compressor running 4 hours adds roughly 9 kWh. Add these to your historical consumption to get the design load.
For off-grid systems, build a detailed load table: each appliance, its wattage, and hours of use per day. Multiply wattage by hours to get watt-hours. Sum everything. Add 20% for inverter losses, phantom loads, and margin. This is your design daily load in watt-hours.
Peak Sun Hours by Region
A peak sun hour (PSH) is one hour of solar irradiance at 1,000 watts per square meter. It is the standard unit for comparing solar resource across locations. Phoenix averages 6.5 PSH per day annually. Seattle averages 3.5. Miami gets 5.5. Chicago gets 4.0. These numbers come from the NREL Solar Resource Database and account for clouds, humidity, and seasonal variation.
PSH is the bridge between your energy needs and your panel count. If you need 30 kWh per day and your location gets 5.0 PSH, you need a system that produces 30 / 5.0 = 6.0 kW under standard test conditions (STC), before accounting for losses. After losses (typically 20-25%), you need 6.0 / 0.78 = 7.7 kW of nameplate panel capacity.
Use the annual average PSH for grid-tied systems with net metering, because overproduction in summer offsets underproduction in winter. For off-grid systems, use the worst-month PSH (usually December or January) to ensure the system meets your needs year-round without excessive generator runtime.
Tilt angle and azimuth affect the PSH your panels actually receive. A south-facing roof at latitude tilt captures the most annual energy. East or west facing panels lose 10-15%. A flat roof with low-tilt racking loses 5-10% compared to optimal tilt. North-facing roofs in the northern hemisphere are generally not viable for solar.
Southwest US: 5.5–6.5 PSH
Southeast US: 4.5–5.5 PSH
Midwest US: 3.5–4.5 PSH
Northeast US: 3.5–4.5 PSH
Pacific NW: 3.0–4.0 PSH
Use NREL PVWatts for site-specific data.
Solar Array Sizing Calculator
Size your solar panel array from daily kWh load, peak sun hours by region, system losses, and tilt derating. Grid-tied and off-grid modes with monthly production estimates.
Choosing Panel Wattage and Type
Residential panels currently range from 370W to 450W per panel. Commercial panels go higher. The wattage determines how many panels you need: a 7.7 kW system requires 18 panels at 430W or 21 panels at 370W. Higher wattage panels cost more per panel but fewer panels means less racking, less wiring, and less labor.
Panel efficiency matters for space-constrained roofs. A 22% efficient panel produces more watts per square foot than a 19% panel. If your roof has plenty of space, efficiency matters less than cost per watt. If space is tight, pay the premium for higher efficiency.
Monocrystalline panels dominate the residential market because they offer the best efficiency in a standard form factor. Polycrystalline panels are slightly cheaper but lower efficiency and increasingly hard to find. Thin-film panels are used in commercial ground-mount applications where space is not a constraint.
Temperature coefficient matters in hot climates. Panels lose output as temperature rises above 25°C (77°F). A panel with a temperature coefficient of -0.35%/°C loses 3.5% of rated output for every 10°C above STC. On a 40°C (104°F) roof surface, the panel might be operating at 65°C, losing about 14% of nameplate output. Higher-quality panels have better (less negative) temperature coefficients.
Panels = System kW × 1000 ÷ Panel Wattage
Example: 7.7 kW system ÷ 430W panels = 17.9 → 18 panels
Roof area needed:
~18 sq ft per panel (standard 60-cell/120 half-cell)
System Losses: The 20-25% You Will Not Produce
A solar system never produces its nameplate rating in real conditions. The losses stack up: temperature deration (5-12%), soiling from dust and pollen (2-5%), wiring losses (2-3%), inverter efficiency (3-4%), module mismatch (1-2%), shading (0-15% depending on site), and degradation over time (0.5% per year). Combined, these losses typically total 20-25% of nameplate capacity.
The standard derate factor used in the industry is 0.78 to 0.82. This means a 10 kW nameplate system produces 7.8 to 8.2 kW under real-world conditions on an average clear day. NREL PVWatts uses 0.86 as a default system derate but adds separate temperature and shading losses on top.
Shading is the biggest variable. Even partial shading on a single panel can reduce output of an entire string because panels wired in series are limited by the weakest link. Module-level power electronics (microinverters or DC optimizers) mitigate this by allowing each panel to operate independently, but they add cost. If your site has significant shading from trees or adjacent structures, microinverters or optimizers are worth the premium.
For system sizing, divide your required production by the derate factor. If you need 30 kWh per day from 5.0 PSH, you need 30 / 5.0 / 0.80 = 7.5 kW nameplate. Rounding up to the nearest panel count gives your final system size.
Grid-Tied vs Off-Grid: Architecture Decisions
Grid-tied systems are simpler and cheaper. Panels feed a string inverter or microinverters, which convert DC to AC and feed it into your panel. Excess production spins your meter backward (net metering) or earns credits. When the sun sets, you draw from the grid. No batteries needed. Cost: $2.50–$3.50 per watt installed.
Grid-tied with battery backup (hybrid) adds a battery bank and hybrid inverter. The battery provides backup during grid outages and can time-shift solar production to avoid peak-rate electricity. Cost: $4.00–$6.00 per watt including battery. Popular with homeowners who experience frequent outages or have time-of-use rates.
Off-grid systems must produce and store all energy independently. They require a larger panel array (sized for worst-month production), a substantial battery bank (sized for 2-3 days of autonomy), a charge controller, an off-grid inverter, and usually a backup generator. Cost: $5.00–$8.00 per watt. The battery bank is the most expensive and maintenance-intensive component.
For most homeowners with grid access, grid-tied is the clear winner on economics. The grid acts as a free, infinite battery via net metering. Off-grid makes sense for remote properties where the cost of running utility power exceeds the cost of a standalone system — typically when the nearest utility pole is more than 500 feet away and the utility quotes $15,000–$50,000 for a line extension.
Roof Space and Mounting Considerations
Each standard residential panel occupies about 18 square feet (roughly 3.5 feet by 5.5 feet). An 18-panel system needs about 324 square feet of unobstructed, south-facing roof. Add setbacks required by fire code (NEC 690.12 and local amendments) — typically 3 feet from the ridge and 18 inches from edges and hips — and your usable roof area shrinks.
Roof pitch affects production. A south-facing roof at 30-40 degrees tilt is ideal for most US latitudes. Steeper pitches (45+) favor winter production. Shallower pitches (15-20) favor summer. Flat roofs use ballasted or tilted racking to achieve the target angle.
Roof condition matters. Solar panels last 25-30 years. If your roof is more than 10 years old, consider re-roofing before installing solar. Removing and reinstalling panels for a re-roof costs $1,500–$3,000 and risks damage. Composition shingle roofs are the easiest and cheapest for solar mounting. Metal roofs work well with clamp-on mounts. Tile roofs require specialty mounts and are more expensive to install on.
Ground-mount systems avoid roof constraints entirely but require dedicated land area, a concrete foundation or driven-post support structure, and longer wire runs. They are common for agricultural and commercial installations and for homes with inadequate roof space or orientation.
Inverter Sizing and Selection
The inverter converts DC from the panels to AC for your home. String inverters handle an entire string of panels (typically 8-15 panels per string). Microinverters sit behind each panel and convert DC to AC individually. Power optimizers pair with a central inverter and provide panel-level optimization without the full cost of microinverters.
Inverter sizing follows the DC-to-AC ratio, typically 1.1 to 1.3. A 7.7 kW panel array might pair with a 6.0 kW inverter (ratio of 1.28). This works because panels rarely produce full nameplate output simultaneously, and clipping losses at peak production are small (1-3% of annual production). Oversizing the array relative to the inverter is standard practice and maximizes energy harvest during non-peak hours.
For string inverters, the input voltage window must match your string configuration. Add panel Voc (open-circuit voltage) values for all panels in the string and verify the total stays within the inverter maximum input voltage at the coldest expected temperature. Cold temperatures increase panel voltage — NEC 690.7 requires temperature correction using the lowest expected ambient temperature and the panel voltage temperature coefficient.
Microinverters simplify design because each panel operates independently. No string voltage calculations needed. They also enable panel-level monitoring and eliminate single-point-of-failure risk. The tradeoff is higher upfront cost and more components on the roof that could eventually need service.
Ratio = Total Panel DC Watts ÷ Inverter AC Rating
Typical range: 1.1–1.3
String voltage check (NEC 690.7):
Max String Voc = Panels in String × Panel Voc × Temperature Correction Factor
Must be ≤ Inverter Maximum Input Voltage
Inverter Sizing Calculator
Match inverter capacity to your load and battery system. Appliance load builder with surge ratings, DC current draw, efficiency curves, and battery drain estimation.
Permits and Utility Interconnection
Every grid-tied solar installation requires two approvals: a building permit from your local jurisdiction and an interconnection agreement with your utility. The building permit covers structural (roof loading), electrical (NEC 690 compliance), and fire code requirements. The interconnection agreement covers the technical requirements for feeding power back to the grid.
The permit process typically requires a site plan showing panel layout, a single-line electrical diagram, equipment spec sheets, and structural calculations or an engineer's letter confirming roof capacity. Many jurisdictions have adopted streamlined solar permit processes (SolarAPP+) that reduce approval time to days instead of weeks.
Utility interconnection involves submitting an application, receiving approval to install, passing inspection, and receiving permission to operate (PTO). Do not energize the system until you have PTO. Operating without PTO can result in disconnection, fines, and voided net metering agreements.
Timeline: permit approval (1-4 weeks), installation (1-3 days for residential), inspection (1-2 weeks), utility interconnection and PTO (2-6 weeks). Total timeline from application to operating system is typically 6-12 weeks. Plan accordingly if you are trying to capture a tax credit in a specific tax year.