Solar PV planning starts with daily energy consumption in kilowatt-hours, but panel count, inverter selection, roof space, battery bank, and interconnection all depend on source data beyond that one number.
This guide walks through early planning steps: auditing loads, finding site-specific solar resource, selecting module wattage, documenting loss assumptions, choosing between grid-tied and off-grid architectures, evaluating roof space, and keeping permit, utility, product, and AHJ source gaps visible.
Understanding Your Energy Needs
Pull 12 months of electric bills and record the kWh consumed each month. Annual total divided by 365 gives an average daily-consumption prompt. Actual project design should use the current load profile, planned load changes, tariff, and utility data.
Do not use just one month. Seasonal variation matters. Heating, cooling, shop equipment, EV charging, well pumps, and occupancy can all shift the design case. Grid-tied projects may be evaluated on annual production and tariff rules; off-grid projects need worst-month and critical-load review.
If you are building new or adding loads such as an EV charger, shop equipment, or heat pump, estimate those loads separately and document the source. Keep uncertain loads as source gaps until they are measured or specified.
For off-grid systems, build a detailed load table: each appliance, its wattage, hours of use per day, surge behavior, criticality, and seasonal use. Battery autonomy and generator backup should be handled in a separate storage/reliability design step.
Peak Sun Hours by Region
A peak sun hour (PSH) expresses daily solar energy as equivalent hours at 1,000 watts per square meter. It is a useful planning unit, but it should come from current site-specific data such as PVWatts, NSRDB, installer modeling, utility data, or measured solar-resource data.
PSH connects energy needs and a panel-count prompt. If a load is 30 kWh/day and a source-backed prompt is 5.0 PSH, the simple starting point is 30 / 5.0 = 6.0 kW before losses. After an entered 0.80 multiplier, the prompt becomes 30 / 5.0 / 0.80 = 7.5 kW nameplate before rounding to whole modules.
Annual average PSH can support early grid-tied discussion, but utility credit rules and export limits matter. Off-grid planning needs monthly or worst-month solar resource, critical loads, battery autonomy, and generator strategy.
Tilt angle, azimuth, roof planes, obstructions, snow, soiling, and shade affect the solar resource your panels actually receive. Treat simple regional rows as source-gap prompts, not as final site data.
Use current PVWatts, NSRDB, installer, utility, or measured site data before design decisions. Off-grid work needs monthly or worst-month solar resource, not just an annual average.
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: Keep the Multiplier Honest
PV production is affected by temperature, soiling, wiring, inverter efficiency, mismatch, shading, snow, module degradation, DC-to-AC ratio, clipping, and equipment-specific behavior. A single loss multiplier is useful for early screening, but it hides the details that matter in a real design.
PVWatts and other modeling tools expose dedicated inputs and assumptions that this simple planning screen does not reproduce. If you use a local multiplier such as 0.80, document where it came from and replace it with a current project model before design use.
Shading is one of the biggest variables. Even partial shading can cause losses that a regional PSH row and one multiplier will not predict. Use a shade survey and equipment-specific modeling where shading, roof planes, or nearby obstructions matter.
For an early prompt, divide required daily production by PSH and the entered multiplier. If you need 30 kWh/day from 5.0 PSH with a 0.80 multiplier, the prompt is 30 / 5.0 / 0.80 = 7.5 kW nameplate before rounding to whole modules.
Grid-Tied vs Off-Grid: Architecture Decisions
Grid-tied systems depend on utility interconnection, meter rules, tariff treatment, inverter settings, export limits, and AHJ inspection. The grid may absorb seasonal imbalance when policy allows, but net-metering assumptions must be verified with the current utility.
Grid-tied with battery backup adds a battery bank and hybrid inverter. Battery value depends on outage goals, time-of-use rates, equipment compatibility, capacity, cycle life, safety listing, and commissioning.
Off-grid systems must produce and store all energy independently. They need worst-month production review, battery autonomy, charge-controller voltage/current limits, generator backup, load shedding, and maintenance planning. A panel-count prompt alone is not enough.
For projects with grid access, compare the current utility extension, tariff, backup-power, and ownership requirements against a standalone system. For remote projects, document line-extension cost, service reliability, fuel logistics, maintenance, and safety responsibilities before choosing an architecture.
Roof Space and Mounting Considerations
A simple module-area prompt can estimate order of magnitude roof space, but actual layout depends on selected module dimensions, roof planes, access pathways, fire setbacks, obstructions, vents, setbacks, walkways, racking, attachment spacing, and local code/AHJ interpretation.
Roof pitch and azimuth affect production. Use site modeling for actual roof planes, not a one-size regional row. Flat roofs, east/west roofs, ground mounts, and carports all need project-specific layout and structural review.
Roof condition matters because removing and reinstalling PV later can add cost and risk. Coordinate roof age, waterproofing, flashing, attachment details, warranty, wind/snow loads, and structural capacity with qualified reviewers.
Ground-mount systems avoid some roof constraints but introduce land-use, foundation, racking, trenching, grounding, conductor, overcurrent, vegetation, and permitting questions.
Inverter Planning Boundaries
The inverter converts DC from the PV or battery side to AC for the loads or grid interface. String inverters, microinverters, hybrid inverters, inverter/chargers, and mobile inverter systems have different listing, wiring, grounding, transfer, interconnection, monitoring, and manufacturer-instruction requirements.
For grid-tied PV, DC-to-AC ratio, clipping, string voltage, rapid shutdown, utility interconnection, and inverter settings depend on selected equipment, module data, adopted code, utility rules, and AHJ review. For off-grid or backup systems, running watts, surge duration, waveform compatibility, DC current, battery limits, transfer equipment, and listed inverter-battery combinations matter just as much as the wattage screen.
Use the inverter planning screen to organize local load assumptions and expose high-current or high-surge paths. Do not treat a calculator row as a product listing, IEEE 1547 interconnection result, NEC design, battery safety approval, conductor selection, transfer-switch approval, or permission to operate.
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 expedited 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.