Solar Panel Sizer
Size a solar system for grid-tied, off-grid, or commercial use. Get panel count, kW DC, battery bank size, and annual savings by state. Free, no signup.
Solar Panel Sizer
Enter your daily load, location, and battery autonomy. Get the minimum solar array size, panel count, charge controller size, and battery bank capacity.
Based on NREL PVWatts methodology. Default composite: ~87% system efficiency (13% total losses). Adjust shading for heavily shaded sites.
- Select a state to auto-fill peak sun hours
- Default: 400W panels, 21% efficiency
- System efficiency: 87% (NREL PVWatts methodology)
- Roof area includes 25% for panel gaps & spacing
Estimated Monthly Production
Based on typical US irradiance distribution for the selected state
Grid-Tied System
We use the NREL PVWatts DC sizing methodology. Your annual energy target is divided by the expected production per kWp to find the required system size, then divided by panel wattage for panel count.
System Size (kW DC) = Annual kWh Target ÷ (Peak Sun Hours × 365 × System Efficiency) Number of Panels = CEILING(System Size × 1000 ÷ Panel Wattage) Off-Grid System
Off-grid arrays must cover daily loads accounting for all conversion losses (inverter, battery round-trip, charge controller, DC wiring). Battery bank is sized for your chosen autonomy days.
Array Size (W) = Daily Load ÷ (Peak Sun Hours × System Efficiency × 0.95) Battery Bank (kWh) = Daily Load (Wh) × Autonomy Days ÷ (DoD × 1000) Commercial System
Same methodology as grid-tied but sized from annual consumption, typically with higher derate factors and ILR ratios. Cost estimate uses ±15% range at your specified $/W, with 30% ITC applied.
System Efficiency / Derate
The composite system efficiency multiplies five loss factors: inverter efficiency, DC wiring losses, soiling, shading, and mismatch/other. Default composite ~87% aligns with real-world residential systems. NREL PVWatts default is 86%.
Derate = Inverter% × Wiring% × Soiling% × Shading% × Mismatch%
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What This Tool Covers
The Solar Panel Sizer determines how large a solar array you need to meet a specific electrical load. Enter your daily energy consumption and location, and the tool outputs the minimum number of panels, array wattage, charge controller size, and battery capacity - covering both off-grid and grid-tied sizing scenarios.
Inputs You Provide
- • Daily load in kWh (or appliance list with wattage × hours)
- • Location or peak sun hours (PSH)
- • System voltage (12V, 24V, 48V)
- • Days of battery autonomy
- • Panel wattage
- • System efficiency (derate factor)
Outputs You Get
- • Minimum array size (W and kW)
- • Number of panels required
- • Charge controller size (amps)
- • Battery bank capacity (Ah and kWh)
- • Estimated system cost range
- • Daily energy generation vs. load balance
Features
Designed for off-grid cabins, RV systems, battery backup installs, and right-sizing residential grid-tied arrays.
Appliance List Mode
Add individual appliances with wattage and daily hours instead of entering a single kWh figure. The tool sums the load automatically.
Battery Autonomy Sizing
Set days of autonomy (1–7 days) and the tool sizes the battery bank to cover that storage window with your chosen depth of discharge.
Location-Based PSH Lookup
Select your state or city to auto-populate peak sun hours rather than looking up irradiance data manually.
How It Works
The sizing process follows the same methodology used by off-grid system designers and NABCEP-certified installers.
Define Your Daily Load
Enter total daily kWh or list appliances individually. Include all loads - lighting, HVAC, water pump, electronics - and their daily run hours.
Set Location & PSH
Select your location or manually enter peak sun hours. PSH values range from about 3.5 hours/day in the Pacific Northwest to 6.5 hours/day in the Southwest desert.
Choose System Voltage & Panel Wattage
Select 12V, 24V, or 48V bus voltage and your target panel wattage. Higher voltage systems use less copper and support larger arrays efficiently.
Set Battery Autonomy Days
Enter how many cloudy days you want the system to operate without solar input. Off-grid homes typically use 2–3 days; critical backup systems may use 5–7 days.
Review Sizing Results
The tool returns array size, panel count, charge controller amperage, battery bank size in amp-hours and kWh, and a rough cost estimate for components.
Use Cases
Off-Grid Cabin & Tiny Home
Size a complete off-grid system with battery storage for a cabin, tiny home, or remote structure where grid connection is impractical or uneconomical.
RV & Marine Solar
Calculate panel count and battery capacity for RV rooftop or boat solar installs, where space constraints and 12V systems require careful sizing.
Grid-Tied Load Matching
Verify that a proposed residential grid-tied array fully offsets annual consumption - or quantify how much of the load will still be purchased from the utility.
Calculation Methodology
Standard off-grid sizing formulas derived from NABCEP and NREL guidelines.
Required Array Size
Array (W) = Daily Load (Wh) ÷ (PSH × System Efficiency)
System efficiency typically 0.75–0.85 for battery systems, accounting for inverter, wiring, and battery charging losses.
Number of Panels
Panels = ceiling(Array Watts ÷ Panel Watts)
Ceiling function ensures you always round up to the next whole panel - never under-size the array.
Battery Bank Capacity
Battery (Ah) = (Daily Load × Autonomy Days) ÷ (System Voltage × DoD)
DoD (depth of discharge) is typically 0.5 for lead-acid and 0.8 for lithium iron phosphate (LFP) batteries.
Charge Controller Size
Controller (A) = (Array Watts ÷ System Voltage) × 1.25
NEC 690.8 requires a 125% safety factor on the calculated short-circuit current for charge controller sizing.
Pro Tips
Use Winter PSH for Off-Grid Sizing
Off-grid systems must work year-round. Size to your worst-month PSH (usually December or January), not the annual average. Annual averages will leave you short in winter.
Choose 48V for Systems Above 1.5 kW
Higher bus voltage means lower current, which means thinner (cheaper) wire and smaller charge controllers. 48V is the standard for any off-grid system that exceeds a basic cabin load.
Add 20% Load Buffer to Appliance Lists
Appliance wattage ratings are often minimums. Compressors, pumps, and motors draw surge current 2–3x their rated wattage at startup. Add 20% to your total load before sizing.
LFP Batteries Change the Battery Bank Math
Lithium iron phosphate batteries support 80–90% depth of discharge vs. 50% for AGM lead-acid. If your customer is using LFP, the battery bank size can be nearly halved for the same autonomy.
Frequently Asked Questions
What is a peak sun hour?
A peak sun hour (PSH) is one hour of sunlight at 1,000 W/m² irradiance - the standard test condition for solar panels. A location with 5 PSH/day receives the energy equivalent of 5 hours of full-intensity sun, even though actual daylight is longer. PSH values come from NREL solar resource data.
What system efficiency should I use?
For grid-tied systems without batteries, use 0.80–0.85. For off-grid battery systems, use 0.75–0.80 to account for charging and inverter losses. If you're using high-efficiency LFP batteries with a quality inverter, 0.82 is a reasonable default.
How many days of autonomy should I design for?
Most off-grid residential systems use 2–3 days. Vacation cabins that sit empty in winter can get away with 1–2 days. Critical infrastructure or year-round remote homes in cloudy climates may need 5–7 days. More autonomy means more battery cost - there's a cost trade-off at every step.
What's the difference between this tool and the System Size Calculator?
The System Size Calculator starts from annual kWh consumption and sizes a grid-tied array to offset it. This Panel Sizer goes deeper - it handles off-grid scenarios, battery sizing, charge controller specification, and appliance-level load building, making it more appropriate for battery backup and off-grid design.
Can I use this for a grid-tied system with no batteries?
Yes. Set battery autonomy days to 0 and the tool focuses solely on array sizing. The battery section outputs will show zero - which is correct for a pure grid-tied system. The panel count and array wattage results are still valid.
Why does system voltage matter for sizing?
System voltage affects battery bank configuration, wire sizing, and charge controller ratings. A 48V system producing the same power as a 12V system runs at one-quarter of the current, which dramatically reduces wire losses and allows the use of smaller-gauge copper.
Related Tools
System Size Calculator
Size a grid-tied array from annual energy consumption.
Battery Size Calculator
Size a battery bank for backup or off-grid storage requirements.
String Sizing Calculator
Size PV strings for inverter MPPT voltage windows.
Panel Layout Estimator
Estimate how many panels physically fit on a roof section.
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