Solar Panel Calculator
Size a PV system, estimate energy output, or find panel count from your usage, sun-hours, and performance ratio — with steps and units.
Calculation Steps
Solar Panel Calculator: How to Size Your Array, Panel Count, and Output
Use the calculator above to translate your energy needs into a right-sized solar array. This guide explains the equations, what each input means, and how to avoid the most common pitfalls—complete with worked examples you can mirror.
Quick Start: From Usage to Panel Count
- 1 Gather usage. From your utility bill, take monthly kWh and divide by 30 to estimate daily \(E_{\text{day}}\) (Wh/day = kWh × 1000).
- 2 Enter Peak Sun Hours (PSH) for your location and tilt (hours/day). PSH represents equivalent full-sun hours.
- 3 Set Performance Ratio (PR). This lumps real-world losses (temperature, wiring, inverter, soiling). Typical residential is ~0.75–0.85.
- 4 Pick a panel wattage (e.g., 400–500 W). The calculator will size the DC array and compute the panel count.
- 5 Adjust for your goals: desired offset (% of bill), space limits, or seasonal needs. Re-run if shading or PSH changes.
Core Equations Used
Required DC array power: \[ P_{\mathrm{req,DC}}=\frac{E_{\mathrm{day}}}{\mathrm{PSH}\cdot \eta_{\mathrm{sys}}} \] Panel count: \[ N=\left\lceil \frac{P_{\mathrm{req,DC}}}{P_{\mathrm{panel}}} \right\rceil \]
Choosing Your Approach
Size by Utility Usage (Most Common)
This approach targets a specific offset of your electricity consumption.
- Pros: Simple, aligns with bills, good for grid-tied homes.
- Cons: Can under- or over-estimate if future usage changes (EV, heat pump, pool).
\( P_{\mathrm{req,DC}}=\dfrac{E_{\mathrm{day}}}{\mathrm{PSH}\cdot \eta_{\mathrm{sys}}} \Rightarrow N=\left\lceil \dfrac{P_{\mathrm{req,DC}}}{P_{\mathrm{panel}}}\right\rceil \)
Size by Load List (Appliances/Devices)
Build a device list with watts × hours/day for each load—useful for off-grid or cabins.
- Pros: Precise for known loads, great for off-grid and critical circuits.
- Cons: More work; easy to miss occasional or seasonal loads.
\( E_{\mathrm{day}}=\sum (P_i \times t_i)\quad\Rightarrow\quad P_{\mathrm{req,DC}}=\dfrac{E_{\mathrm{day}}}{\mathrm{PSH}\cdot \eta_{\mathrm{sys}}} \)
What Moves the Number (Key Drivers)
Variables & Symbols
- \(E_{\mathrm{day}}\) — Average daily energy use (Wh/day).
- \(\mathrm{PSH}\) — Peak sun hours (h/day).
- \(\eta_{\mathrm{sys}}\) — Overall system efficiency (PR, 0–1).
- \(P_{\mathrm{panel}}\) — Module nameplate power (W).
- \(P_{\mathrm{req,DC}}\) — Required DC array power (W).
- \(N\) — Number of panels (rounded up).
Unit Conventions
The calculator accepts energy as Wh or kWh, power as W or kW, and time in hours. Be consistent: \(1~\text{kWh}=1000~\text{Wh}\). For metric/imperial design, conversions do not change the electrical math—just ensure all inputs are in compatible units.
Worked Examples
Example A — U.S. Grid-Tied Home
A home uses 900 kWh/month. Site has \( \mathrm{PSH}=5.5~\text{h/day} \). Assume \( \eta_{\mathrm{sys}}=0.78 \) and \( P_{\mathrm{panel}}=420~\text{W} \).
- Daily energy: \(E_{\mathrm{day}}=\frac{900\times 1000}{30}=30{,}000~\text{Wh/day}\).
- Array power: \[ P_{\mathrm{req,DC}}=\frac{30{,}000}{5.5\times 0.78}\approx 6{,}993~\text{W}\approx 7.0~\text{kWdc}. \]
- Panel count: \[ N=\left\lceil \frac{6{,}993}{420}\right\rceil= \left\lceil 16.65 \right\rceil = 17~\text{panels}. \]
Rounding: Panel counts round up to whole modules; array power is typically quoted to one decimal in kW.
Example B — Small Off-Grid Cabin
Load list totals \(E_{\mathrm{day}}=4{,}800~\text{Wh/day}\). PSH \(=4.0\). Off-grid PR is lower; take \( \eta_{\mathrm{sys}}=0.70 \). Use \(P_{\mathrm{panel}}=200~\text{W}\) 12-V modules.
- Array power: \[ P_{\mathrm{req,DC}}=\frac{4{,}800}{4.0\times 0.70}=1{,}714~\text{W}. \]
- Panel count: \[ N=\left\lceil \frac{1{,}714}{200}\right\rceil=\left\lceil 8.57 \right\rceil = 9~\text{panels}. \]
- Optional battery energy for 2 days autonomy at 80% DoD: \[ C_{\mathrm{batt}}(\text{Wh}) =\frac{4{,}800\times 2}{0.8}=12{,}000~\text{Wh}. \] At \(48~\text{V}\): \( C_{\mathrm{batt}}(\text{Ah})=\frac{12{,}000}{48}\approx 250~\text{Ah}. \)
Note: Batteries are optional for grid-tie but essential off-grid. Inverter/charger design changes PR.
System Variations & Their Sizing Impact
| Variation | When It Fits | Impact on Sizing | Notes |
|---|---|---|---|
| Grid-tied, no battery | Bill offset on typical homes | Use annual usage; PR ~0.75–0.85 | Check service-panel limits and interconnection rules. |
| Grid-tied + battery | Backup, TOU shifting | Array often slightly larger to recharge storage | Size battery in kWh to cover critical loads duration. |
| Off-grid | Remote cabins/RVs | Use worst-month PSH; lower PR (0.6–0.8) | Include generator/alt energy for low-sun periods. |
| Microinverters/optimizers | Partial shade, complex roofs | Improves real-world yield; PR can be higher | Higher hardware cost; easier module-level monitoring. |
| Ground mount | Ample land, limited roof | Similar array size; tilt can be optimized | Extra racking, trenching, and fencing considerations. |
| High-efficiency panels | Small roof area | Fewer modules for same kW | Cost per W may be higher; check warranty and degradation. |
Buying, Logistics & Practicalities
Selection Criteria
- Module: Power rating, efficiency, dimensions, warranty (product + performance).
- Inverter: String vs micro/optimizer, efficiency, monitoring features, warranty.
- Racking: Roof material compatibility, wind/snow ratings, corrosion resistance.
- Balance of System: Conductors, combiners, disconnects, surge protection.
Installation & Operations
- Layout to minimize shade; respect roof setbacks and fire pathways.
- Route homeruns cleanly; size conductors for voltage drop and code.
- Plan maintenance: safe access, periodic cleaning where soiling is high.
- Document string maps, serials, and commissioning test results.
Sanity Checks & Standards
- Verify main service-panel capacity and interconnection limits.
- Coordinate with local permits/inspection; follow applicable electrical codes.
- If adding storage, confirm transfer switch/backup loads panel design.
Assumptions & Limitations
- Do: Use realistic PSH and PR for your site; round panel counts up.
- Don’t: Ignore seasonal variation, shading, or roof structural limits.
Frequently Asked Questions
How many solar panels do I need for a typical home?
Divide daily Wh by \(\mathrm{PSH}\cdot \eta_{\mathrm{sys}}\) to get array watts, then divide by panel wattage and round up. Many homes land between ~12–24 modern panels depending on usage, PSH, and losses.
What are Peak Sun Hours (PSH)?
PSH is the equivalent number of hours per day when solar irradiance averages 1,000 W/m². Higher PSH means more energy from the same array size.
Should I size to monthly or annual energy?
For grid-tie, annual energy is a good baseline. If winter performance is critical or off-grid, size using worst-month loads and PSH.
What’s a good Performance Ratio (PR) to use?
Residential PR commonly ranges ~0.75–0.85. Hot climates, long wire runs, or older components may justify using a lower PR.
How do I account for shade?
Model the worst shade window and consider module-level electronics (optimizers or microinverters). Adjust PR downward or increase array size if shading is frequent.
How much roof area will I need?
Multiply panel count by module area (length × width) and add spacing for walkways and setbacks. High-efficiency modules reduce area for the same kW.
Does the calculator handle batteries?
Primary sizing is for the PV array. For storage, estimate energy needed during outages and use \(C_{\mathrm{batt}}=\dfrac{E_{\mathrm{day}}\times \text{Days}_{\mathrm{aut}}}{\mathrm{DoD}}\) as a rough starting point.