# Solar panel calculator

> How many solar panels you need: system size in kW, panel count, yearly kWh, installed cost and payback from your use and peak sun hours.

Interactive version: https://www.calcopenly.com/construction/solar-panel-calculator
Subject: Construction and home calculators

System size is the daily energy to cover divided by the peak sun hours and by one minus the system losses. Dividing that by the panel rating and rounding up gives the panel count, and yearly generation is the rounded size × sun hours × (1 − losses) × 365. Cost is the size in watts times the installed price per watt, less incentives, and simple payback is that cost divided by the yearly saving.

With the defaults, 30 kWh a day at 5 peak sun hours and 14% losses needs 6.98 kW, which rounds up to 18 panels of 400 W (7.2 kW). They make about 11,300 kWh a year. At 2.80 a watt the system costs 20,160 and saves 1,808 a year at 0.16 per kWh, a payback of 11.2 years.

The 14% default is the PVWatts figure for soiling, shading, wiring and similar losses. PVWatts models the inverter and cell heating separately, so including a 96%-efficient inverter raises the loss to about 17.4%.

## Inputs

- **Electricity use**: Monthly kWh from your bill ÷ 30.
- **Share of use to cover**
- **Peak sun hours**: Yearly average kWh/m²/day for your location, e.g. from PVWatts or the Global Solar Atlas.
- **System losses**: Soiling, shading, wiring, mismatch and downtime: PVWatts uses 14%. Its inverter (96%) and heat losses are extra; 17.4% includes the inverter.
- **Panel rating**
- **Installed cost per watt**
- **Incentives and rebates**
- **Electricity price per kWh**

## Results

- System size (kW) — main result
- Panels (panels)
- Size before rounding to panels (kW)
- Generation per year (kWh)
- System cost after incentives
- Savings per year
- Simple payback (years)

## Formula

$$
P = \frac{E_{\text{day}} \times \text{offset}}{H_{\text{sun}} (1 - \text{losses})},\quad N = \left\lceil \frac{P}{P_{\text{panel}}} \right\rceil,\quad \text{payback} = \frac{\text{cost}}{E_{\text{year}} \times \text{tariff}}
$$

## Worked examples

### 30 kWh a day, 5 sun hours, 400 W panels

- Electricity use: 30 kWh/day
- Share of use to cover: 100%
- Peak sun hours: 5 h/day
- System losses: 14%
- Panel rating: 400 W
- Installed cost per watt: 2.8
- Incentives and rebates: 0%
- Electricity price per kWh: 0.16
- **Size before rounding to panels: 6.98 kW**
- **Panels: 18 panels**
- **System size: 7.2 kW**
- **Generation per year: 11,300 kWh**
- **System cost after incentives: 20,160.00**
- **Savings per year: 1,808.06**
- **Simple payback: 11.2 years**
- Checked against: Python decimal: 30 ÷ (5 × 0.86) = 6.977 kW → 17.44 → 18 panels = 7.2 kW; 7.2 × 5 × 0.86 × 365 = 11,300.4 kWh; 20,160 ÷ 1,808.06

### Edge: need is an exact number of panels

- Electricity use: 17.2 kWh/day
- Share of use to cover: 100%
- Peak sun hours: 5 h/day
- System losses: 14%
- Panel rating: 400 W
- Installed cost per watt: 2.8
- Incentives and rebates: 0%
- Electricity price per kWh: 0.16
- **Size before rounding to panels: 4 kW**
- **Panels: 10 panels**
- **Generation per year: 6,278 kWh**
- **Simple payback: 11.2 years**
- Checked against: Hand calculation: 17.2 ÷ 4.3 = 4.0 kW = 10 × 400 W exactly; payback is independent of size at a fixed cost per watt

### 10 kWh a day, 5.5 sun hours, 20% losses, 540 W panels at 50 per watt

- Electricity use: 10 kWh/day
- Share of use to cover: 100%
- Peak sun hours: 5.5 h/day
- System losses: 20%
- Panel rating: 540 W
- Installed cost per watt: 50
- Incentives and rebates: 0%
- Electricity price per kWh: 8
- **Panels: 5 panels**
- **System size: 2.7 kW**
- **Generation per year: 4,336 kWh**
- **System cost after incentives: 135,000.00**
- **Simple payback: 3.9 years**
- Checked against: Python decimal: 10 ÷ 4.4 = 2.27 kW → 4.21 → 5 panels = 2.7 kW; × 4.4 × 365 = 4,336.2 kWh; 135,000 ÷ 34,689.6

### Cover half the use with a 30% rebate

- Electricity use: 30 kWh/day
- Share of use to cover: 50%
- Peak sun hours: 4.5 h/day
- System losses: 14%
- Panel rating: 400 W
- Installed cost per watt: 3
- Incentives and rebates: 30%
- Electricity price per kWh: 0.2
- **Panels: 10 panels**
- **System size: 4 kW**
- **System cost after incentives: 8,400.00**
- **Simple payback: 7.4 years**
- Checked against: Python decimal: 15 ÷ (4.5 × 0.86) = 3.88 kW → 10 panels; 4,000 W × 3 × 0.7 = 8,400; 5,650.2 kWh × 0.2 = 1,130.04 a year

## Questions

### How many solar panels do I need?

Divide your daily use by the peak sun hours and by one minus the losses to get the system size, then divide by the panel rating and round up. 30 kWh a day with 5 sun hours and 14% losses needs 30 ÷ (5 × 0.86) = 6.98 kW, which is 18 panels of 400 W (7.2 kW). Monthly kWh from your bill divided by 30 gives the daily figure.

### What are peak sun hours?

The day's solar energy on the panels written as hours of full sun at 1,000 W/m²: 5 kWh/m² a day is 5 peak sun hours. Panel ratings refer to 1,000 W/m² and a 25 °C cell, so a 1 kW array makes about 1 kWh per peak sun hour before losses. PVWatts, now run by the US National Laboratory of the Rockies (formerly NREL), gives yearly averages for any location.

### What system losses should I use for solar panels?

PVWatts uses 14% by default: soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1% and availability 3%, combined by multiplying rather than adding. It models the inverter (96% efficient by default) and cell heating separately, so 1 − 0.86 × 0.96 ≈ 17.4% here covers the inverter as well.

### How long do solar panels take to pay for themselves?

Divide the installed cost after incentives by the yearly saving. The default 7.2 kW system at 2.80 a watt costs 20,160 and saves 1,808 a year at 0.16 per kWh, so it pays back in 11.2 years. The PVWatts manual puts long-term panel degradation at about 0.5% a year, and power exported for less than the retail price also stretches the payback.

### Is there still a federal tax credit for home solar?

Not for new systems. The IRS says the 30% Residential Clean Energy Credit covered property installed from 2022 through 31 December 2025 and is not available for property placed in service after that date. State, utility and local rebates may still apply; enter them as a percentage in the incentives box to update the cost and payback.

### How accurate is the solar panel calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “30 kWh a day, 5 sun hours, 400 W panels” is checked against Python decimal: 30 ÷ (5 × 0.86) = 6.977 kW → 17.44 → 18 panels = 7.2 kW; 7.2 × 5 × 0.86 × 365 = 11,300.4 kWh; 20,160 ÷ 1,808.06.

### Where does the method come from?

Dobos, PVWatts Version 5 Manual (NREL, 2014) — system losses default 14%, inverter efficiency 96%; PVWatts calculator, National Laboratory of the Rockies (location-specific sun hours); IRS — Residential Clean Energy Credit.

## Sources

- [Dobos, PVWatts Version 5 Manual (NREL, 2014) — system losses default 14%, inverter efficiency 96%](https://pvwatts.nlr.gov/downloads/pvwattsv5.pdf)
- [PVWatts calculator, National Laboratory of the Rockies (location-specific sun hours)](https://pvwatts.nlr.gov/pvwatts.php)
- [IRS — Residential Clean Energy Credit](https://www.irs.gov/credits-deductions/residential-clean-energy-credit)

_Note: financial information, not professional advice._