# 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.

交互版本：https://www.calcopenly.com/zh/construction/solar-panel-calculator
主题：建筑与家居计算器

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%.

## 输入

- **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**

## 结果

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

## 公式

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

## 计算示例

### 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**
- 核验来源：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**
- 核验来源：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**
- 核验来源：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**
- 核验来源：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

## 常见问题

### 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.

### “Solar panel calculator”有多准确？

准确性取决于输入值和方法的假设。十进制运算使用50位有效数字，但估算、数值方法和源数据的精度可能较低；显示时的舍入并不能消除这些限制。 已按独立来源核验的计算示例：4。 例如，“30 kWh a day, 5 sun hours, 400 W panels”根据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进行核验。

### 这种方法出自哪里？

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.

## 来源

- [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)

_仅用于规划。贷款机构、税务机关和市场采用各自的舍入方式、费用和规则；作出承诺前，请向相关机构确认数值。_