Số chữ số thập phân tối đa: 2; Gần nhất; nếu cách đều thì ra xa số không
Panels
18panels
Size before rounding to panels
6.98kW
Generation per year
11,300kWh
System cost after incentives
$20,160.00
Savings per year
$1,808.06
Simple payback
11.2years
Simple payback ignores panel degradation (about 0.5% a year in the PVWatts manual), price changes and financing, and assumes every kWh offsets a kWh bought at the full price.
Covering 30 kWh a day takes 6.98 kW, which rounds up to 18 panels of 400 W (7.2 kW). They make about 11,300 kWh a year, worth $1,808.06 at $0.16 per kWh, so the $20,160.00 system pays for itself in about 11.2 years.
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%.
Nguồn đối chiếu: 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
Câu hỏi
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” chính xác đến mức nào?
Độ chính xác phụ thuộc vào dữ liệu nhập và giả định của phương pháp. Phép tính thập phân dùng 50 chữ số có nghĩa, nhưng ước lượng, phương pháp số và dữ liệu nguồn có thể kém chính xác hơn; làm tròn khi hiển thị không loại bỏ các giới hạn đó. Ví dụ có lời giải đã đối chiếu với nguồn độc lập: 4. Ví dụ, “30 kWh a day, 5 sun hours, 400 W panels” được kiểm tra bằng 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.
Phương pháp này lấy từ đâu?
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.
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