The calculator answers one question: how many watts of panels are needed to produce a given amount of energy per day. It gives a planning estimate. A real design also checks the roof, the shading, the inverter and the local rules.
Formula
Panels = array (W) / panel rating (W), rounded up
Peak sun hours (PSH) are the number of hours per day at an irradiance of 1,000 W/m² that would deliver the same energy as the real day. A site with 5 kWh/m² of daily sunlight has 5 peak sun hours. Values of 3 to 6 are typical, depending on latitude, climate and season. Look up your site in a tool such as NREL’s PVWatts or the Global Solar Atlas and use the figure for the plane of your array, not for flat ground.
Performance ratio (PR) bundles every loss between the panel’s rated output and the energy you can use: heat, dust, wiring, mismatch, inverter conversion. About 0.75 is a common planning value for a well-built system, and a poor one can be lower.
Worked example
A household uses 30 kWh per day and has 5 peak sun hours. With a PR of 0.75:
Array = 30,000 / (5 × 0.75) = 8,000 W.
With 400 W panels that is 8,000 / 400 = 20 panels. The calculator rounds up, so a result of 7.8 panels becomes 8, and it reports the expected daily output of the rounded-up array.
Getting the inputs right
- Daily energy. Take a year of electricity bills, divide the total kWh by 365, and decide whether you want to cover all of it or only part. Off-grid and winter-limited systems need the worst month, not the average.
- Peak sun hours. An annual average hides the winter shortfall. If your goal is year-round cover, size from the weakest month, or accept that the grid or a generator fills the gap.
- Panel rating. The rating is measured under standard test conditions of 1,000 W/m² and 25 °C cell temperature. Real cells run hotter, and silicon loses roughly 0.3 to 0.4 % of its output per °C above 25 °C, which is part of the PR.
What this does not cover
Roof area is a practical limit: a 400 W panel is typically close to 2 m². Shading from chimneys, trees and neighbouring buildings can cut output far more than the PR assumes, especially when one shaded panel limits a string. Panels also lose a fraction of a percent of their output each year. Grid-connected systems also face export rules and inverter size limits that depend on the utility and the country.
Common mistakes
- Multiplying panel watts by 24 hours. Panels produce at full rating only around midday in clear conditions.
- Forgetting the PR. Using the nameplate overstates output by a quarter or more.
- Using a sunny-month figure for a winter-critical load.
Questions
How many panels do I need for a 2,000 kWh monthly bill?
That is about 66.7 kWh per day. At 5 peak sun hours and a PR of 0.75, the array is 17.8 kW, or 45 panels of 400 W.
How many kWh does a 5 kW array produce per day?
At 5 peak sun hours and a PR of 0.75, about 18.8 kWh.
Does the calculator size the inverter?
No. See the inverter size calculator, and for a battery system the off-grid calculator.