Solar Basics #1: What Wattage Really Means, and What to Check Before You Buy Panels
Every solar panel has a number on its label: 540 W, 550 W, 600 W. Most buyers treat it as the whole story. It isn’t, but it is the right place to start, because every other calculation in a solar project is built on it.
This is the first article in a Solar Basics series where we break down the terms purchase managers meet on every datasheet, starting with wattage.
Part 1: What wattage means
Wattage (W) is the power a panel produces at one moment, under test conditions. Think of it as the speed of a car, not the distance it travels.
Those test conditions are called Standard Test Conditions (STC): sunlight of 1,000 watts per square metre, a cell temperature of 25°C, and a fixed light spectrum. A rooftop in Gujarat in May is almost never at those conditions, so a panel rarely produces exactly its rated wattage. The rating is best used to compare panels with each other, not to promise what you will see on the meter.
Watt versus kilowatt-hour. Watts measure power at an instant. Kilowatt-hours (kWh), the “units” on your electricity bill, measure energy over time. A 1,000 W appliance running for one hour uses 1 kWh. A panel’s wattage tells you how fast it can work; the sunshine hours tell you how long it works at that speed.
How much does one panel generate in a day?
You need three numbers:
- Panel wattage, converted to kilowatts (550 W = 0.55 kW).
- Peak sun hours, the number of hours per day with the equivalent of full 1,000 W/m² sunlight. In most of India, 4 to 5.5 is a reasonable range; check the figure for your location.
- Performance ratio, which covers real-world losses from heat, dust, wiring and the inverter. A typical figure is 75 to 85%.
Daily energy (kWh) = Panel kW × Peak sun hours × Performance ratio
Worked example: a 550 W panel, 5 peak sun hours, 80% performance ratio.
0.55 × 5 × 0.80 = 2.2 kWh per day
| Peak sun hours | Daily output | Monthly (30 days) | Yearly |
|---|---|---|---|
| 4.0 | 1.76 kWh | 53 kWh | 642 kWh |
| 5.0 | 2.20 kWh | 66 kWh | 803 kWh |
| 5.5 | 2.42 kWh | 73 kWh | 883 kWh |
(All at 550 W and 80% performance ratio.)
Using it to size a project: if a facility needs 100 kWh a day, divide by the output of one panel: 100 ÷ 2.2 = 45.5, so about 46 panels of 550 W, or roughly 25 kW of capacity. Round up and leave a margin.
A simple rule of thumb: 1 kW of installed solar produces about 4 to 5 units a day in good locations.
Part 2: What a purchase manager should check before buying
Wattage is only the starting point. Before you issue a purchase order, check the following.
1. Price per watt is not the real price. Compare the cost per unit of energy over the plant’s life, not just the quote. A panel with better temperature behaviour and lower degradation can win over 25 years even if it costs a little more today.
2. Wattage, tolerance and the datasheet. Ask for the full datasheet, not a brochure. Check the power tolerance (a positive tolerance such as 0 to +5 W is better than a negative one), module efficiency, and the temperature coefficient of power. In hot regions a lower (less negative) coefficient means more energy in summer.
3. Cell technology and size. Know whether you are being offered PERC, TOPCon or another cell type, and the module dimensions. Larger modules can change your mounting, structure and handling costs.
4. Certifications and test reports. Ask for certificates for the exact model, such as IEC 61215 (design qualification) and IEC 61730 (safety), plus any Indian standards that apply to your project. Request recent third-party reliability test reports.
5. Rules for your project type. Government schemes, net-metering and subsidised projects can require listed manufacturers or domestic content. These rules have been changing, so confirm the current requirements for your project before you order.
6. Warranty, read in full. There are two: a product warranty (defects, usually 10 to 12 years or more) and a performance warranty (guaranteed output, typically 25 to 30 years). Check the first-year degradation, the yearly degradation, and how a claim is actually made and who pays shipping.
7. Manufacturer strength. A warranty is only as good as the company behind it. Look at financial health, years in business, production capacity and references from similar-sized buyers.
8. Quality control on your order. Ask for flash-test reports (the measured output of each panel) for your batch, a pre-shipment inspection, and a policy on breakage and replacement in transit.
9. Supply and payment terms. Confirm lead time, delivery schedule, price validity, and payment terms such as advance versus letter of credit, and what happens if the supplier delays.
Our Take
Buyers who compare panels on price per watt alone often end up comparing the wrong thing. A better question is how much energy each panel will deliver per rupee over its life, and whether the supplier will still be there to honour the warranty. Start with wattage, but don’t stop there.
In the next article in this series we’ll look at the temperature coefficient: why a panel’s real output in summer depends on a number most buyers skip.