Why the array should be bigger than the inverter

Explain to a customer that you are putting 6 kW of panels on a 5 kW inverter and you will get one of two reactions. Either they think you are cheating them out of an inverter, or they think you have made an error.

It is neither. It is usually the better design, and it is worth being able to explain in one minute.

What the ratio means

The DC to AC ratio, or inverter loading ratio, is the module array's rated capacity divided by the inverter's AC rating. Six kilowatts of modules on a five kilowatt inverter is a ratio of 1.2.

The reason it works is that modules almost never produce their rated output. That rating is measured at 1000 watts per square metre of irradiance and a cell temperature of 25 degrees, a combination that an Indian rooftop sees rarely and briefly.

Real output is reduced by:

• Cell temperature. Modules lose output as they heat, and a rooftop module in May in most of India runs far above 25 degrees.
• Irradiance. Mornings, evenings, haze, monsoon cloud. Most operating hours are well below the rated condition.
• Soiling, which on Indian rooftops is continuous rather than occasional.
• Cable, mismatch and inverter losses.
• Degradation, which reduces output slightly every year for twenty five years.

So an array rated at 6 kW spends its life delivering considerably less than 6 kW. An inverter sized to the nameplate spends its life underused.

What you gain, and what you lose

The gain is at both ends of the day and across the weaker months. A larger array reaches useful output earlier in the morning, holds it later in the evening and produces more on overcast days. Those are exactly the hours a nameplate matched system gives away.

The loss is called clipping. On the clearest, coolest, highest irradiance days around midday, the array may briefly try to deliver more than the inverter can pass, and the excess is simply not produced.

The asymmetry is the whole point. You gain a little on most hours of most days, and lose a little on a few hours of a few days. In Indian conditions the trade is usually favourable up to around 1.2, and often further.

Where it stops paying

Push far enough and the arithmetic turns. Things to check before you go past a modest ratio:

• The inverter's own limits. Every inverter has a maximum DC input power and a maximum input current per MPPT. Exceeding either is not a design choice, it is a warranty problem.
• The customer's tariff and export rules. Where exported units are worth less than imported ones, extra midday generation is worth less than it used to be, which weakens the case for a large array aimed at the middle of the day. That direction of travel is covered in net metering is being rewritten.
• Sanctioned capacity. The approval you get may be written against a specific capacity, and the inspection checks against it. See sanctioned load is the ceiling nobody checks.
• Subsidy rules, which for residential schemes are written against a capacity and a price, and which are not the place to improvise.

The other reason it comes up

Module wattages do not divide neatly into system sizes. A 5 kW design with 540 watt modules is either nine modules at 4.86 kW or ten at 5.4 kW. Neither is 5 kW.

For PM Surya Ghar applications this used to cause pointless rejections when the declared DC capacity did not match the inverter rating exactly. MNRE has since allowed a tolerance of up to 10 percent on the applied DC capacity, with the inverter kept within sanctioned limits, precisely so that a real module count can be approved.

That change matters more to your application success rate than to your design, but it removes an argument you used to have to have.

What to tell the customer

Short version, in their language: the panels are rated for perfect laboratory conditions that a roof never sees, so we deliberately fit slightly more panel than inverter. It gives you more units in the early morning, the evening and the monsoon, and costs you a few minutes of trimmed output on the clearest days of the year. The inverter is sized correctly for what actually arrives.

Then show it rather than saying it, because a generation curve with and without the extra modules ends the conversation faster than any explanation.

The short version

• DC to AC ratio is array capacity divided by inverter rating. Around 1.1 to 1.2 is normal and usually optimal in Indian conditions.
• Modules rarely produce their rated output because of temperature, irradiance, soiling, losses and degradation, so a nameplate matched inverter is underused.
• Oversizing adds generation in mornings, evenings and weak weather, and costs a little clipping on the best days.
• Limits: the inverter's DC input power and current per MPPT, export economics, sanctioned capacity and subsidy rules.
• MNRE now allows up to 10 percent tolerance on declared DC capacity under PM Surya Ghar, which fixes the module count problem.
• Fix a house standard, and show the customer the curve rather than explaining the theory.