A common question asked of Enphase installers is, “Why are you installing a 260 watt solar module on a 215 watt microinverter?” But, there is no "common answer" to this question.
Though the practice of “over-sizing” solar pannels relative to the inverter has existed for decades, it remains one of the trickiest topics to explain. And, even among solar professionals, conversations about over-sizing become more like religious debates than scientific discussions (trust me, I’ve been there).
The reason for all the confusion: there is no right answer.
No matter what sizing ratio you recommend, there are costs and benefits. And, the “right” choice depends on where you see the most value.
For example, over sizing solar inverter to equal the module’s nameplate rating ensures there will almost never be any energy lost to inverter saturation (aka “Solar clipping”). At the same time, using a large inverter increases the overall cost of the system.
Conversely, selecting a smaller inverter lowers the system cost, but it also limits the output of the modules at select times, when conditions are optimal (mostly in the early spring).
In both cases, we are balancing cost with performance.
Enphase recently weighed-in on this balancing act with a new study: Bigger is Better: Sizing Solar Modules for Microinverters.
In our study, we examined the cost/benefit trade-offs and came to the conclusion that there are two reasons to recommend over-sizing: one based on the fundamentals of solar product ratings and the other based on the economics of today’s solar industry.
Fundamentals: The Tale of Two Tests
The first reason to over-size is simply that you are anticipating how the solar module will actually perform in the field.
The power ratings of modules are based on a very brief indoor “flash test” with ideal sunlight and temperature conditions. Yet, in the field, module output actually declines as the module heats up from the sun.
As a result, the module’s peak output in the field is typically 10-15 percent lower than its rating. Further, these peaks continue to decline over time, as the module degrades and collects dust.
Conversely, the nameplate rating of an inverter (aka the “max output power”) is based on a series of tests performed at high temperatures and across a range of electrical conditions to reflect the worst-case scenarios the inverter will face. And throughout all of these test, the inverter must consistently meet or exceed it's nameplate rating.
In this sense, the inverter's nameplate rating is a minimum hurdle which the inverter must be able to consistently and continuously clear when operating in the field.
For this reason, most inverter products are capable of producing 4-5% more power than their rating. In addition, inverter performance does not degrade over time, unlike module performance.
Thus, the nameplate ratings of modules and inverters represent completely different things, even though they are both stated in Watts.
Sizing your inverter to match the module would mean you are paying for inverter capacity that rarely, if ever, gets used.
Which brings up the second factor: the economics of today’s industry.
Economics: “AC Systems” in 2013 (and beyond…)
One of the unique--and often overlooked--advantages of using microinverters is the impact they have on the electrical design of the solar system.
With microinverters, all wiring and electrical equipment is sized based on the output rating of the microinverter (i.e the AC capacity), rather than the module.
In contrast, most of the electrical design in a central inverter system is based on the size of the array (i.e. the DC capacity). Consequently, increasing the array size also require increasing conductor sizes, fuse counts and more.
When you break the link between DC capacity and electrical design, this means that module size has no influences on the conductors, circuit breakers and other downstream electrical equipment.
This also means that over-sizing with microinverters directly reduces the electrical balance of system cost on a $/W basis.
With this in mind, we evaluated the cost-effectiveness of different over-sizing ratios, and found a clear trend: bigger is better.
This finding is especially significant within today’s industry environment, where solar module prices are declining more rapidly than any other component cost.
As the decline continues, the economically optimal sizing ratio actually increases, due to its tendency to improve returns.
In other words, the goal of a good system design should be to regularly saturate the inverter, and not just in year one, but also in years 10 to 15, once degradation and soiling have taken effect on the module.
If the economics are encouraging us toward more over-sizing, clearly we need to consider reinventing the term itself. “Over-sizing” and “inverter clipping” imply that you’ve made a mistake in designing the system. This terminology is an outdated vestige of an industry focused solely on module cost and module performance.
It’s time we empower solar professionals with terms to support smart choices, based on sound economics. Welcome to the era of “right-sizing” and “inverter saturation.”