Does the 120% rule apply to microinverters as well as string inverters?

Yes. The rule governs the total backfed current at the panel, not the inverter topology. Microinverters aggregate onto one or more branch circuits and the combined output is what counts against the limit — the same ceiling applies on a standard 200A service.

Can I install a larger DC array than my inverter allows?

Generally yes, and it is common practice — DC-to-AC ratios of 1.2 to 1.3 are normal, because panels rarely produce their full rated output. The inverter clips the peaks on the sunniest hours and captures more energy across mornings, evenings and cloudy days. The constraint is that the inverter's AC output must stay within what the service panel permits.

Is a 400A service upgrade worth it just to fit more solar?

Rarely, on solar economics alone. At $3,000-8,000 with no federal credit to offset it, the extra production has to cover the upgrade before it starts paying you back — which usually takes longer than simply installing a smaller array. It makes more sense when you need the capacity anyway for an EV charger, a heat pump or a shop, and the larger array is a bonus rather than the justification.

Solar Panel Sizing for a 200 Amp Service: Full Breakdown

Your electrical panel, not your roof, is often what caps system size. Here is the 120% rule, the ~7.6 kW ceiling it creates on a 200A service, and the four ways around it.

Most homeowners size a solar array against their roof and their electricity bill, then discover late in the process that their electrical panel is the real limit. On a standard 200 amp service, the National Electrical Code's interconnection rules typically cap a backfed solar system at around 7.6 kW AC — regardless of how much roof you have.

Home battery system connected at the main service panel — Solar inverters and home batteries both backfeed the service panel, and both fall under the same interconnection limits.

The 120% rule, in plain terms

When solar backfeeds your panel, power enters from both ends of the busbar — the utility at one end, the solar breaker at the other. The code allows the busbar to be loaded slightly beyond its rating because those two sources rarely peak together, but only within a defined margin:

(Busbar rating × 1.2) − main breaker rating = maximum solar backfeed breaker

For a standard 200A panel with a 200A main breaker:

200A busbar × 1.2 = 240A allowable
240A − 200A main = 40A maximum backfeed breaker
A breaker must be sized at 125% of continuous inverter output, so 40A ÷ 1.25 = 32A of inverter output current
32A × 240V = 7,680W AC, or about 7.6 kW

That 7.6 kW is AC output at the inverter. Because DC arrays are routinely oversized relative to the inverter, the DC array behind it can reasonably be 9-10 kW — but the inverter is what the panel sees, and the inverter is what the rule constrains.

Why this catches people out

An average home needing 10,800 kWh a year wants roughly an 8-9 kW system in most climates — just above the ceiling a 200A service allows. Homes in cloudier regions, or with an EV or heat pump, land well above it. The array that matches your consumption and the array your panel will accept are frequently not the same array, and finding out at the interconnection application stage is expensive.

Four ways around the limit

Option	What it does	Typical cost	Trade-off
Derate the main breaker	Swap the 200A main for 175A or 150A, freeing headroom (240 − 175 = 65A backfeed ≈ 12.5 kW)	$200 – $600	Only viable if your load calculation supports the smaller main
Supply-side (line-side) tap	Connects solar ahead of the main breaker, outside the 120% rule entirely	$1,000 – $3,000	More complex; some utilities restrict it
Service upgrade to 400A	Raises the busbar rating and the whole ceiling	$3,000 – $8,000+	Expensive; usually needs utility coordination
Size the system to fit	Design to 7.6 kW and accept partial offset	$0	Often the right answer — and cheaper than the alternatives

That last row deserves more weight than it usually gets. Since the federal tax credit ended, spending $5,000 on a service upgrade to enable an extra 2 kW of panels has to pay for itself out of pocket, with no 30% offset. In many cases a 7.6 kW system covering 85% of usage returns more per dollar than a 10 kW system that required rewiring the service first.

The load calculation matters as much as the busbar

Derating the main breaker is only permitted if a proper load calculation shows your home's demand fits within the smaller main. A house with electric heat, an electric range, an EV charger and central air will not survive a derate to 150A. A gas-heated house with modest loads often will, comfortably.

This is exactly the kind of calculation a licensed electrician performs as part of the permit package, and it is not a good place to estimate. Getting it wrong means nuisance trips on the coldest day of the year.

Batteries count against the same limit

A home battery with backup capability is another source feeding the same busbar, and it consumes the same headroom. A plan for 7.6 kW of solar plus a battery later frequently does not fit a 200A service without one of the options above — worth designing for at the start rather than discovering when you add storage.

Before assuming a number, work out what you actually need: calculate your daily kWh usage, then turn it into a panel count, and compare that against the 7.6 kW ceiling before you talk to anyone about a service upgrade.

One caveat throughout: NEC section numbering and the specific allowances have changed between code editions, and your jurisdiction may be on an older cycle or have local amendments. Everything above describes the common case — confirm against the code edition your building department actually enforces.