You already have rooftop solar. Read this first

Why the arithmetic changes

For everyone else, the case for plug-in solar rests on avoiding a 33-cent purchase. You make power, you use it, you don't buy it.

If you already have rooftop solar, your daytime consumption is probably already covered. A plug-in panel's output arrives at a moment when your home isn't buying anything anyway. So it doesn't avoid a purchase — it adds to an export.

And exports are compensated poorly. California's net billing tariff, adopted by the CPUC in December 2022, pays new solar customers avoided-cost rates for exported energy — typically in the range of 5 to 8 cents rather than the retail 33.

So the same panel that saves a renter 33 cents a kilowatt-hour earns you perhaps 6.

Estimated. Full working.

The interconnection wrinkle

There's a second issue, and it's the one to take seriously.

You have an interconnection agreement with your utility covering a system of a specific size. Adding generation can, depending on how it's characterised, put you outside that agreement.

SB 868 exempts compliant plug-in devices from interconnection requirements, which suggests the plug-in device shouldn't disturb your existing arrangement. But the interaction between the exemption and an existing net-billing agreement isn't something we've seen addressed anywhere — there is no published CPUC or utility guidance on it as of 1 September 2026 — and getting it wrong could affect the compensation on your main system, which is worth far more than a $600 panel.

What we'd do: ask your installer, and ask your utility, before buying. Get the answer in writing.

When it might still make sense

Your roof system is undersized and you're still importing at midday. Look at a sunny weekday on your utility's hourly usage data. If you're importing between 11am and 3pm, a plug-in panel is displacing real purchases and the ordinary arithmetic applies.

You want it somewhere the roof doesn't reach — a detached workshop or garage on its own circuit that you'd rather not run a new feed to.

You're on legacy net metering with retail-rate credits. If you're on an older agreement where exports are credited at or near the retail rate, exporting isn't the loss it is under net billing. Check which agreement you're on before assuming.

The better version of this idea

If your goal is more solar and you already have a roof full of it, the productive question is usually storage rather than another panel. A battery lets your existing system's exports — currently earning 6 cents — get used in the evening instead, when they'd otherwise cost you 33 or more.

That's a bigger purchase and a different article, but it's where the money is for you.

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