If you own or manage affordable multifamily housing in California, you’ve probably heard of SOMAH — the Solar on Multifamily Affordable Housing program. It’s a well-intentioned state program that subsidizes solar installations on affordable apartment buildings, with the goal of putting real energy savings in the hands of low-income residents.
The promise is simple: put solar on the roof, lower everyone’s bills.
The execution? Surprisingly complicated.
The Problem Nobody Talks About: Who Gets the Solar Credit?
Here’s the challenge that most SOMAH program guides gloss over.
When a solar array is installed on a multifamily building, the power it generates doesn’t go to one meter — it flows into a shared system serving both tenant units and common areas (hallways, elevators, parking lot lighting, laundry rooms, etc.).
California’s SOMAH program has a clear rule about how to split those benefits:
- 51% of solar generation must go to tenants — and they must receive it at no cost. This is non-negotiable. It’s the social equity core of the entire program.
- 49% can go to common areas, which benefits the property owner.
This sounds straightforward. But the moment you try to turn it into an actual billing system, questions multiply fast:
- How do you prove the split is 51/49?
- How do you bill the property owner for their 49% without accidentally touching the tenants’ 51%?
- How do you make sure tenant VNEM credits aren’t being recaptured somewhere in the accounting?
- And how do you do all of this in a way that survives a SOMAH audit?
The Wrong Way: Estimating from the Inverter
The intuitive approach is to pull generation data straight from the solar inverter — a SolarEdge or Enphase device that sits on the roof and reports how many kilowatt-hours the panels produced.
The problem: inverter data is an estimate. It’s not the number the utility uses. It doesn’t match the bill. And when a SOMAH auditor pulls up the utility’s records and compares them to your billing reports, they won’t reconcile — because you were working from a different data source.
We’ve seen this create real compliance headaches. Not fraud, just misalignment — and in a program designed to protect low-income residents, misalignment is enough to trigger scrutiny.
The Right Way: Start Where the Utility Starts
The breakthrough insight is simple: use the same data the utility uses.
Every SOMAH solar installation in California is connected to a Net Generation Output Meter (NGOM) — a revenue-grade utility meter that measures exactly how much power the solar array produces. This is the meter SDG&E, PG&E, and SCE use to calculate VNEM (Virtual Net Energy Metering) credits.
And critically: the utility already handles the 51/49 split for you.
When SDG&E reads the NGOM, it automatically allocates:
- 49% of solar generation → common area meter (as VNEM credits)
- 51% of solar generation → tenant meters (as VNEM credits, at no cost to residents)
The 51% never touches a billing intermediary. It flows directly from the utility to tenant bills. Done.
The Energy311 Billing Architecture
With the NGOM as the data foundation, here’s how the billing logic works for the common area (owner) side:
Step 1 — Capture Common Area Usage at 15-Minute Intervals Energy311 pulls the common area grid usage plus the 49% NGOM solar generation at 15-minute intervals. Combined, this gives us the total common area consumption — what the building would have used if the solar panels weren’t there.
Step 2 — Build the Calculated Bill (Without Solar Credits) That usage data is run through the applicable utility rate schedule — for SDG&E, that’s the AL-TOU rate, which is the standard tariff for common area meters on 200+ unit multifamily buildings. The result is a Calculated Bill: a precise, to-the-penny figure representing what the property owner would have paid with no solar.
This isn’t an estimate. It uses the same rate tables the utility uses, applied to revenue-grade interval data. It’s exact.
Step 3 — Pull the Actual Utility Bill (With Solar Credits) Energy311 also pulls the real SDG&E/PG&E/SCE bill for the common area meter. This bill already reflects the VNEM credits the utility applied — the 49% solar allocation showing up as a credit.
Step 4 — Calculate the Solar Value
Solar Value = Calculated Bill − Actual Utility BillThe difference between what the owner would have paid (no solar) and what they actually paid (with solar credits) is the dollar value the solar system delivered that month.
Step 5 — Invoice at a Guaranteed Discount
Solar Provider Invoice = Solar Value − negotiated discount %The property owner pays the solar provider for the energy delivered, but is mathematically guaranteed to pay less than they would have paid the utility — every single month.
Why This is SOMAH-Compliant by Design
This architecture doesn’t just happen to be compliant — it’s compliant by construction.
Compliance RequirementHow This Architecture Satisfies ItVerified 51/49 splitNGOM data is the utility’s own meter — the same source used for VNEM statementsTenants receive benefit at no costThe 51% flows directly from utility to tenant bills — Energy311 never touches itAudit transparencyEnergy311 reports match utility statements perfectly — same data sourceNo estimationNGOM is a revenue-grade meter — no inverter approximationsWorks across all utilitiesSDG&E, PG&E, and SCE all use NGOM — only the rate schedule changes
The SOMAH audit question becomes easy: “Show us your data source.” The answer is: “The same meter the utility uses.”
The Bigger Picture
SOMAH is a genuinely important program. California has set an ambitious goal of bringing solar to low-income multifamily residents who have historically been left out of the clean energy transition — renters in affordable housing who can’t put panels on a roof they don’t own.
But good policy intent doesn’t automatically produce good billing infrastructure. The gap between “51% must go to tenants at no cost” and “here is a billing system that proves that in real-time, every month, in a format that survives an audit” is where projects fail or stall.
Getting the data architecture right — starting from the utility’s own meter, honoring the utility’s own allocation, and building the billing logic on top of that foundation — is what makes the whole thing work.
is a co-founder at Energy311, a software platform that automates solar billing and savings verification for multifamily properties across California.