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Checking My Calculator Against NLR's System Advisor Model

by David St. Martin · October 28, 2026

A reviewer's critique of this calculator made a fair point: I had tested it mostly against my own calculations. So Claude and I ran it against an outside one, the System Advisor Model (SAM) from the National Laboratory of the Rockies, the former NREL. Researchers and many in the industry use it. It's free.

What I compared

The same 50 homes on both sides: real Marin County homes, simulated over a full year by NLR's ResStock. The same production, from NLR's PVWatts, for an array sized to each home's yearly use. The same plan, PG&E's E-ELEC, hour by hour. SAM then did its own billing, with its own code, and I compared first-year bill savings. I ran three export rules: a flat 6¢, PG&E's published export prices by the hour, and net metering.

The calculator ran each home two ways: on the home's own full year, as if loaded from a Green Button file, and on the five typical homes it uses when it has nothing else, with each home given the usage pattern a visitor would pick for it.

What I found

For each of 50 homes, how far the calculator's first-year savings landed from SAM's: within 0.7% with the home's own data, between 12% low and 18% high with five homes like it-20%-10%0%+10%+20%Your own meter data-0.2%-0.3%-0.2%-0.2%-0.2%-0.4%-0.2%-0.4%-0.6%-0.3%-0.2%-0.2%-0.2%-0.3%-0.2%-0.2%-0.3%-0.3%-0.4%-0.6%-0.3%-0.5%-0.6%-0.2%-0.4%-0.3%-0.4%-0.2%-0.5%-0.4%-0.5%-0.2%-0.1%-0.2%-0.2%-0.4%-0.2%-0.3%-0.2%-0.2%-0.4%-0.2%-0.3%-0.2%-0.6%-0.1%-0.3%-0.2%-0.3%-0.2%Five homes like yours-2.2%-8.4%+17.6%+1.1%+7.4%-10.1%+17.2%-11.8%+12.6%+1.8%+4.9%-6.2%-4.8%-3.5%-1.5%-1.9%+3.2%+11.1%+1.7%-2.9%+7.5%+17.9%-0.0%-0.7%+11.4%+0.4%-0.5%-6.7%+0.2%-2.0%+6.7%+7.9%-2.0%+11.0%+5.5%+3.1%-0.4%-2.1%+3.5%-2.1%-0.1%+0.3%+1.3%-2.0%-4.2%-12.4%+1.4%-8.9%+7.4%-8.4%
For each home, how far the calculator's first-year savings landed from SAM's. PG&E's export prices, south-facing array.

With a home's own data, the calculator and SAM agree within 0.74% for every one of the 50 homes, under all three export rules. With the five typical homes, they agree on average, within 0.65% at the median, but miss individual homes: four in five land within 10%, and the worst is 18% high.

Export ruleSAM, median savingsOwn data, furthest from SAMFive homes, middle 80% of homes
Flat 6¢$1,138 a year0.47%7% low to 10% high
PG&E's export prices$1,0470.63%8% low to 11% high
Net metering$2,3410.04%0.04%

Net metering agrees for every home either way, for a simple reason: when every exported kWh earns retail, it doesn't matter when the house uses power. A west-facing array smaller than the home's use gave a similar picture as the table.

Batteries

SAM models a battery in far more detail than I do, down to its cells. I set it to charge only from the panels and cover the house, which is what the calculator does too. For a 13.5 kWh battery, SAM puts the median home's battery at $857 a year; the calculator says $831 with the home's own data. Both batteries give back almost exactly the same energy, 2,865 and 2,851 kWh a year. The calculator's figure is about 3% lower because I take another 8% off for a battery that isn't always there when you'd want it, on purpose.

What this means for you

The calculator's arithmetic holds up against NLR's. What separates a good answer from a rough one is your data. Typical homes get the average right and can still be 10% or more off for your home, in either direction. If your utility offers Green Button, a year of your own meter data closes that gap.

What this check doesn't cover

Both sides used the same production from PVWatts, so this tests the energy and billing arithmetic, not the production model. The homes are all in one county, on one utility's plan. And it covers the first year's savings; the money side after that, growing the savings and comparing them with investing the cash, is simpler arithmetic that the calculator's own tests check.

The full method is on the How it works page.