Shell rose from #7 to #2 in during the oil shock with this method
3 minutes.
That's how long it took Peta-Gaye Hardy to build a three-scenario sensitivity model (bull, bear and base) inside Excel during a live masterclass last week.
This is the same exercise that took Shell's planning team months in 1971, but later helped them anticipate the 1973 oil shock when every competitor was left struggling.
Shell went from #7 in the global oil industry to #2 because Pierre Wack made them rehearse three outcomes instead of just forecasting for one.
Most people don’t do this. They just build one forecast and then change one number.
Then on Friday at 5PM someone asks a question about a different scenario.
You know the answer is somewhere in the data. But this means three hours of rebuilding the model, and hoping a formula does not error.
I've done this, and what’s even more painful is, the CEO does not even want a model! She wants three numbers and one sentence telling her what to do about them.
Peta-Gaye shared a single ChatGPT-in-Excel prompt that does exactly that.
And today I’m going to show you.
You are much more dangerous that you realize
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Mike, an interim CFO, told me last month that he built a full financial model in 15 minutes using AI, one that used to take him hours. Here’s what I said back to him:
The experience is the moat. AI just gives you back the hours you used to spend formatting it.
Multiply this by every "what if" that question people email you each week. And by Friday you're wondering why you never get to do real analysis.
The problem is you're building one model and changing it manually every time someone asks, when you should be asking AI to build three scenarios at once.
But, the good news is, ChatGPT now exists in Excel and Google Sheets.
Which makes not only the model easier to build.
But the answer you give is much faster too.
Providing you are building for the right answers…
Wack's 3-scenario method
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In 1971 Pierre Wack made Shell's executives stop predicting. And he made them rehearse instead.
His core principle was simple:
A single forecast is a single bet. But, three scenarios force you to decide on (and plan for) what to do in each situation.
This is the difference between numbers on a spreadsheet that don’t mean anything. And a set of actions you can guide the business through.
The reason it worked at Shell in 1973 is that the executives had already mentally lived the worst case.
So when things changed for the worse, they didn’t panic. They just executed the plan they'd already rehearsed.
This is what I do when I give masterclasses. I work with the team to plan for what happens when the internet is bad, or when there’re tech issues.
This is what your CEO wants on Friday at 5PM. A plan instead of just a forecast.
And until recently, building one took finance teams half a day. Now it takes one prompt.
The 3-minute scenario Prompt
1. Get ChatGPT for Excel
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You can get the ChatGPT for Excel in the Microsoft Marketplace here: ChatGPT for Excel. Make sure you are logged in on your Microsoft Account.
After installation, there is a separate log in for the ChatGPT.
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You will have to log in your ChatGPT credentials to use the AI chat inside Excel. Click "Continue with ChatGPT" and it will redirect you to the login page.
2. Open it and Select a Model
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Use Standard or Heavy for bigger tasks. For a mini table Fast is fine. But for modelling you need a model that’s more intelligent. Use Fast only when you need quick responses for small tasks or edits.
3. Point it at the data
Direct ChatGPT to the tab with the data you want to work with.
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Peta’s demo used a 6-month P&L tab built earlier in the session.
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4. Use this prompt as a template
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Apply these assumptions in a clearly labelled Assumptions block:
[list yours].
Add a clustered column chart comparing net contribution across scenarios.
Below the chart, write three sentences:
1. In the bull case we should do X,
2. In the bear case we should do Y,
3. Either way we should do Z.
Use formulas only, no hard-coded values.
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5. Ask for the formulas, not the values.
The "use formulas only" line is important. Without it, ChatGPT will hard-code numbers and you will end up with problems with your audit trail.
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6. Save the prompt so you never write it again.
If your first attempt doesn't quite work, iterate until it does. Then ask ChatGPT to write the prompt that would have got you there the first time.
I call this the system-prompt trick.
The One Thing to Remember
In 1971, Pierre Wack had to convince Shell's board to spend weeks building three scenarios.
In 2026, you can build them in three minutes.
The method hasn't changed in 55 years. What's changed is who gets to use it.
Scenario planning used to be a tool for Fortune 500 CFOs with consulting budgets. Today it's a prompt.
But there’s a big difference between an analyst and a 20-year CFO.
Anyone can write a prompt. But only you know what to put in it to make the biggest impact.
The prompt is free. The judgement is what they pay you for. So use it 😉
Best,
Your AI Finance Expert,
– Nicolas
P.S. – Have you tried using AI for Financial Modelling yet? Reply and let me know. I read all replies.
P.P.S. – For a video on how to model using AI, watch my video here – How to Use ChatGPT 5 to Build INSANE Financial Models
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