The Big Idea
When you’re thinking about going solar, everyone gives you averages.
Average bill.
Average production.
Average demand.
Average savings.
But here’s the truth no one tells you:
Nobody lives in the “average” home. Not even close.
In fact, there’s a classic example from the 1940s: US Air Force pilots kept crashing because cockpits were built for the “average” pilot — but no single pilot matched the average across all 10 body measurements. Designing for an average can be actively harmful; the cockpit story is a memorable cautionary tale. See the original write-up
And when you live in Phoenix—under SRP or APS demand pricing—the difference between “average” and your actual reality can become a $100+ mistake every single month.
You won't crash, but your bill may take off.
This is the story of our home, our first summer after solar, and the moment I realized that demand charges weren’t just a line item… they were a problem to solve.
The Setup: What We Thought We Were Buying
When we installed solar, we intentionally chose 40–60% daytime coverage and one Powerwall, not three or four:
- We didn’t need full offset during the day
- We wanted a battery mainly for demand charges + emergencies
- Multiple Powerwalls were expensive
- Our solar array wasn’t big enough to recharge multiple batteries during an outage
- And honestly, we thought one battery would be plenty
Everyone told us we’d be fine. On average, we knew we would be.
At the time, it all made sense.
The First Bill: “Wait… How Is This Calculated?”
After the system went live, we got our first SRP bill.
We expected a demand charge.
We did not expect how confusing it would be.
The math behind demand billing felt like trying to calculate taxes in a foreign language:
- 30-minute intervals
- Summer tiers, winter tiers
- First 3 kW vs next 7 kW vs everything above
- Peak windows
- Shoulder seasons
- Solar backfeed impacts
- Battery logic that sometimes helps and sometimes hurts
We actually called SRP and had them walk us through it step by step.
In fact, I recommend you call them now to understand your bill: (602) 236-4448
And because our system went live in winter, our AC wasn’t running much.
Our winter demand was tiny.
We thought:
“Oh, this is annoying, but really isn’t bad at all.”
We weren’t ready for what summer would bring.
Phoenix Summer: Where Theoretical Math Meets 120°F Reality
If you’re reading this in Arizona, you know.
Summer isn’t a season — it’s an extreme sport.
110°F is normal.
115°F is common.
120°F happens.
And overnight, sometimes it barely dips below 100°.
Your AC isn’t optional.
It’s life support.
And here’s our setup:
- Two AC units
- A 5-ton unit for most of the home
- A 3-ton unit for the addition
- Two small kids
- Windows that face the sun
- A house that heats up fast
- A Powerwall that drains faster
Running both AC units pulls:
- ~5 kW (5-ton)
- ~3 kW (3-ton)
8 kW total.
Here’s where things get real:
At 8 kW of demand, our Powerwall drops 1% per minute.
That’s:
- 100 minutes of full AC runtime
- But SRP’s peak window is six hours (2 p.m.–8 p.m.)
- And solar falls off a cliff around 3:30–5:00 p.m.
Meaning:
- We need AC
- We can’t run them nonstop
- And one battery isn’t enough to carry the evening
This is where the “average home” advice completely falls apart.
The Math That Shocked Me
SRP’s summer demand tiers are:
- $11.98/kW for first 3 kW
- $20.05/kW for next 7 kW
- $36.13/kW above 10 kW
Just 30 minutes of both AC units running during peak sets a recorded demand of 8 kW.
Here’s what that looks like:
- First 3 kW → 3 × 11.98 = $35.94
- Next 5 kW → 5 × 20.05 = $100.25
Total: $136.19 — just in demand fees.
Not energy charges.
Not taxes.
Not service charges.
Just… a 30-minute AC moment.
The part that bothered me wasn’t even the money.
It was the lack of control.
I felt like my bill was a mystery waiting to happen.
My Reality: Living by the 30-Minute Timer
At first, I tried brute-forcing it.
Every 30 minutes:
- Check battery
- Toggle between grid/battery
- Mentally calculating what my demand was (i.e., 8 kW for 15 minutes = 4 kW demand)
- Hoping I didn't miscalculate (a cloudy day fluctuates solar production)
I was setting alarms like a madman, flipping through apps, trying to predict a system designed to be unpredictable.
It was exhausting.
And unless someone does this themselves, no one realizes how mentally draining demand charges can be.
This was not a sustainable way to live.
The Turning Point: “Doesn’t Tesla Have an API?”
One afternoon, my wife casually said:
“You know Tesla probably has an API.”
That sentence changed everything.
I opened my laptop.
And I built DemandGuard for myself.
My software engineering colleagues refused to merge it to production.
It had hard-coded values everywhere.
But it worked.
It did one thing:
It targeted my demand to 1.5 kW.
If my house wanted 8 kW, the battery provided 6.5 kW, and the grid only saw 1.5.
That created a guaranteed minimum demand charge of:
1.5 × 11.98 = $17.97
From $136 → $17.97.
An 86% reduction.
And the battery lasted the full peak window.
That moment — watching my demand line flatten — felt like magic.
It was the first time I felt in control of my bill.
From Personal Hack → DemandGuard → Grid Getter
After that, I kept refining the script.
I automated the solar behavior.
I expanded the automations to work for everyone.
I fixed edge cases.
I added safety checks.
Soon, friends were asking for it.
So I built:
- A cleaner algorithm → DemandGuard
- A UI
- Integrations
- A coordinating layer
- Notification mobile apps
- And eventually… a full automation platform
That platform became Grid Getter.
The goal is simple:
Make demand pricing predictable and manageable for real people — not average ones.
No One Is Average — Especially Not in Phoenix
Here’s what I want you to take away:
- Your home isn’t average
- Your AC isn’t average
- Your solar setup isn’t average
- Your battery capacity isn’t average
- Your lifestyle isn’t average
Even a simple change — like when we replaced our older 5-ton AC with a more efficient one — dropped its draw from ~5 kW to ~3.5–4 kW. That alone reshaped our demand profile.
You need to know your numbers.
Your usage.
Your needs.
Your comfort level.
And I’m here to help you understand it.
Want Help With Your Setup?
If you’re feeling confused, overwhelmed, or blindsided by SRP or APS demand pricing, reach out anytime:
Grid Getter’s basic automation is:
- Free to try
- No credit card required
- Unlimited Premium automations cost $5–$10/mo
A tiny price to avoid $100+ per month in unnecessary charges.
I built Grid Getter because I needed it.
I offer it now because you might need it too.
Create your profile now — Sign up to connect your system, try Grid Getter for free, and start reducing demand charges today.
Appendix: More Demand Charge Math (For Those Who Want It)
1. What 8 kW of Demand Actually Means
If your home pulls 8 kW at any single point during SRP’s summer peak:
- Your demand charge locks to that number
- It doesn’t matter if the rest of the month you stayed at 0–2 kW
- One spike = full fee
2. How SRP Tiers Stack
For an 8 kW spike:
| Portion | Tier Rate | kW in Tier | Subtotal |
|---|---|---|---|
| First 3 kW | $11.98 | 3 | $35.94 |
| Next 7 kW | $20.05 | 5 | $100.25 |
| Total | — | — | $136.19 |
3. Powerwall Drain Reality
At ~8 kW:
- Battery drains ~1% per minute
- ~100 minutes total
- Peak window is 6 hours
- Solar tapers down around 3:30
- Near-zero production by 5:00
4. Why DemandGuard’s 1.5 kW Target Worked
Setting a demand target of 1.5 kW meant:
- 1.5 × 11.98 = $17.97 demand charge
- Battery covers the difference
- Long runtime because it’s not draining at 8 kW
- Predictable cost, no surprises
5. Simple Rule of Thumb
Every 1 kW of prevented peak saves ~$12–$20 per month on SRP.
And peaks happen faster than you think.
If you want, I can now create:
- A companion article (“How to Measure Your Real Demand”)
- A graphic showing the 1%/minute battery drain
- A calculator widget for your blog
- A downloadable PDF version
Just tell me!