SRP E-27 and the Summer Demand Problem
If you're on SRP's E-27 plan and you've ever opened your June or July bill and wondered why it jumped $80 compared to April, the answer is almost certainly your demand charge — not your energy usage.
E-27 bills you three ways: a basic service charge (flat, irrelevant to this conversation), an energy charge per kWh consumed, and a demand charge based on your peak grid draw. The demand charge is where summer gets brutal. SRP's summer on-peak demand rate runs $13.07/kW (as of early 2026 — verify the current figure against the E-27 rate schedule at srpnet.com before making decisions based on this), and it applies to your peak demand during the 2–8 PM weekday window.
That measurement method trips people up. SRP doesn't capture your instantaneous peak. It measures your single highest demand interval during on-peak hours across the entire billing period. SRP has used both 15-minute and 30-minute intervals depending on meter generation, so check your current E-27 rate schedule to confirm which applies to you. One bad afternoon sets your demand charge for the cycle. You don't get a do-over because you were careful every other day.
Summer on-peak runs May through October on E-27 (confirm the exact dates on your current rate schedule, since SRP occasionally adjusts plan terms). Weekdays only, 2–8 PM. Those six hours are the most expensive electricity window SRP offers, and if your Powerwall isn't specifically configured to manage demand, not just shift energy, you're probably leaving money on the table.
Why Summer Bills Spike Even When You're Trying
Your AC unit pulls somewhere between 3–5 kW on its own when it's running hard. On a 110°F day, it's running hard constantly. If your house has been sitting unshaded all day and you walk in at 5 PM and crank the thermostat, you might see 5+ kW from the AC alone for an extended stretch while the house catches up.
Now layer in real life: someone starts the dryer at 5:30 (3–5 kW), somebody else preheats the oven at 6 (2–3 kW), and your EV needs to charge (7+ kW if you're not throttling it). For 30 minutes, your total grid draw hits 12–15 kW. That interval gets logged. At $13.07/kW, a 12 kW demand reading costs $157 in demand charges alone, before you've paid for a single kWh of energy.
The painful part is you probably don't even know it happened. There's no alarm when your demand spikes. You just see it three weeks later on your bill.
Time-Based Control Isn't Enough
Tesla's Time-Based Control mode is the typical starting point for TOU customers like those on E-27 — it tells the Powerwall to discharge during peak hours to offset your energy draw. That helps with the kWh portion of your bill. It does not cap your demand.
Here's the distinction: Time-Based Control will discharge your Powerwall during the 2–8 PM window, but if your loads pull 12 kW and your Powerwall 2 can only supply 5 kW continuous, you're still drawing 7 kW from the grid. Powerwall 3 delivers 11.5 kW continuous, which changes the math considerably. If you're running a PW3, your ceiling holds further before the grid picks up slack. But if that grid draw interval gets recorded as your billing cycle peak, you pay for it regardless.
What you actually need is a demand ceiling — a hard limit on how much grid power you pull at any given moment during peak hours, regardless of what your loads are doing. That's what Grid Getter's DemandGuard feature does. You set a target (say, 6 kW), and if grid draw approaches that ceiling during on-peak hours, the system signals the Powerwall to discharge harder to keep your grid draw in check.
The math is simple. Say your current on-peak demand peak is 8 kW. With DemandGuard capped at 6 kW:
$13.07/kW × 2 kW reduction = $26.14/month saved, just from that 2 kW difference. (These examples use the $13.07/kW rate cited above — verify your current rate schedule before running your own numbers.)
Hold that ceiling all summer and you're looking at $130–$160 in savings from June through October, on top of whatever you're already saving from Time-Based Control shifting your energy costs.
Pre-Cooling: The Free Demand Reduction
Before any automation does its job, there's a strategy that costs nothing: pre-cool your house before the peak window opens.
The goal is to get your home down to around 74°F by 1:30 PM. Run the AC hard before the 2 PM cutoff — that's still off-peak, so the demand charge doesn't apply. Then let your thermostat drift up to 78°F through the peak window. Thermal mass does the work; your walls and furniture hold the cool, and a well-insulated house won't heat up fast enough to force the AC to run constantly.
A house that's already at 74°F at 2 PM needs far less cooling than one that's at 78°F and rising. The AC still runs, but it cycles more and pulls less sustained power. That's a direct reduction in your on-peak demand.
This works even better when your Powerwall is ready for it. Grid Getter's scheduling tools let you set your Powerwall to hold a larger reserve ahead of the peak window, so it has capacity to absorb the load when the 2 PM cutoff hits and you stop pre-cooling. You still manage the thermostat yourself, but the battery is staged and waiting.
Four Practical Moves for This Summer
Stagger your heavy appliances. This sounds obvious but the execution matters. During the 2–8 PM window, never run the dryer and the oven at the same time. Pick one. If you can push laundry to after 8 PM, do that instead. EV charging should be scheduled for overnight — most EVs have built-in scheduling, or you can set it through your charger's app. (Automatic load scheduling is something we're working on — if that's a feature you'd want, let us know.).
Pre-cool to 74°F, then let it float. Set a schedule in your thermostat to hit 74°F by 1:30 PM on weekdays. During peak hours, raise the setpoint to 78°F. The house may feel slightly warm by 7:30 PM but you'll recover fast once peak ends at 8. On days above 115°F you might need to hold 76°F — that's a judgment call depending on your home's insulation.
Set your DemandGuard ceiling gradually. Pull your current on-peak demand from your SRP account — say it reads 9 kW. Don't start at 5 kW. You'll drain the battery before the peak window closes and the ceiling won't hold when you need it. Start at 7 kW, run it for a week, confirm it's holding, then bring it to 6 kW. Gradual beats optimizing everything at once.
Check your demand numbers weekly, not just at bill time. Your demand profile shifts as summer progresses. June is different from August. Heat patterns change, schedules change, and as your Powerwall cycling patterns settle in, you'll see which weeks had outlier intervals and why. Log into your SRP account and pull your usage history weekly. Look for the intervals above your target ceiling. Those are your action items.
One Number to Pull Right Now
Log into your SRP account and find your current billing cycle's on-peak demand reading. It should be listed as your "on-peak demand" in kW under your bill details or usage summary. That number times $13.07 is your demand charge this month.
Now ask yourself: what was happening in your house during the interval that set that number? If you can answer that question, you can probably prevent it from happening again.
Getting this right is mostly a setup problem. Once your DemandGuard ceiling is configured and your pre-cooling schedule is running, the system handles the peak window automatically. You stop thinking about it, and you stop seeing the bill spikes.
Grid Getter's free plan gets DemandGuard running on your Powerwall — set your ceiling, and the system holds it through the peak window automatically.
Start there. Set the ceiling, let it run, and check your bill next month.