Powerwall ULO Arbitrage in Toronto: The Full Math

# Powerwall ULO Arbitrage in Toronto: The Full Math

Toronto Hydro's Ultra-Low Overnight (ULO) rate is the most arbitrage-friendly residential electricity tariff in Canada. The spread between off-peak and peak is 12x — far above any other major utility. A home battery that charges at 2.4¢/kWh and discharges during 28.6¢/kWh peak captures real money on every cycle.

This guide is the engineering-honest math. We work through best case, realistic case, and the cases where the math does not work. If you want the broader battery context, see [Home Battery & Powerwall Toronto Complete Guide](/blog/home-battery-powerwall-toronto-2026-complete-guide). For the no-solar grid-charge case, see [Powerwall Without Solar Toronto](/blog/powerwall-without-solar-toronto-grid-charge).

ULO Rate Schedule (2026)

There is also a flat regulatory + delivery charge layer that does not change with time of use. The ULO arbitrage applies only to the commodity portion of your bill (the per-kWh number above), not to delivery charges. We use the commodity numbers throughout this analysis because that is the part the battery actually shifts.

The Best-Case Cycle

Battery charges 13.5 kWh from grid at 2.4¢/kWh:

• Cost in: 13.5 × 2.4¢ = 32.4¢

Battery discharges all 13.5 kWh during 4pm-9pm peak, displacing grid at 28.6¢/kWh:

• Cost out displaced: 13.5 × 28.6¢ = $3.86

Net daily savings: $3.86 − $0.32 = $3.54

Annualized at 5 weekdays × 52 weeks = 260 cycles:

• Best case annual savings: $3.54 × 260 = $920

That is the marketing number. Real life is 70–85% of it.

Why You Don't Get the Best Case

Round-Trip Efficiency Loss (~10%)

The Powerwall 3 round-trip efficiency is 89%. To deliver 13.5 kWh out, you must put 15.2 kWh in. Real charging cost:

• 15.2 × 2.4¢ = 36.4¢

Net daily savings becomes $3.50 instead of $3.54. Small loss.

You Don't Always Use 13.5 kWh During Peak

The 4pm-9pm window is 5 hours. Most Toronto homes use about 6-12 kWh during that window — not the full 13.5 kWh the battery can deliver. Excess battery capacity sits unused unless you also extend discharge into mid-peak hours (12.2¢, lower spread).

Realistic discharge: 9-11 kWh during peak window. Net daily savings: $2.30 - $2.85.

Weekends Have No Peak Rate

Saturdays and Sundays charge at ULO (2.4¢) and run all day at off-peak (7.6¢). Spread is only 5.2¢/kWh, vs 26.2¢ on weekdays. Most apps default to "skip weekends" for grid-only arbitrage to preserve cycle life.

Holidays Skip Peak

Statutory holidays use weekend rates. About 9 holidays per year are excluded from peak arbitrage.

Cycle-Life Considerations

Powerwall 3 is rated for 10 years at 70% capacity retention under typical use. One cycle per day is well within spec. Cycling 1.5x per day to capture mid-peak as well shortens battery life — the arbitrage gain doesn't justify the warranty risk.

Realistic Annual Savings — Three Scenarios

Scenario A: Average Toronto Household

• 8,500 kWh/year total usage
• ~4 kWh average during peak window
• Battery cycles ~210 days/year (skips weekends, holidays, low-use days)
• 9 kWh discharge per cycle on average

Annual savings:

• Charging cost: 9 / 0.89 × 2.4¢ × 210 = $51
• Displaced peak: 9 × 28.6¢ × 210 = $540
• Net: $489/year

Scenario B: Heavy Peak User (Heat Pump + EV + Cooking 4–9pm)

• 13,000 kWh/year, EV at home Mon-Fri 5pm-7am, cooks at 6pm
• 11 kWh average peak discharge
• 240 cycle days/year

Annual savings:

• Charging cost: 11 / 0.89 × 2.4¢ × 240 = $71
• Displaced peak: 11 × 28.6¢ × 240 = $755
• Net: $684/year

Scenario C: Light User (Couple, Gas Heat, Often Out 4–9pm)

• 4,500 kWh/year
• 2.5 kWh peak discharge average
• 180 cycle days/year

Annual savings:

• Charging cost: 2.5 / 0.89 × 2.4¢ × 180 = $12
• Displaced peak: 2.5 × 28.6¢ × 180 = $129
• Net: $117/year

For Scenario C, ULO arbitrage alone has a 130-year payback. The battery is wrong unless outage backup or future EV/heat-pump load is the driver.

Payback Math at Realistic Numbers

This is why we tell customers honestly: ULO arbitrage alone does not justify a battery for most Toronto homes. The math improves with:

• Greener Homes Loan at 0%: turns the cash payback into a cash flow comparison vs the loan payment
• HRSP $5K rebate: drops effective cost to $10,500 — heavy user payback drops to 15 years
• Outage avoidance value: $1,500 of food + hotel + sump pump backup per major outage, multiplied by your real outage frequency
• Solar pairing: changes the entire economic structure (see [Powerwall with Solar Toronto](/blog/powerwall-with-solar-panels-toronto))

Outage Avoidance: The Number Most Analyses Ignore

Toronto outages are not rare. Recent significant events:

• May 2022 Derecho: 1.2M households, 3-7 days
• December 2022 ice storm: ~250K households, 1-3 days
• August 2024 thunderstorm: ~180K households, 12-24 hours
• February 2025 ice storm: ~90K households, 6-18 hours

A single 24-hour outage costs an average household $300-$1,500 in spoiled food, hotel costs, sump-pump-flooded basement risk, and missed work. A 72-hour outage costs $1,500-$5,000+.

If your area sees one significant outage every 2-3 years, the avoidance value alone is $200-$700/year — comparable to the ULO arbitrage savings, and additive.

Combined "ULO + outage avoidance" math for an average Toronto household:

• Annual benefit: $689-$1,189/year
• Powerwall 3 cost: $15,500 (or $10,500 net of HRSP)
• Payback: 9-15 years
• Greener Homes financing: $0 cash out, monthly loan payment $129

That is the realistic case. It is reasonable but not spectacular without solar.

The Math With Solar Pairing

Solar changes the arbitrage equation entirely:

• Daytime solar generates kWh at near-zero marginal cost
• Battery stores solar instead of exporting to net meter
• Discharges during peak at 28.6¢/kWh
• Effectively replaces grid kWh, not just shifts them

Annual savings for an 8 kW solar + Powerwall 3 system in Toronto: $1,800-$2,400 in avoided grid purchases, vs $489 for grid-only ULO arbitrage.

That is why we recommend solar pairing whenever the roof allows — see [Powerwall with Solar Panels Toronto](/blog/powerwall-with-solar-panels-toronto). Solar PV requires an NRCan-registered installer for federal incentive eligibility.

How to Configure ULO Charging in the App

Tesla Powerwall (3 or 2):

• 1. Open Tesla app, select Powerwall
• 2. Operations → Customize
• 3. Time-Based Control mode
• 4. Set "Off-Peak" rate to 2.4¢ for hours 11pm-7am all days
• 5. Set "Peak" rate to 28.6¢ for hours 4pm-9pm Mon-Fri
• 6. Set Reserve to 20% (preserves backup capacity)

FranklinWH:

• 1. FranklinWH app, select aPower
• 2. Operation Mode → Time of Use
• 3. Configure ULO schedule
• 4. Save and confirm

Enphase IQ Battery 5P:

• 1. Enlighten app, Settings → Profile
• 2. Time of Use Profile
• 3. Add Toronto Hydro ULO rate plan
• 4. Set backup reserve

We program these schedules during commissioning. Customers can adjust later as their habits change.

Verify Your Rate Plan First

Most Toronto residential customers are on the default tiered or TOU rate — not ULO. Switching to ULO is a free plan change through Toronto Hydro. Call 416-542-8000 or use the My Toronto Hydro portal. Allow one billing cycle for the change to take effect.

If you install a battery and forget to switch to ULO, your arbitrage savings collapse to near zero. We confirm rate plan as part of project intake.

Get the Real Numbers for Your House

We pull your last 12 months of Toronto Hydro hourly data, model arbitrage savings against your actual usage profile, and show you the realistic payback before you sign anything. Book a [home battery consultation](/services/hvac-energy/home-battery-powerwall) for the personalized math.