What are time-of-use (TOU) electricity rates?

Time-of-use rates charge different prices for electricity depending on when you use it. Peak periods (typically 4–9 PM on weekdays) cost 2–3x more than off-peak periods (nights and weekends). Solar panels produce most energy during off-peak daytime hours, so a battery lets you store cheap solar power and use it during expensive peak periods.

Does solar help with TOU rates?

Solar alone helps during the day but the biggest TOU savings come from combining solar with a battery. Solar produces most energy 10 AM–3 PM (off-peak). A battery stores this cheap solar energy and discharges during peak hours (4–9 PM), replacing expensive grid electricity. Without a battery, you export cheap solar and buy expensive peak power.

What size battery do I need for TOU rate arbitrage?

For a typical home on TOU rates, 10–13.5 kWh of battery storage (one Powerwall or equivalent) covers most peak-hour loads. Larger homes with high evening demand benefit from 20+ kWh. The key metric is your evening peak usage in kWh — size the battery to cover 80–90% of those hours.

Time-of-Use Solar Calculator

On TOU rates? Find out how much your solar and battery can save by shifting energy away from expensive peak hours.

TOU rates

Peak rate?

$/kWh

Off-peak rate?

$/kWh

Super off-peak rate?

$/kWh

Peak hours per day?

hrs

Your system

Daily usage?

kWh/day

Solar production?

kWh/day

Battery capacity?

kWh

TOU optimization savings

$247.95/month savings

Monthly cost without optimization$247.95

Monthly cost with solar + battery$0.00

Solar value (peak avoidance)$165.00/mo

Battery arbitrage savings$92.61/mo

Annual TOU savings$2,975.40/yr

Battery discharges during peak10.5 kWh/day

Optimal daily schedule:

Super off-peak (4h overnight): Charge battery to full
Morning off-peak: Solar begins producing, battery conserves charge
Peak hours (5h, 4PM-9PM typical): Battery discharges to avoid peak rates
Evening off-peak: Low-rate grid use after battery depleted

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How to Use This Calculator

Find your TOU rates

Check your utility's website or recent bill for peak, off-peak, and super off-peak rates. California utilities (PG&E, SCE, SDG&E) have the most complex TOU structures with rates varying from $0.12/kWh overnight to $0.65/kWh during summer peak. Texas and other deregulated markets offer variable pricing. If your utility has only two tiers (peak and off-peak), set super off-peak equal to off-peak.

Peak hours per day

Enter the number of hours per day that peak rates apply. Typically 4-6 hours in the late afternoon and evening (4-9 PM in California, 3-8 PM in Texas). Weekends often have no peak rates on many tariffs.

Battery strategy

With a home battery, the optimal strategy is charge during super off-peak overnight (cheapest power) and discharge during peak (most expensive power). This "price arbitrage" can save $50-200+/month in high-rate states. The calculator assumes this optimal strategy automatically.

The Formula

Usage distribution (typical): Peak hours (35% of daily usage): charged at peak rate Off-peak hours (50%): charged at off-peak rate Super off-peak (15%): charged at super off-peak rate Without solar/battery: Daily cost = Sum of (usage fraction × period rate) Battery arbitrage savings (per day): Discharge during peak = MIN(battery kWh, peak usage) Savings = Discharge kWh × (Peak rate - Super off-peak rate) × 0.92 round-trip efficiency Solar savings: Solar during peak = Solar production × 0.40 (40% during peak window) Solar during off-peak = Solar production × 0.60 Solar value = respective kWh × applicable rate

Example: San Diego Home with Tesla Powerwall

SDG&E TOU-DR-1 rate with 10kW solar and Powerwall 2

Daily usage: 30 kWh. SDG&E peak: $0.65/kWh (4-9 PM). Off-peak: $0.35/kWh. Super off-peak: $0.25/kWh. 10kW solar produces 40 kWh/day. Powerwall 2: 13.5 kWh.

Peak rate$0.65/kWh

Off-peak rate$0.35/kWh

Super off-peak$0.25/kWh

Battery13.5 kWh (Powerwall 2)

Result

Without solar/battery~$380/month

With solar + Powerwall~$40/month

Monthly TOU savings~$340/month

Annual savings~$4,080/year

At these high California rates, the combination of solar and battery pays back the Powerwall ($9,000 after 30% ITC) in about 2-3 years on TOU savings alone, not counting the additional value of backup power during outages.

FAQ

Is TOU pricing better or worse for solar customers? ▼

TOU pricing can be much better for solar customers — but only if your solar production timing aligns with peak rates, or you have a battery to shift solar energy to peak periods. In California, peak rates are 4-9 PM when solar production is declining. Without a battery, you export cheap off-peak solar and buy back expensive peak power. With a battery, you store midday solar and discharge it during peak hours, capturing the full peak rate value. In high-peak-rate states, a solar+battery combo on TOU is dramatically more valuable than flat-rate solar.

What time should I charge my EV on TOU rates? ▼

Always charge your EV during super off-peak hours — typically midnight to 6 AM. Most EVs have programmable charging timers. Tesla owners can use scheduled charging in the car app. At $0.12/kWh super off-peak vs $0.45/kWh peak, charging a 75 kWh battery pack costs $9 at super off-peak vs $33.75 at peak. Annual savings for a typical EV driver: $500-1,000 just from timing charges correctly.

Which battery is best for TOU optimization? ▼

Any home battery with smart inverter capabilities works: Tesla Powerwall 2 (13.5 kWh, ~$12,000 installed), Enphase IQ Battery 10T (10.1 kWh, ~$13,000), Franklin WH10 (10 kWh, ~$10,000), or SolarEdge Home Battery. Key features to look for: TOU scheduling (automatic peak/off-peak optimization), grid-tied operation, and backup power capability. The Powerwall's "Time-Based Control" mode automatically optimizes for your TOU schedule.

Time-of-Use Solar Calculator

How to Use This Calculator

Find your TOU rates

Peak hours per day

Battery strategy

The Formula

Example: San Diego Home with Tesla Powerwall

SDG&E TOU-DR-1 rate with 10kW solar and Powerwall 2

Result

FAQ

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