Time-of-Use (TOU) Explained: How to Size a ホームバッテリー to Reduce Peak Costs. At the end of the day—when you turn on cooling, cook dinner, and possibly charge an EV—you may concentrate a large share of your consumption into your utility’s on-peak window. Under 利用時間（TOU） pricing, those hours are billed at higher rates than off-peak, and in some tariffs the peak price can be roughly 2–3× the off-peak rate (the exact spread depends on your utility, season, and plan). A properly sized home battery can reduce costs through エネルギー裁定取引 by charging during lower-cost periods (or from surplus solar) and discharging during peak pricing. This guide explains how TOU works, why solar alone often doesn’t align with peak demand, and how to size storage for reliable, bill-based savings.
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What Is Time-of-Use (TOU) Pricing? (Demystifying the Rates)
Time-of-Use pricing means that electricity costs different amounts at different times of day, typically based on grid demand (and, increasingly, on the utility’s cost to procure and deliver energy during those periods). When demand is lower, rates are usually lower. When demand surges, rates rise.
Most TOU plans break the day into zones such as:
- Off-Peak (often late night to midday/afternoon): Lower rates. Often the best time to charge batteries and EVs.
- On-Peak (commonly late afternoon to evening): Higher rates. This is the danger zone for your bill.
- Super Off-Peak (plan- and region-specific): Ultra-low pricing that may appear seasonally or during certain hours (sometimes midday in solar-heavy regions, sometimes overnight), depending on the utility.
The exact time windows vary by utility, season, and even weekday vs. weekend—so your bill or rate sheet is the source of truth.
Flat Rate vs. TOU
| 特徴 | Flat Rate | TOU |
|---|
| Cost consistency | Predictable | Variable by time window |
| User control | 低い | High (you can shift usage) |
| Grid impact | Less load-shifting incentive | Load-shifting friendly |
| Smart meter required | Not required (but often installed) | Typically required (interval/AMI metering) |
Flat rates reward indifference. TOU rewards strategy.
The “Solar Gap”: Why Solar Panels Alone Are Not Enough
Solar panels usually produce the most energy around midday. Unfortunately, many homes use the most electricity in the late afternoon and evening—right when TOU prices often peak. This mismatch is the solar gap, and it’s a big reason solar-only homes can still see high bills on TOU.
Grid operators popularized this challenge with the “duck curve,” which illustrates a familiar pattern in solar-heavy regions: midday oversupply followed by steep evening ramps in net demand. Solar helps, but without a way to time-shift energy, you still end up buying expensive kWh when the sun is down.
Policy changes can amplify the gap. Under newer net-billing structures (often discussed in the context of NEM 3.0), exported solar is credited based on avoided costs and can land in single-digit cents per kWh at many hours—especially when the grid is already saturated with solar. The practical result is that without storage, you may be “selling” solar for low credits midday and “buying” grid power at high prices at night. That’s the trade batteries are designed to fix.
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How Home Energy Storage Solves the TOU Puzzle
A ホームバッテリー changes the math by letting you move energy across time.
Strategy 1: Peak Shaving (Load Shifting)
The battery charges when electricity is cheaper—either from solar or during off-peak—and discharges during the expensive window. From 4 PM to 9 PM (or whatever your plan defines as peak), your home runs more on stored energy and less on premium-priced grid power.
結果 あなた avoid buying kWh at peak prices, shifting much of that consumption to off-peak (minus round-trip losses and normal battery wear).
Strategy 2: Self-Consumption (The Net-Billing Fix)
Instead of exporting solar for low credits, you store it and use it later. This increases your self-consumption and reduces exposure to TOU volatility.
In practice, most systems blend both strategies automatically using time-based controls.
Calculating Your ROI
Let’s run a realistic scenario from a high-TOU market.
- Peak rate: $0.54 / kWh
- Off-peak rate: $0.24 / kWh
- Spread: $0.30 / kWh
Now assume a ~13–15 kWh class battery (for example, systems in the Powerwall category).
Here’s the key correction that makes ROI math honest: you rarely “shift” the full nameplate kWh every day. Real savings depend on (1) how much energy you can actually move from peak to off-peak, (2) the battery’s round-trip efficiency, and (3) your control settings (like reserve for backup).
A more realistic daily savings model
- Shifted energy (kWh/day): what you actually offset during peak hours
- Effective spread ($/kWh): peak rate minus your charging cost (off-peak or solar opportunity cost)
- システム効率： round-trip losses reduce delivered peak kWh
A practical estimate looks like this:
Daily savings ≈ (Shifted kWh/day) × (Peak rate − Charge rate) × (Round-trip efficiency)
(and then you can mentally subtract “some” for long-term battery wear—exact values vary by product and usage)
例
If you can reliably shift 12 kWh/day (common once you account for reserve settings and real peak-window usage):
- Daily savings ≈ 12 kWh × $0.30 × 0.9 ≈ $3.24/day
- Monthly savings ≈ ~$97/month
If that energy is charged from solar you’d otherwise export for low credits, the effective savings can be higher—because you’re avoiding expensive purchases at night while giving up cheap export value midday.
From experience with energy-savvy homeowners, this is often the “aha” moment: batteries stop looking like backup gear and start looking like rate-plan tools.
Reality check box (don’t skip):
- If you set a backup reserve (e.g., 20%), you can’t spend that energy on arbitrage.
- If your peak window is short, you may be power-limited (kW) even if you have enough kWh.
- Savings are highest when your home’s peak-window usage is consistent and the spread is large.
Key Considerations for Configuring Your System
Battery Capacity (kWh)
Start with your on-peak energy usage, not your daily total. If your home uses 12 kWh between 4 PM and 9 PM, a 10 kWh battery won’t fully cover that window—especially after you account for reserve settings and losses. Undersizing is the fastest way to leave savings on the table.
Rule of thumb: Aim to cover the energy you want to shift, not the battery’s marketing number.
Power Output (kW)
Capacity answers どのくらい. Power answers 何. If your battery can’t run major loads during peak hours, you’ll still pull from the grid.
For many homes, ~5 kW continuous is a practical baseline, but it’s not universal:
- Larger homes, electric cooking, multiple HVAC zones, or well pumps may need higher kW または multiple batteries.
- Also watch surge/starting loads—motors and compressors can briefly demand far more than their steady-state draw.
Chemistry Matters
For TOU applications with frequent cycling, LiFePO₄ (LFP) chemistry is often attractive because it’s known for strong cycle-life potential and thermal stability. That said, the best choice still depends on the full system design (warranty terms, temperature environment, inverter limits, and your cycling profile), not chemistry alone.
Smart Controls
探す Time-Based Control, Cost Savings Mode, or similar features in the app. These automate charging/discharging based on your rate schedule so you don’t have to babysit the system. The best controllers also let you set:
- Peak/off-peak windows aligned to your tariff
- Backup reserve percentage
- Export behavior (where allowed/desired)
Future Trends: VPPs and Dynamic Pricing
TOU is often just the beginning.
- Virtual Power Plants (VPPs): Utilities or aggregators pay battery owners to deliver power during grid stress events (heatwaves, emergencies, critical peak events).
- AI-Driven HEMS: Home Energy Management Systems increasingly factor in weather forecasts, pricing signals, and usage patterns to optimize charging automatically.
These trends reward flexible, well-sized battery systems even more over time—especially systems that can respond to events without sacrificing homeowner comfort.
結論
Time-of-Use rates can feel like a penalty for the unprepared—but they reward homes that can shift energy. If you’re on TOU (especially with solar), a correctly sized battery isn’t just backup; it’s a cost-control tool that helps you avoid peak purchases and reduce exposure to volatile rates. The sizing rule is simple: match 容量（kWh） to what you use during the peak window, ensure enough 出力（kW） for real loads, and let smart controls automate the schedule. Check your bill today—if your on-peak price is consistently above $0.40/kWh (or the spread is large), run a quick sizing calculation or request a sizing review before you buy, so you get predictable savings without paying for an “almost big enough” system.
よくあるご質問
Do I need solar to save money on TOU rates?
No. You can charge a battery from the grid during off-peak hours and discharge during peak. Solar often increases the upside (especially in net-billing environments), but it’s not required.
What is the best battery mode for TOU?
Use “Time-Based Control,” “Cost Savings Mode,” or the equivalent feature that aligns charging and discharging with your utility rate schedule—plus a reserve setting that matches your outage risk.
Does TOU pricing apply on weekends?
Usually yes, but peak windows and prices often differ by utility and season. Some plans have lighter weekend peaks or different definitions entirely. Always check your utility’s rate card.
Can I charge my EV during peak hours?
You can, but it’s usually expensive. Smart EV chargers (and many EVs) can schedule charging to off-peak hours to minimize cost—often delivering most of the savings without changing your routine.