In Ontario, Ultra-Low Overnight (ULO) pricing offers significantly lower overnight electricity rates. This makes it particularly attractive for EV charging, where a large portion of usage can be scheduled overnight.
However, achieving actual savings depends on more than just shifting EV charging.
This leads to an important question:
How much total household electricity needs to be shifted to overnight hours to achieve overall savings?
Achieving savings requires shifting a substantial portion of total house electricity usage to overnight hours.
While EV charging helps, it may not be sufficient on its own, other loads – such as water heating can tip the overall usage balance and determine whether overall savings are achieved.
Under Time-of-Use (TOU) and Ultra-Low Overnight (ULO) pricing, electricity cost is determined by when electricity is used, not just how much is used.
(Note: Under the Tiered Plan, higher rates apply once monthly usage exceeds the Tier 1 threshold. For simplicity, the following discussion uses Tier 1 as a reference, making the estimated savings slightly conservative.)
The following case examines total household usage to show whether EV charging alone leads to overall savings – and how water heating can determine the outcome.
The results are based on actual household data from a two-person household using:
an electric storage water heater
a plug-in hybrid electric vehicle (PHEV)
The household is subscribed to Ontario’s ULO pricing plan.
Total electricity usage is derived from utility bills
Water heating usage is measured separately using a controller
The data covers one full year, along with a recent four-month period, allowing for consistent comparison.
| Period | Nov 2024 - Oct 2025 | Nov 2025 - Feb 2026 |
|---|---|---|
| Total Usage (kWh) | 14,941 | 4,773 |
| Overnight Usage (kWh/%) | 7,179 | 2,410 |
| 48.0% | 50.5% | |
| Water Heating (kWh%) | 1,577 | 514 |
| 10.6% | 10.8% |
Across both periods, approximately 48–50% of total electricity usage was consistently shifted to overnight hours—above the threshold required for savings under ULO pricing. This result highlights the importance of actively managing water heating.
At first glance, this suggests that the household should benefit from lower electricity costs.
However, this overnight usage includes water heating.
Without shifting water heating to overnight hours, total overnight usage would fall close to—or below—the required threshold. In that case, expected savings may be significantly reduced or even eliminated.
This shows that EV charging alone does not guarantee savings, even when scheduled overnight.
Water heating represents a significant portion of household electricity —about 10% of the total in this case. At around 50% total overnight usage, the 10% contribution from water heating is sufficient to tip the balance whether overall savings are achieved.
Without control, electric water heaters operate automatically. They often reheat water during periods of higher household activity, which typically coincide with higher electricity rates.
Under Ontario’s ULO pricing, the on-peak rate is exceptionally high while the overnight rate is exceptionally low. The wide price difference amplifies the effect of usage timing, so even relatively small portions of electricity use, depending when it occurs, can significantly influence overall cost.
Shifting electricity usage to overnight hours is essential for achieving overall savings under ULO pricing.
While EV charging can contribute significantly, it may not be sufficient on its own. Water heating, when combined with other household electricity use, can determine whether the overnight share remains above the threshold required for overall savings
Managing both EV charging and water heating is key to achieving consistent and reliable overall cost savings.
Overnight EV charging can help reduce electricity cost under Ultra-Low Overnight(ULO) plan, as overnight are significantly lower, However, whether it leads to actual savings depends on how the rest of the household electricity is used. If a significant of usage still occurs during higher priced periods, the expected savings may be reduced or offset.
Water heating can represent a significant portion of household usage, and its impact depends on when it occurs. Under ULO pricing, electricity during on-peak hours is much more expensive than overnight usage. Because water heating often occurs during periods of higher household demand, this coincide with on peak hours. This combination of relatively high energy use and unfavorable timing can significantly reduce or even eliminate the expected savings.
Under Ontario’s Ultra-Low Overnight (ULO) plan, electricity used during on peak hours is substantially more expensive.The on-peak rate can be several times higher than Tier rates and up to ten times higher than the overnight rate. As a result, even a relatively small amount electricity used during on-peak periods can significantly offset savings gained from shifting usage to overnight hours.
To reduce water heating costs under TOU and ULO plans, the most important requirement is having a storage water heater with sufficient capacity. Without enough storage, hot water must be reheated during higher-priced periods, limiting potential savings.
Improving effective capacity—such as insulating the tank—helps retain heat longer and extend usable hot water.
Further savings come from two types of shifting:
- Usage shifting: moving hot water activities to lower-cost periods (more practical under TOU)
- Heating shifting: scheduling water heating before higher-priced periods (especially important under ULO)
In practice, sufficient storage is the foundation. Without it, the effectiveness of heating shifting is limited.
