Formula Used
Temperature rise: ΔT = target temperature − inlet temperature.
Useful heat: Q = m × c × ΔT.
Here, m is water mass in kilograms. The value c is 4.186 kJ/kg°C.
Daily useful kWh: Q ÷ 3600.
Electric yearly input: annual useful kWh ÷ electric efficiency + standby kWh.
Gas yearly therms: annual useful kWh ÷ gas efficiency ÷ 29.300111 + standby therms.
Recovery time: full tank useful kWh ÷ useful heater output kW.
Total yearly cost: energy cost + yearly maintenance.
How to Use This Calculator
Choose volume and temperature units first. Enter daily hot water use and tank size. Add inlet and target water temperatures. Then enter your local electricity and gas rates. Adjust efficiency, standby loss, power rating, installation cost, maintenance cost, and emission factors. Press the calculate button. The result appears above the form and below the header section.
Example Data Table
| Input |
Example Value |
Meaning |
| Daily hot water |
64 gallons |
Estimated household hot water demand |
| Tank size |
50 gallons |
Stored water reheated after heavy use |
| Temperature rise |
65 °F |
Target temperature minus inlet temperature |
| Electric efficiency |
95% |
Useful heat divided by purchased electric energy |
| Gas efficiency |
62% |
Useful heat divided by fuel heat input |
Water Heater Choice Matters
A water heater turns energy into stored hot water. The physics is simple, yet the bills can differ a lot. Water mass, temperature rise, heater efficiency, and local rates all shape the final cost. A small change in inlet temperature can change yearly energy use. A long shower schedule can change it even more.
How the Comparison Works
Electric units usually convert purchased energy into heat very directly. Standard resistance models often have high site efficiency. Gas units burn fuel and lose some heat through exhaust. That loss is included through the gas efficiency field. The calculator also adds standby loss, because tanks lose heat while waiting. You can enter higher loss for older tanks or colder spaces.
Useful Physics Behind It
The main heat equation is Q equals mass times specific heat times temperature change. Water has a high specific heat. That means it needs meaningful energy for every degree of heating. The tool converts volume into water mass. It then converts heat from kilojoules into kilowatt hours. Gas use is converted through therm energy. This keeps both heaters on the same energy basis.
Cost and Recovery Meaning
Annual cost is not only fuel cost. Maintenance, standby energy, and installation difference can matter. The payback field compares total first cost against yearly savings. Recovery time estimates how long each heater takes to reheat a full tank. A larger burner can recover faster than a smaller element. But faster recovery does not always mean lower cost.
Using Results Wisely
Use real utility rates from your bill. Enter realistic daily hot water demand. Try summer and winter inlet temperatures. Also test several efficiency values. New models may perform better than old units. Location, venting, wiring, and safety rules can affect the true choice. Treat the result as a planning estimate. Ask a licensed installer before making final changes.
Key Inputs to Review
Daily volume should match real habits. Dishwashing and laundry may add hidden demand. Tank size affects recovery energy for a full refill. Power rating affects recovery speed only. It does not change useful heat needed. Emission factors are optional planning values. They help compare local grid power against gas combustion for climate estimates and reports.
FAQs
1. Which heater is cheaper to run?
The cheaper heater depends on local utility rates, efficiency, standby loss, and hot water demand. Electric models can be cheaper where power is low cost. Gas models can be cheaper where gas is inexpensive.
2. Why does the calculator use temperature rise?
Water heating energy depends on the difference between inlet water temperature and target hot water temperature. A higher temperature rise needs more energy, even when the same water volume is used.
3. What is standby loss?
Standby loss is energy wasted while hot water sits in the tank. Older tanks, colder rooms, and poor insulation can increase this loss. It affects both yearly cost and total energy use.
4. What does recovery time mean?
Recovery time estimates how long the heater takes to reheat a full tank after heavy use. It depends on tank size, temperature rise, input power, and efficiency.
5. Why is gas measured in therms?
A therm is a common billing unit for natural gas. This calculator converts useful water heating energy into therms so gas cost can be compared against electric cost.
6. Are emissions exact?
No. Emissions are planning estimates. Electric emissions depend on the local power grid. Gas emissions depend on fuel type and combustion assumptions. Edit the emission fields for your region.
7. Should installation cost be included?
Yes, when comparing a new purchase. Installation cost can change the first year result and payback. It is especially important when switching fuel type or adding new venting.
8. Can this compare tankless heaters?
It can estimate energy cost if you enter suitable efficiency and standby values. However, tankless sizing, flow rate, and installation limits need separate professional review.