Energy & Utilities · 8 calculators
Energy & Utilities questions, answered
Price appliance electricity, EV charging and fuel economy, and size a battery for a load. 8 questions, one per calculator, each naming its formula and figure at documented inputs. Open any one to swap in your own numbers. Every answer in Energy & Utilities states the calculator’s formula, the worked-example inputs behind the figure, the result at those inputs and the point where the model stops being a reasonable approximation, so each number can be checked against your own situation before you rely on it. The questions in Energy & Utilities run from the ones most people ask first to the ones that only matter once the basics are settled, and every answer links to the calculator that produced its figure, so the reasoning and the arithmetic can be read side by side and challenged separately.
Common energy & utilities questions
Is my electric rate the same for every kilowatt-hour?
Not always. Many utilities charge tiered or time-of-use rates and add delivery fees and taxes. For an all-in rate, divide your whole bill by the kilowatt-hours you used.
What is a kilowatt-hour?
The energy used by a 1,000 watt device running for one hour. A 100 watt bulb left on for ten hours uses one.
Why is standby power worth counting?
Devices left plugged in draw a few watts around the clock. Across a house of chargers, screens and set-top boxes that adds up, and it runs all 8,760 hours of the year.
Do EV efficiency figures include charging losses?
It depends on how the figure was measured, so check the source. The EV calculator lets you add a loss percentage, so the cost reflects what the wall supplies and not only what reaches the battery.
How do I measure my real gas mileage?
Fill the tank, note the odometer, drive normally, then fill again. Miles since the first fill divided by the gallons added is your mileage for that tank. Average a few tanks.
Why do solar panels produce less than their rated watts?
Ratings come from lab conditions of strong sun and cool panels. Real roofs face heat, dirt, shade, wiring losses and an angled sun, so a planning loss of around 14 percent is a common allowance.
How long will a battery really last?
Runtime falls as the load rises and as the battery ages, cools or drains deeply. The calculator shows the best case at your chosen usable depth, so plan on somewhat less.
What is a peak sun hour?
One hour of sunlight at 1,000 watts per square meter, the standard test strength. A day with five peak sun hours delivers the same energy as five hours at that strength, even if daylight lasts much longer.
Energy & Utilities at a glance: each result, and the input that moves it most
| Calculator | Result at the worked example | Input that moves it most |
|---|---|---|
| Electricity Cost Calculator | Cost for the period: $61 | Hours used per day ($15 (25%)) |
| EV Charging Cost Calculator | Charging cost per month: $57 | Electricity price per kWh ($13 (24%)) |
| MPG Calculator | Miles per gallon: 28 | Gallons of fuel used (6.8 (24%)) |
| Battery Runtime Calculator | Runtime: 4.8 hours | Load (watts) (1.0 hours (20%)) |
| Solar Panel Output Calculator | Energy per day (kWh): 31 | Peak sun hours per day (6.9 (22%)) |
| Solar Payback Calculator | Simple payback: 8.2 years | Electricity price per kWh (2.0 years (24%)) |
| LED Bulb Savings Calculator | Savings per year: $190 | Electricity price per kWh ($45 (24%)) |
| Generator Size Calculator | Minimum generator size (watts): 5,000 | Running watts of everything on together (600 (12%)) |
How to calculate each energy & utilities figure
How do I calculate electricity cost?
kWh = watts ÷ 1,000 × hours per day × days; cost = kWh × price per kWh. Find which appliance is worth replacing by pricing its running cost per month and per year. The prefilled example returns a cost for the period of $61.
How do I calculate EV charging cost?
kWh from the wall = miles ÷ 100 × kWh per 100 miles ÷ (1 − charging loss); cost = kWh × price. Compare the monthly charging cost with what you spend on gasoline today. The prefilled example returns a charging cost per month of $57.
How do I calculate MPG?
MPG = miles driven ÷ gallons used; cost per mile = gallons × price ÷ miles; L/100 km = 235.215 ÷ MPG. Track real mileage against the sticker figure by measuring one full tank to the next. Using the worked example on the page, the miles per gallon works out to 28.
How do I calculate battery runtime?
Runtime (hours) = capacity (Ah) × voltage × usable share ÷ load (watts). Size a battery bank for an RV, a workshop or a backup power supply before you buy it. Using the worked example on the page, the runtime works out to 4.8 hours.
How do I calculate solar panel output?
Daily kWh = panels × panel watts ÷ 1,000 × peak sun hours × (1 − system losses). Compare a proposed system's production against what your household actually uses. Run it at the prefilled values and the energy per day (kwh) comes out at 31.
How do I calculate solar payback?
Payback (years) = (system cost × (1 − incentive)) ÷ annual savings; annual savings = annual kWh × price per kWh. Test a quote by checking how long the savings take to repay what you spend. At the worked-example inputs the simple payback is 8.2 years.
How do I calculate LED bulb savings?
Annual kWh saved = (old watts − LED watts) ÷ 1,000 × hours per day × 365 × bulbs; savings = kWh saved × price. Rank the rooms worth relamping first by how many hours their lights stay on. At the worked-example inputs the savings per year is $190.
How do I calculate generator size?
Starting watts needed = running watts of everything on + the largest single starting surge; add 20 percent headroom. Size a portable generator so a starting motor does not trip it. At the worked-example inputs the minimum generator size (watts) is 5,000.
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