Track 4: Energy

22 diagrams.

BatteryTemperature

Where a battery lives matters. Most do best near room temperature; cold cuts capacity and can stop LFP charging, and heat shortens life.

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A temperature scale from about minus 10 to 110 degrees Fahrenheit, split into zones. Below 32 °F (0 °C), lithium iron phosphate batteries must not be charged unless they have a built-in heater or are kept warm, and lead-acid batteries lose a large share of their capacity. From 32 to 50 °F, it is still cold and capacity is lower. From about 50 to 80 °F (10 to 27 °C) is the range where most batteries do best; ratings are usually given at about 77 °F (25 °C). Above about 80 °F, heat shortens battery life for all types. A note says to check the maker's range for your battery.

CloudyWeekCharge

A battery through a cloudy week. Each cloudy day it refills less than it gives back at night, until the sun returns or a generator tops it up.

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A chart of battery state of charge over seven days, with illustrative values. On days 1 and 2, which are sunny, the battery drops at night and refills to full each afternoon. Days 3 to 6 are cloudy: each day it charges only a little, and the charge steps down day by day from about 90 percent toward a dashed floor near 20 percent, the lowest charge planned for, set by the depth of discharge. Early on day 6 a generator tops it up to about 70 percent before it reaches the floor. On day 7 the sun returns and it refills. A bracket from the start of the cloudy spell to the generator start is labeled days of autonomy, how long the battery carries the loads with little sun.

ExampleLoadList

An example load list. The bars show energy per day. The microwave has the biggest watts but runs briefly; the fridge and pump use the most energy.

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An example winter load list for a small off-grid cabin, with columns for watts, hours per day, days per week, and watt-hours per day, which is watts times hours times days per week, divided by 7. Fridge, which cycles on and off: 120 watts, about 8 hours of running a day, 960 watt-hours. Well pump: 1,000 watts, 1 hour, 1,000 watt-hours. Router and standby loads: 15 watts, 24 hours, 360 watt-hours. Microwave: 1,100 watts, a quarter hour, 275 watt-hours. LED lights: 50 watts, 5 hours, 250 watt-hours. Clothes washer: 500 watts, 1 hour, 3 days a week, about 214 watt-hours a day. Laptop and phones: 60 watts, 3 hours, 180 watt-hours. The daily total is about 3,240 watt-hours, or 3.2 kilowatt-hours. The fridge and the well pump together are more than half of it.

GeneratorPlacement

A running generator belongs outside, at least 20 feet from the house, with its exhaust pointed away from doors, windows and vents. Never in a garage or shed.

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A plan view, seen from above and not to scale. A house has an attached garage, doors, windows and a vent. A portable generator sits outside, at least 20 feet (6 meters) from the house and garage, as the CPSC advises, with its exhaust pointing away from the building. A generator drawn inside the garage is crossed out, and so is one inside a shed: never run a generator indoors, in a garage, basement, crawlspace or shed, even with doors or windows open. Inside the house, CO alarms with batteries or battery backup are marked. Notes say carbon monoxide cannot be seen or smelled and can kill within minutes, and fans and open doors do not make it safe.

GridTiedDayNight

A grid-tied system without batteries uses the grid like a battery, so it has no power of its own in an outage.

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Three small block diagrams of a grid-tied solar system with no battery. On a sunny day, solar power runs the house first, and extra power flows out to the grid; many utilities credit this, often called net metering, under rules that vary. At night there is no solar, and the grid supplies the house. In a grid outage, the inverter shuts off, even in full sun, so no power flows anywhere; this protects utility line workers. A hybrid system with a battery is the option for backup.

HeatPumpCop

A resistance heater turns each unit of electricity into one unit of heat. A heat pump uses that unit to move more heat in from outside, here three units in all.

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Two energy-flow sketches. Left, an electric resistance heater, such as a baseboard heater, with a coefficient of performance, or COP, of 1: one unit of electricity goes in and one unit of heat comes out. Right, a heat pump with a COP of 3: one unit of electricity goes in, two more units of heat are moved in from the outside air, and three units of heat come out into the house. A note says a heat pump's COP falls as the outdoor air gets colder, and cold-climate models are designed to keep a useful COP at lower temperatures.

LedVsIncandescent

Same light, very different energy. Energy is the area: watts tall, hours wide.

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Two area charts for bulbs that give about the same light, each on for 5 hours a night. The incandescent bulb draws 60 watts, so its rectangle is tall: 60 watts times 5 hours is 300 watt-hours. The LED bulb draws 9 watts, so its rectangle is short: 9 watts times 5 hours is 45 watt-hours. The LED uses about 85 percent less energy for the same light.

LossChain

Each step between the panels and the plug loses a little. Often only about 70 to 85 percent of what the panels make reaches the plug, so a kilowatt-hour saved at the plug saves more at the panels.

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A band of energy flows from left to right through six stages: panels, wires, charge controller, battery, inverter and plug. The band starts at 100 percent at the panels and gets a little narrower at each step, because each step loses some energy, mostly as heat, shown by small arrows rising off the band. At the plug, often only about 70 to 85 percent of what the panels made is left; the end of the band shows that range. A note says that saving 1 kilowatt-hour at the plug saves more than 1 kilowatt-hour at the panels.

MicroHydroHeadFlow

Micro-hydro power comes from head, the vertical drop, and flow, how much water passes. A small, steady source runs day and night.

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A side view of a hillside, not to scale. At the top, an intake takes water from a stream. A pipe carries it down the slope to a small turbine house at the bottom, and the water then returns to the stream. Head is the vertical drop from the intake to the turbine; net head is what is left after friction in the pipe, about 5 to 10 percent less for a first estimate. Flow is how much water passes, in gallons per minute. A box gives the DOE rule of thumb for small systems: net head in feet times flow in gallons per minute, divided by 10, is roughly the watts. Example: 100 feet times 20 gallons per minute, divided by 10, is about 200 watts, all day and night, about 4.8 kilowatt-hours a day.

MotorSurge

A motor draws a short surge as it starts. The inverter must handle that peak, but it adds almost no energy.

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A sketch of a pump motor's power over a few seconds, not to scale. Before it starts, it draws nothing. As it starts, power jumps for a second or less to the surge, or starting watts, often 2 to 4 times the running watts, and sometimes more. Then it settles to the steady running watts. The shaded area under the line is the energy used; the surge spike adds only a sliver. Notes say the inverter must be able to handle the surge, and that a load list records surge watts for motor loads.

PanelTilt

Panels face true south. A tilt about equal to your latitude works across the year; steeper favors the low winter sun and sheds snow; flatter favors summer.

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A side view of a ground-mounted solar panel facing true south, to the right, at an example latitude of 40 degrees north. The solid panel is tilted at about the latitude, 40 degrees from flat, which gives good output across the year. A dashed steeper position, about latitude plus 10 to 15 degrees, favors the low winter sun and sheds snow better. A dashed flatter position favors summer. The high summer noon sun and the low winter noon sun are shown to the south, with sunlight reaching the panel.

PeakSunHours

Ten hours of daylight can add up to only four peak sun hours. The curve and the box hold the same energy.

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A chart of sunlight strength through one day, from 7 in the morning to 5 in the evening, 10 hours of daylight. The real sunlight rises slowly in the morning, peaks around noon well below full strength, and falls in the afternoon; the area under this curve is shaded. A dashed box shows the same amount of energy as if it came at full test strength, 1,000 watts per square meter, for 4 hours around midday. The two areas are equal, so this day has 4 peak sun hours.

PowerAndEnergySizing

Power and energy size different parts. Loads running at the same moment set the inverter size; the day's watt-hours set the battery and the array.

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Two rows. Top row, power, in watts, meaning how fast electricity is used right now: a well pump, a microwave and a kettle running at the same time have watts that add up, and that total sets how big the inverter must be. Bottom row, energy, in watt-hours per day: the load list total, each load's watts times its hours, added up for the day, sets how big the battery and the solar array must be.

SeasonalSources

In many places, wind and streams are strongest when solar is weakest. Pairing them with solar can shrink the winter gap.

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A chart of relative output by month, a common pattern rather than data for any one site. Solar output is low in winter, rises through spring and peaks in June and July. Wind is often strongest from late fall through early spring and weakest in summer. Stream flow for micro-hydro often peaks in spring, stays fair in winter, and drops in late summer and early fall; streams can also freeze. A shaded winter band shows where wind and hydro can fill in for solar.

SizingWorkflow

The sizing path: a load list gives daily kWh for the worst month, which sets the array, the battery and the generator hours. Then a licensed installer turns it into a design.

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A flow diagram of the planning steps. Start with winter and summer load lists. They give daily kilowatt-hours; off grid, use the worst month. If the numbers feel too big, a loop arrow goes back to the load list: cut loads first. From daily kilowatt-hours, three results follow. Array size in kilowatts: daily kilowatt-hours divided by the sun hours for the worst month times the derate. Battery size in kilowatt-hours: daily kilowatt-hours times the days of autonomy, divided by the depth of discharge. Generator hours: to cover long cloudy spells and keep the rest smaller. All three go to a licensed installer, who turns this planning estimate into a real design.

SystemTypes

The three system types as blocks. What changes is whether there is a battery and a grid connection, and so what happens in an outage.

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Three block diagrams side by side, showing power flows only, not wiring. Off-grid, with no utility connection: solar panels feed a charge controller, which charges a battery bank; an inverter-charger turns battery power into household power for the house loads, and a backup generator can charge the battery through the inverter-charger. In an outage there is no grid to lose; it keeps running from the battery. Grid-tied, usually with no battery: solar panels feed an inverter, which supplies the house loads; power flows both ways between the house and the utility grid. In an outage it shuts off, even in full sun, to protect line workers. Hybrid, a grid connection plus a battery: solar panels feed a hybrid inverter, which charges and draws from a battery bank, supplies the critical loads, and exchanges power with the utility grid. In an outage the critical loads keep running from the battery. A note says every system needs an electrical permit and inspection and a licensed installer, and grid-connected systems need utility approval.

TapAndBucket

Power is how wide the tap is open; energy is how much water ends up in the bucket. A small load for a long time can use as much energy as a big load for a moment.

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Two taps fill two identical buckets to the same level. On the left, the tap is open a little, a thin stream, standing for low power used for a long time: a 10 watt LED bulb on for 5 hours, which is 10 watts times 5 hours, or 50 watt-hours. On the right, the tap is wide open, a thick stream, standing for high power used for a short time: a 1,000 watt kettle for 3 minutes, which is 1,000 watts times 0.05 hours, also 50 watt-hours. A dashed line joins the two water levels, marked same energy, 50 watt-hours. The width of the stream is power, in watts. The water in the bucket is energy, in watt-hours.

TransferSwitchConcept

The safe way to power house circuits from a generator: through a transfer switch or interlock that connects only one source at a time. Backfeeding through an outlet is never safe.

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A block diagram of the concept, not a wiring diagram. The utility grid and a generator both lead to a transfer switch or approved interlock, installed by a licensed electrician, which connects only one source at a time. From it, power goes to a critical-loads panel that feeds the circuits you most need, such as the fridge, the well pump and a few lights. A crossed-out path shows a generator plugged into a wall outlet with a homemade cord, called backfeeding: never do this. It can send power out to utility lines and kill line workers, and it can overload house wiring and start a fire.

UsableCapacity

Usable energy is the nameplate times the depth of discharge. A 10 kWh bank used to 50 percent depth of discharge gives about 5 kWh.

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A battery drawn as a long bar with a nameplate capacity of 10 kilowatt-hours. A scale below shows state of charge from 0 percent at the left to 100 percent at the right. The right half, from full down to 50 percent, is shaded as the part you use: a 50 percent depth of discharge, about 5 kilowatt-hours. The left half is kept in reserve and not used. A note says that deeper use gives more usable energy each cycle but wears the battery out sooner, so cycle life is shorter.

WindTowerHeight

Wind near the ground is slow and choppy. DOE's rule of thumb puts the bottom of the blades at least 30 feet above anything within about 300 to 500 feet.

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A side view, not to scale. On the left, trees and a house break up the wind near the ground, making it slow and choppy, shown by wavy arrows. Higher up, the wind is smoother and faster, shown by straight arrows. On the right, a small wind turbine stands on a tall tower. A dashed line marks the top of the tallest obstacle nearby. The bottom of the turbine blades is at least 30 feet (9 meters) above that line, for anything within about 300 to 500 feet (90 to 150 meters) of the tower, following the DOE rule of thumb. A note says tower design and raising need a structural engineer and an experienced installer.

WinterShade

A tree that is harmless in June can shade the array in December, when the low sun casts long shadows.

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A side view, not to scale, with north to the left and south to the right. A ground-mounted solar array faces south. A tree stands some distance to its south. The high June noon sun casts a short shadow that ends well before the array. The low December noon sun casts a shadow several times longer, which reaches across the array. Shade on part of a panel can pull down the output of the whole string, so check the site at different times of day and year.

WinterSummerGap

The same array through the year. Off grid, the winter months set the size; in summer the array makes far more than the house uses.

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A bar chart of daily energy from one example array, a 2 kilowatt array with an 80 percent derate, month by month, with illustrative sun hours. It makes about 3.2 to 3.5 kilowatt-hours a day in December and January and about 8.6 in June. A line shows daily use, a little higher in winter, about 4 to 4.3 kilowatt-hours, and about 3.3 in summer. In November, December and January the bars fall short of the line, shaded as the winter gap: size for this, or plan generator hours. From spring to fall the bars are well above the line, a summer surplus that is often unused off grid unless you add summer loads.