Energy Fundamentals

Units and Conversions

• Watt (W): Rate of energy use/generation (power)
• Watt-hour (Wh): Energy used over time. 1W for 1 hour = 1Wh
• Kilowatt-hour (kWh): 1000 Wh. Standard unit for electricity billing.
• Joule (J): SI unit of energy. 1 Wh = 3600 J
• BTU: British Thermal Unit. Heat energy. 1 BTU ≈ 0.29 Wh
• Horsepower (HP): 1 HP ≈ 746 W

Power (W) = Voltage (V) × Current (A)

Energy (Wh) = Power (W) × Time (h)

Typical Power Requirements

• LED light bulb: 5-15W
• Phone charger: 5-20W
• Laptop: 30-100W
• Refrigerator: 100-400W running (surge higher at startup)
• Well pump: 500-1500W
• Electric water heater: 3000-4500W
• Electric stove burner: 1500-3000W
• Window AC unit: 500-1500W
• Space heater: 1500W (standard)
• Power tools: 500-2000W

US household uses ~30 kWh/day (900 kWh/month). Off-grid can function on 3-10 kWh/day with conservation and efficient appliances.

How Solar Works

1. Solar panels (PV): Convert sunlight to DC electricity. Typical panel: 100-400W, 15-22% efficiency.
2. Charge controller: Regulates power to batteries. Prevents overcharge. MPPT controllers are more efficient (~15-30% better).
3. Batteries: Store energy for use when sun isn't shining. Lead-acid (cheaper, shorter life) or lithium (expensive, longer life).
4. Inverter: Converts DC battery power to AC for standard appliances. Pure sine wave for sensitive electronics.

Sizing a System

Step 1: Calculate daily energy need (Wh/day). Add up all devices × hours used.

Step 2: Determine solar array size. Divide Wh/day by peak sun hours (3-6 depending on location) to get Watts needed. Add 20-30% for inefficiency.

Step 3: Size battery bank. Wh/day × days of autonomy (1-3 typical) ÷ depth of discharge (50% for lead-acid, 80% for lithium) = battery Wh needed.

DIY Example: Need 2000Wh/day, 4 peak sun hours, 1 day autonomy, lead-acid batteries.
Solar: 2000Wh ÷ 4h × 1.25 = 625W of panels (say 3 × 200W panels)
Battery: 2000Wh × 1 ÷ 0.5 = 4000Wh (say 4 × 100Ah 12V batteries)

Panel Types

• Monocrystalline: Most efficient (18-22%), most expensive, black color. Best for limited space.
• Polycrystalline: Less efficient (15-18%), cheaper, blue color. Good value for space available.
• Thin-film: Least efficient (10-13%), flexible, works better in shade/heat. Special applications.

Source: Panels from solar suppliers, Amazon, Home Depot. Price has dropped to $0.50-1.00/Watt for panels alone. Complete systems $2-4/Watt installed.

Panels last 25-30 years with minimal degradation (~0.5%/year). Inverters and batteries need replacement (5-15 years depending on type).

Installation

• Orientation: South-facing in northern hemisphere, north-facing in southern. Tilt angle ≈ latitude for year-round production.
• Mounting: Roof-mounted (use existing structure), ground-mounted (adjustable angle, easier maintenance), pole-mounted (tracking possible).
• Shade: ANY shade kills production. Panels in series - one shaded panel affects all. Microinverters or optimizers help.
• Wiring: Use UV-resistant cable, proper gauge for current and distance. Fuses/breakers on all circuits.

Solar panels ALWAYS produce power when light hits them. Cover panels when working. DC electricity can arc and start fires if improperly connected.

How Wind Works

Wind spins turbine blades → turns generator → produces electricity. Power increases with cube of wind speed - double the wind = 8x the power.

Power (W) = 0.5 × ρ × A × v³ × Cp

Where: ρ = air density (~1.225 kg/m³), A = swept area (m²), v = wind speed (m/s), Cp = efficiency (~0.35 max)

Wind Reality

• Small turbines (400W-2kW): Often disappointing. Need steady 15+ mph winds. Output much lower than rated.
• Medium turbines (2-10kW): Can be worthwhile with good site. Tower height critical.
• Large turbines (10kW+): Significant power but expensive, need permits, maintenance.

Most residential sites have insufficient wind for wind to make sense. Solar usually better investment. Wind best as supplement in windy locations.

Wind Requirements

• Average wind speed 10+ mph minimum, 12+ mph good, 15+ mph excellent
• Consistent wind (not gusty)
• Clear approach (no trees/buildings for 200+ feet upwind)
• Tower height at least 30ft above anything within 300ft

DIY: Savonius (vertical axis) turbines easier to build, lower efficiency. Homemade from 55-gallon drums or PVC pipe. Good for pumping water, battery trickle charging. Not primary power.

How Hydro Works

Flowing water turns turbine → turns generator → produces electricity. Most consistent renewable if you have suitable water source.

Power (W) = Head (m) × Flow (L/s) × 9.81 × Efficiency (~0.6-0.8)

Head = vertical drop. Flow = water volume per time.

Hydro Requirements

• Head: Minimum 3-5 feet for low-head systems. 10+ feet for standard. More is better.
• Flow: Minimum ~20 gallons/minute for small systems. 100+ gpm for meaningful power.
• Consistency: Year-round flow ideal. Seasonal streams require battery backup for dry periods.

DIY: Simple impulse turbine from PVC pipe and car alternator. More sophisticated: Pelton wheel (high head, low flow), crossflow (medium head/flow), or Kaplan (low head, high flow). Search "DIY hydroelectric generator."

Hydro Advantages

• Runs 24/7 (unlike solar)
• Small systems can produce significant power
• Battery bank can be smaller (constant charging)
• Long equipment life with maintenance

Generators

Internal combustion engine → turns generator → produces electricity. Reliable but requires fuel.

Types:

• Gasoline: Common, cheap, fuel degrades in 6-12 months. 2-5kW portable, larger stationary.
• Propane: Stores indefinitely, cleaner burning. 2-10kW common. Need large tank.
• Diesel: Most efficient, longest lasting engine, fuel stores 1-2 years with treatment. 5-20kW+ common.
• Natural gas: If pipeline available, unlimited fuel. Doesn't work if grid down (pipelines need electricity).
• Dual/tri-fuel: Runs on multiple fuel types. Flexibility.

Source: Honda (quiet, reliable), Generac, Champion. Portable $500-2000. Whole-house standby $3000-10000+ installed.

DIY: Car alternator + small engine = generator. Lawnmower engine, alternator, battery for field excitation, inverter. Low efficiency but works.

Fuel Storage

• Gasoline: 6-12 months with stabilizer. Store in approved containers, rotate stock.
• Diesel: 12-24 months with biocide and stabilizer. Algae grows in diesel.
• Propane: Indefinite storage. Tanks 20lb (grill) to 500+ gallon. Never goes bad.
• Rotation: Use oldest fuel first. Add fresh stabilizer when storing new fuel.

NEVER store fuel inside living space. Fire hazard. Approved containers only. Ventilation critical.

Battery Types

• Flooded lead-acid (FLA): Cheapest per Wh. Require maintenance (water). Vent hydrogen gas. 3-7 year life.
• Sealed lead-acid (AGM/Gel): No maintenance, can't spill. More expensive. 4-8 year life.
• Lithium iron phosphate (LiFePO4): Most expensive upfront. 80% depth of discharge (vs 50% lead-acid). 10-15 year life. No maintenance. Safer than other lithium chemistries.
• Nickel-iron (NiFe): Edison batteries. Extremely long life (20-50 years). Low efficiency, high maintenance. For permanent installations.

Source: Lead-acid from auto parts, battery stores. Lithium from solar suppliers, Amazon. Golf cart batteries (6V) often best value for FLA systems.

Battery Care

• Never fully discharge: Lead-acid: don't go below 50%. Lithium: don't go below 20%.
• Temperature: Batteries hate extremes. Ideal 50-80°F. Cold reduces capacity, heat shortens life.
• Equalization: Periodic overcharge for flooded lead-acid. Mixes electrolyte, desulfates plates.
• Watering: Flooded batteries need distilled water. Check monthly.
• Clean terminals: Corrosion increases resistance. Baking soda + water for cleaning.

Human Power

Healthy human: 75-100W sustained, 200-400W for short bursts, 1000W+ for seconds.

• Bicycle generator: 50-150W while pedaling. Good for charging devices, small batteries.
• Hand crank: 10-30W. Emergency only. Very tiring.
• Treadle: Foot-powered. 50-100W sustained. Used historically for machinery.

DIY: Stationary bike + car alternator + battery. Or bike + DC motor (acts as generator when spun). Many plans online.

Human power for electricity is inefficient use of food energy. Better to use human power directly for mechanical tasks (pumping, grinding, etc).

Animal Power

Draft animals: Horse 1-2 HP sustained, ox 0.5-1 HP, donkey/mule 0.5 HP.

• Plowing: Primary historical use. Still viable for agriculture.
• Transport: Cart pulling. 10-20 miles/day.
• Stationary power: Treadmill or sweep (circular walking) to power machinery. Grinds grain, pumps water, runs saws.

DIY: Horse/ox-powered sweep mechanism connects to any rotating shaft. Used to run threshers, grist mills, butter churns historically.

Reducing Energy Needs

• Insulation: Best investment. Reduce heating/cooling loads 30-50%.
• LED lighting: 1/10th the power of incandescent.
• Efficient appliances: Energy Star rated. Modern fridge uses 1/4 of 20-year-old model.
• Line dry clothes: Saves 3-6 kWh/load.
• Wood heat: Renewable, doesn't require electricity.
• Propane appliances: Stove, fridge, water heater can run on propane, reducing electrical load.

Every Watt you don't use is a Watt you don't need to generate. Conservation is always cheaper than generation.