Chapter 17 Exercises: Solar, EV Chargers, and Whole-Home Generators

These exercises build the analytical and practical skills needed to make informed decisions about modern electrical additions. Several involve calculations; bring a calculator or use a spreadsheet.

Exercise 17.1: Solar Payback Calculation for Your Home

Objective: Calculate the approximate payback period for a solar installation at your specific location.

What you need: Your most recent 12 months of utility bills, calculator or spreadsheet, internet access to NREL PVWatts Calculator

Steps: 1. From your utility bills, determine: - Annual electricity consumption in kWh (sum of 12 monthly bills) - Average electricity rate (total annual cost ÷ total annual kWh) 2. Go to pvwatts.nrel.gov. Enter your address. 3. Using your annual consumption and 4.5 peak sun hours as a starting assumption (adjust based on PVWatts' solar resource data for your location), calculate the system size that would cover approximately 90% of your annual consumption: System size (kW) = (Annual kWh × 0.90) ÷ (365 days × peak sun hours × 0.80 derate) 4. Estimate installed cost: multiply your system size (kW) by $3.00/watt for a current market estimate. (e.g., 7 kW × $3,000/kW = $21,000) 5. Apply the 30% ITC: multiply installed cost by 0.70 for after-credit cost 6. Calculate simple payback: after-credit cost ÷ annual savings (annual kWh covered × electricity rate)

Reflection: Is your payback period under 12 years? Over 15? What would change if your electricity rate went up 3% annually? Does your roof orientation or shading significantly affect the estimate?

Exercise 17.2: Net Metering Research

Objective: Determine your utility's actual net metering policy.

What you need: Your utility bill (shows utility name), internet access

Steps: 1. Find your utility's website. 2. Search for "net metering," "solar interconnection," or "distributed generation policy." 3. Determine: - Does your utility offer net metering? - At what rate does it credit excess production? (Retail rate? Avoided cost? Fixed credit?) - Is there a capacity limit on your system size for net metering eligibility? - Is there a waiting period or application process? 4. If the website is unclear, call the utility's customer service and ask directly: "I'm considering solar. What is your current net metering rate for residential customers?"

Why this matters: The difference between full retail net metering ($0.15–0.32/kWh credit) and avoided cost net metering ($0.03–0.06/kWh credit) can change a solar system's effective payback period by 5–8 years.

Exercise 17.3: Solar Quote Evaluation

Objective: Evaluate a solar installer's quote critically using independent data.

What you need: A solar quote (either a real one you've received, or a sample from a solar installer's marketing material), your PVWatts calculation from Exercise 17.1

Steps: 1. From the quote, identify: - System size (kW) - Estimated annual production (kWh) - Installed price before incentives - Projected payback period 2. Compare the installer's production estimate to your PVWatts estimate for the same system size. Do they agree within 10–15%? Large discrepancies (installer's estimate is 20%+ higher than PVWatts) suggest optimistic projections. 3. Calculate price per watt: installed price ÷ system size in watts. Is it between $2.50 and $4.00/watt? (Below $2.50 may indicate quality shortcuts; above $4.00 may indicate overpricing for your market.) 4. Verify the payback calculation yourself using your numbers from Exercise 17.1.

Red flags to note: Production estimates significantly above PVWatts, payback calculations that assume continuously rising electricity rates of 5%+/year, or quotes that include lease/PPA structures without clearly explaining that you don't own the system.

Exercise 17.4: Battery Storage Assessment

Objective: Determine whether battery storage makes financial or practical sense for your situation.

Steps: 1. Answer the following questions: - How many times per year does your power go out for more than 4 hours? (0–1 per year suggests backup power value is low; 4+ suggests meaningful value) - Do you have medical equipment requiring continuous power? (If yes, backup value is high regardless of outage frequency) - Does your utility have time-of-use (TOU) rates with a significant peak/off-peak differential? (Check your bill or call your utility) - What is the current installed cost of one battery unit (Tesla Powerwall or equivalent) in your area? 2. If you're also installing solar: calculate how many kWh of typical evening consumption a battery would need to cover (your consumption from 5pm–midnight, approximately) 3. Apply the 30% ITC to your battery cost estimate 4. Based on your TOU differential (if applicable) and outage frequency, estimate what the annual financial value of battery storage would be

Honest assessment: For most grid-reliable locations without TOU pricing, battery storage payback exceeds 15–20 years on financial merit alone. The decision to add batteries is often driven by backup power value (a non-financial consideration) rather than pure economics.

Exercise 17.5: EV Charging Level Assessment

Objective: Determine whether Level 1 charging is adequate for your driving patterns, or whether Level 2 is necessary.

What you need: If you have an EV: recent driving log or odometer readings. If evaluating for future purchase: your typical daily mileage.

Steps: 1. Determine your average daily driving distance (miles). 2. Calculate Level 1 charging recovery: daily miles ÷ 4 miles/hour = hours of charging needed. 3. Do you typically have that many hours between arriving home and next departure? - If you arrive home at 6pm and leave at 7am (13 hours available), and need only 8 hours of Level 1 charging: Level 1 is likely adequate. - If you regularly need more charging hours than you have, Level 2 is warranted. 4. Calculate Level 2 cost: estimate $1,000 installed (typical for a straightforward garage installation). How long would the convenience benefit take to "pay back" given your driving patterns?

Additional consideration: Level 1 may be inadequate during cold weather (below 32°F), when battery charging slows and heating systems draw more range. If you live in a cold climate and drive frequently in winter, Level 2 is a stronger recommendation.

Exercise 17.6: Generator Load Calculation

Objective: Calculate the correct generator size for your critical loads.

What you need: List of appliances you'd want to run during a power outage, the load table from Section 17.5

Steps: 1. List every appliance you want to run during a power outage (be realistic — include essentials: refrigerator, well pump or water, heating, communications, medical devices, basic lighting). 2. For each appliance, note: running watts AND starting watts (from the table in Section 17.5 or the appliance's nameplate). 3. Calculate total running watts (all appliances running simultaneously). 4. Identify the single appliance with the highest starting watts. 5. Minimum generator size = total running watts + the single highest starting surge (since you won't start everything simultaneously). 6. Round up to the next standard generator size (3,500W, 5,500W, 7,500W, 10,000W, 14,000W, etc.).

Example calculation: - Refrigerator: 200W running / 800W starting - Well pump (1HP): 750W running / 2,800W starting - Gas furnace fan: 600W running / 800W starting - Lighting: 400W running / no surge - Phone/laptop chargers: 100W running / no surge - Total running: 2,050W - Highest starting surge: well pump at 2,800W - Minimum size: 2,050W + 2,800W = 4,850W → select 5,500W or 6,500W generator

Exercise 17.7: Carbon Monoxide Detector Audit

Objective: Verify adequate CO protection throughout your home.

Steps: 1. Locate every CO detector in your home. 2. For each detector: - When was it last tested? (Press the test button now — it should alarm) - When was it installed or last replaced? (CO detector sensors wear out in 5–7 years; check the manufacture date on the back) - Is it on every level of the home? 3. Required CO detector locations: every level of the home (including basement), outside every sleeping area. 4. Test every CO detector right now. Replace any that don't respond, any that are over 7 years old, or any that are missing from required locations.

Why this matters for this chapter specifically: Portable generator CO poisoning claims dozens of lives annually. A working CO detector is your warning system if generator exhaust migrates into the house despite outdoor placement. It is not optional.

Exercise 17.8: Research Your State's Solar Incentives

Objective: Identify any state or local incentives that supplement the federal ITC for solar installations.

What you need: Internet access

Steps: 1. Go to dsireusa.org (Database of State Incentives for Renewables & Efficiency — the authoritative national database of state and utility incentive programs). 2. Select your state. 3. Look for: state income tax credits, property tax exemptions for solar equipment, sales tax exemptions on solar equipment, utility rebates. 4. For each incentive found, note: the incentive amount or percentage, whether it stacks with the federal ITC, and any income or system size limitations.

Sample outcome: In some states, state incentives add 10–25% on top of the federal 30% ITC, dramatically improving solar economics. In other states, the federal ITC is the only incentive. Knowing which situation you're in is fundamental to accurate payback calculations.

Exercise 17.9: Evaluate Your Panel Capacity for Future Additions

Objective: Determine whether your current electrical panel can accommodate planned additions without an upgrade.

What you need: Access to your electrical panel (open the door only — don't touch any wiring), your circuit directory

Steps: 1. Open your panel door and count: - Total circuit spaces in the panel - Used circuit spaces (breakers installed, excluding the main breaker) - Empty/available spaces 2. Note your service amperage: usually on the main breaker (100A, 150A, or 200A). 3. For each planned addition, note the required dedicated circuit: - EV charger (Level 2): 50A dedicated circuit - Solar system: 30–50A dedicated circuit (check inverter specs) - Battery storage: 20–50A dedicated circuit - Hot tub: 50–60A dedicated circuit 4. Do you have enough empty spaces for your planned additions? Is your service amperage adequate?

If you're at or near capacity: Factor a panel upgrade into your planning. The cost of upgrading during installation of the first new system is much lower than upgrading twice.

Exercise 17.10: Portable Generator Readiness Check

Objective: If you own or plan to own a portable generator, evaluate your readiness to use it safely.

For those who own a generator: 1. Start the generator (outdoors, at least 20 feet from any opening). Does it start within 3 pulls? If it doesn't start reliably, service it before you need it in an emergency. 2. Check the fuel: is there fuel in the tank? Is it stale (more than 30 days old without stabilizer)? Drain and replace stale fuel; add stabilizer to fresh fuel. 3. Check the oil level. 4. Do you have a transfer switch or interlock kit installed? If not, note that safely connecting the generator to your home's wiring requires professional installation. 5. Where will you place the generator during use? Mark or photograph the location — at least 20 feet from any door, window, or vent. Can you run a heavy-duty extension cord from that location to your critical loads?

For those who don't own a generator but are considering one: Review the load calculation from Exercise 17.6 and the generator sizing table in Section 17.5. What size do you need? What is the approximate cost? Would a standby generator (automatic, propane or natural gas) or portable generator better suit your situation and budget?