Home Solar

This video explains what my solar project is:

Due to cold snap, the Permit To Operate was slipped to next week. Between the snow induced power outages and below freezing temperatures, everything got pushed off a week.

Bob Wilson

 

Some final paper work has held up the solar panels. But this is were we were Friday evening:

• 13 kWh battery - solar buffer for night.
• 16 kW natural gas fueled generator - handles any outage longer than the battery and solar panels can handle
• (16) 400 W microinverter solar panels - each panel produces split-phase, 240 VAC, pending final paper work
• 200 A, split-phase, 240 VAC, grid service - backup for low solar production and battery at 20%. Also takes any excess power produced, hopefully careful load management will make this very\, very small.
• Master Switch - mechanically disconnects house from grid.
• System Controller - coordinates all power sources feeding the house panel
• SPAN house panel (not shown) - circuit breakers for all load circuits and sheds non-critical loads to extend battery life

Snapshot during testing:


First solar panel test:


Charging the Tesla:



First EV charging result:

As I learn how to best operate, this will become more common. In this case, I had not configured the Tesla Universal Charger with the local grid cost per kWh.

Bob Wilson

 

I hired the contractor because they have experience designing and building multiple systems. They have paid the "learning curve" tuition. But looking at home-brew and my system, there are two conflicting architectures:

• individual solar panel AC inverters - each panel makes 240 VAC, split phase, and coordinate the AC frequency and phase with the other units. This means ordinary 120/240 VAC wiring and connectors for the rest of the system. Also, each panel inverter has its own solar-power controller to get the maximum output independent of the other panels that may be fully, partially, or not exposed. Regular code inspectors know how AC works as do ordinary electricians.
• large arrays of panels DC wired to single AC inverters - the 48 V panels are wired into a couple of groups with only one solar-power controller for each array. It turns out that in series, one shaded panel becomes a current limiter. In parallel, there will also be panels under producing. These DC bus systems are often wired with huge, often welder, cables, both uncommon and expensive. Then if the inverter goes down, you're toast until a replacement arrives. High power DC systems are not as common as regular house wiring AC systems.

The Saturday wind storms are here and the old tin shed is rolling itself in a ball. I heard a power pole fuse blow and the automatic 16 kW, natural gas generator is powering the house . . . the second time. There is a 30 second delay until the generator cames on. The wind is still blowing, hard.

Right in the center of the line of power outages East of Redstone Arsenal.

Bob Wilson

 

No damage visible but there will be ‘proof of operation’ when they come back for the final Permit To Operate.

Let me suggest, a couple of panels, ground mounted, with micro inverters for each panel. This will give 240 VAC, split phase or 120 VAC. Then feed a ‘luggable’ power block. Use that to handle some isolated loads like a UPS. Make sure it has grid charging.

The advantage of micro inverters is the rest of the circuit uses regulate house wiring instead of high current, DC. The micro inverters manage esch solar panel load for maximum production. Both parallel and series DC solar panels don’t optimize each panel production and one stinker screws the whole array.

Bob Wilson

 

May 31, 2025, close enough to the first full month of solar:

• My use: (293.5 kWh + 272.5 kWh) *0.17/kWh = $96.22 electricity savings
  • 79% of solar electricity used by me
  • 21% given "free" to the utility company

Electric car charging:

• 523 kWh * 4 mi/kWh = 2,092 miles of free EV driving
  • 2,092 miles / 52 mi/gal ~= 40.25 gallons of not bought Prius gas
  • $2.50/gal at COSTCO today * 40.25 gal = ~$100 gas savings over my previous Prius

The biggest change is developing new habits of using most of my home electricity once the solar panels start producing enough to match the load. This mean home "load shifting":

• shower hot water an hour after sun rise
• cloths washing and drying an hour after sun rise
• cooking while the sun shines
• EV charging 10 AM to sun set
  • maximum EV charge rate 20 A to match solar panel peak rate
• air conditioning afternoon
• better light and TV usage (turn off what is not in use)

There were two, unusual EV charging events that required drawing grid power. These were associated with testing and tuning my replacement Tesla battery. I won't need to do this again.

Based on the first month, it looks like I'll easily save, tax free, ~$1,000 per year in electricity costs which matches my expectation.

Bob Wilson

 

The experiment continues:

• The Tesla L2 charger runs about 85% efficiency
  • Battery charge rate, 16 A, does not include the 15% overhead
  • Battery cabin cooling with A/C runs about 1.1 kW
    • Fan only is about 600 W
• Maximum solar output is 20 A with any available charged battery 22 A
  • Tesla max battery rate 32 A means 38 A circuit current

My plan is to monitor the solar panel output with a current sensor and adjust the EV charger rate to 85% of that value. This will adjust the charging circuit load to follow the solar panel power yet preserve the solar battery charge. I'll probably use an average technique to handle passing clouds.

There will be an optional, "MAX" rate that will signal 32 A. This will take everything in order:

1. All solar panel output
2. All solar battery output
3. Grid power if needed (paying $0.17/kWh for speed)

There remains the question of interfaces:

• hard wired - will cause "angst" for electricians
• Wire-less or combinations
  • LED lights and buttons
  • Bluetooth
  • WiFi
  • Zigbee

Bob Wilson

 

Probably a good time to discuss charging and use metrics:

• North America electricity
  • 240 VAC to homes using split phase transformer, L2 voltage
  • 120 VAC one side of the split phase wall outlet
  • 208 VAC typical commercial voltage for commercial, three phase
  • 110 VAC two phases of three phase commercial wall outlet
  • Amps is universal for circuit breaker
    • Best practice is 80% of breaker rating to avoid heat stress
    • 12-15 A usable on wall outlets for 15-20 A circuit breaker
  • kW = Amps * VAC
• Amps (battery) - in the Tesla and BMW charging settings, the charge going into the battery.
• Amps (circuit) - the actual draw on the circuit including the charger inefficiency, ~85% at L2 voltages
  • 10 A (battery) / 85% efficiency = 11.8 A (circuit) the actual load on the circuit breaker
  • 16 A (battery) / 85% efficiency = 18.8 A (circuit), lowest BMW i3-REx charge rate
  • 20 A (battery) / 85% efficiency = 23.5 A (circuit)
  • 30 A (battery / 85% efficiency = 35.3 A (circuit), highest charge rate BMW i3-REx
  • 32 A (battery) / 85% efficiency = 37.6 A (circuit), highest charge rate Tesla
• kWh (charger) - the power unit for grid charging which may not include overhead but a home owner pays
  • Tesla chargers do not report the efficiency or cabin climate loss!
  • All other chargers report the actual grid charging, kWh
• Miles / kWh - a very useful metric from the BMW for trip planning
  • Wh / mile - Tesla's nearly useless metric shown while driving
    • 1000 / (Wh / mile) = Miles / kWh - converting Tesla to a useful trip metricmetric
    • 200 Wh / mile = 5 miles / kWh :: efficient configuration and driving for Tesla
    • 250 Wh / mile = 4 miles / kWh :: the EPA metric, 25 kWh/100 miles
    • 3.6 miles / kWh :: my measured BMW i3-REx efficiency, driving 63 mph (*)
• kWh/100 miles - the EPA metric given to each electric car
  • 25 kWh/100 mi - 2019 Tesla Model 3 Standard Range Plus (my car)
    • 4 miles / kWh = 100 miles / 25 kWh
  • 26 kWh/100 mi - 2019 Tesla Model 3 Standard Range
  • 30 kWh/100 mi - 2017 BMW i3-REx (my other car)
    • 3.3 miles / kW = 100 miles / 25 kWh

Sad to say, the three metrics for driving efficiency makes head-to-head comparisons and real world operations more confusing than they need to be. Of the different metrics, "miles / kWh" is the most useful for a clever driver. It is also how I evaluate tuning and driving tricks.

The EPA metrics are useful to compare stock cars when buying but otherwise ... not so much unless trying to tune the car for efficient driving. Careful because manufacturers are known to "sandbag" the numbers the wrong way like the "Standard Range" and "Standard Range Plus" Teslas.

Tuning and driving tricks:

• 50 psi - inflate the tires to maximum sidewall and instantly gain more steering authority and efficiency with lower rolling resistance.
• replace Tesla tires, "400 A A" wear rating with "600 A A" or higher - the longer wearing tires have lower rolling resistance. At 63 mph, there is little to no heat induced pressure rise from the tire pressure system (TPS)
• 63 mph - decades of Prius driving revealed this is the steady state, benchmark speed to give EPA results on any car. Higher speeds reduce efficiency and can lead to tire pressure (and temperature) rise, the mark of inefficiency.
  • If tire pressure increases, 2-3 psi, while driving, you are losing range, efficiency, and tire rubber
  • If tire pressure remains within 1 psi of the inflation pressure found in first mile, you are at a good speed
• Narrower and lower diameter tires - improves vehicle efficiency from lower tire drag. However, tire shops often have a "black book" scale of what tires can go on what cars. So I order my tires by lying about what car they go on and take the old tire and rims to the shop for new TPS sensors, mounting, and balancing.
• Alignment - tire shops use an "idiot proof" machine but the gold standard is tire temperature measured right after higher speed, typical driving. Race car drivers use this to tune the suspension and tire inflation for the track.
  • Tire alignment machines require using the door jam pressure so always test and inflate to maximum sidewall after they are done. Either ask or borrow an air hose to get it back to maximum sidewall pressure. If they do it for you, drop off a dozen hot donuts as a tip the next day.

Bob Wilson

 

Yes on the insurance but don't know about the property taxes. Insurance rate went up because fire, lightning and storm damage are always a possibility. Removing the tall trees took away natural lightning rods for more solar access. The panels are on the roof so high winds could cause damage, apparently hail not so much, As for fires, always a risk with or without a solar roof. However, there is a solar storage battery that poorly mounted in the sun could fail early or otherwise pose an additional fire risk.

Huntsville Utilities requires a higher insurance rate if the system is sized for "stand alone" or "off grid" power levels. However, mine was sized to "minimize grid $0.12/kWh charges" which is less than the utility threshold for extra insurance. I will pay much less for the little electricity I occasionally draw from the grid. But my first month, avoided at least $100 of EV charging cost.

The Huntsville permitting system is easily abused to delay the "permit to operate." I recommend you buy the permit and identify the contractor doing the work ahead of time. Home inspection often want the permit "posted in plain view." Don't be afraid to visit the utility customer office and force a "chat" with the powers that be. I lost February, March, and April solar panel operation thanks to the nitpicks in the inspection department followed by permit delays and extra costs.

Assume the home inspections and utility are deliberately going to throw procedural hurdles to block operation but counter by being a polite, "this home owner pays your salary", customer who wants to make sure things go OK. Yes, the inspector found one flaw in the circuit breaker panel which the contractor corrected (they didn't put a physical circuit breaker in on the EV charging line.) But insist on getting the inspector's identification and phone number and 'be interested' in what is going on. YOU CAN ALWAYS NEGOTIATE!!!

Bob Wilson