Before anyone thinks about installing a solar photovoltaic system in their house, they should be thinking about improving the building envelope. That means no leaky, drafty and bad insulation house. Hopefully, no HVAC system in a hot or cold attic either. A tight building envelope is were the best return on investment is, ROI. A tight building envelope, low heating and cooling loads, energy star appliances and lighting, is easy to achieve on a new house, but could be difficult and expensive depending on how bad the existing house is.
THE GOAL FOR NEW OR EXISTING HOMES IS TO HAVE THE LOWEST ENERGY DEMAND POSSIBLE
TO INSTALL THE SMALLEST PV SYSTEM POSSIBLE
Several other factors are needed to be looked at before making your selection on which PV system is best for your home; roof orientation and space, angle, system cost, efficiency and carbon offset. Typical efficiency for regular silicone PV panels is between 15-23%. I happen to like SunPower systems with 20-23%% efficiencies, depending on model, and great warranty. SunPower Equinox is the only home solar + storage system designed by one company. With today’s PV systems cost, it makes no sense to install solar water heater systems.
In December 2020, Congress passed an extension of the ITC, which provides a 26% tax credit for systems installed in 2020-2022, and 22% for systems installed in 2023. (Systems installed before December 31, 2019 were eligible for a 30% tax credit.) The tax credit expires starting in 2024 unless Congress renews it.
SIZING A SOLAR SYSTEM
With our houses, an Energy Rater energy efficiency analysis of your house will determine the projected daily or yearly kWh requirement. A PV system retailer uses it to divide by your peak sun-hours to calculate the kW output needed. Then he’ll divide the kW output by the panel’s efficiency to get the number of solar panels needed for your system.
The PVWatts Calculator estimates the energy production and cost of energy of grid-connected photovoltaic energy system. It allows homeowners and industry to estimate the performance of a potential PV installation. See the National Renewable Energy Laboratory’s Calculator
Additional factors to consider:
- Solar resources, or the amount of kWh/m²/day you have in your area.
- Energy storage, as in batteries.
- System availability options, vendors, installers, net metering, and grid connectivity.
- Zoning and permitting regulations.
- Environmental impacts.
- Tax credits and incentives.
Assuming you have a PV panel system of the same solar efficiency, the production effectiveness of a system has more to do with the amount of “sun hours”, or solar resources, a geographical area gets during the year, and how much cloud cover, especially in winter. The Northern states receive less sun hours than the South, and even lesser sun hours than the Southwest, like AZ, NM, and TX. During the summer months, the entire USA receives abundant amounts of Global Horizontal Irradiance (GHI).
The amount of solar radiation per day is measured in kWh/m²/day. The PNW and the Great Lake areas, receive less solar radiation because they get more cloud cover during winter, denser and heavier air, with higher moisture, stop more sun rays. On average, the far Northern regions of the USA get less than 4kWh/m²/day, whereas the far Southwest of the USA gets more than 5.75 kWh/m²/day. As you move towards the Southwest, where the air is dryer and there is less clouds, the solar radiation penetration increases. See: NREL Solar Annual GHI
All this means is that, if you are planning to produce a percent amount of you total energy needs of kWh per year for a 2,000 ft² house in Seattle or Buffalo, you could need up to 50% more PV panels than if the same house is located in Tucson, Las Vegas or Albuquerque.
If anyone wants to geek-out and explore a plethora of solar resource data, tools, and maps, check NREL website. It’s all FREE information.
ALL-IN-ONE SOLAR SYSTEMS
I’ll start out by saying that for most grid-tied PV systems, battery back-up makes no economic sense in the Southern half US. In other words, the ROI is not there yet, for most folks. In northern climate zones, a lot depend on grid infrastructure. Older municipalities with overhead powerlines have many problems in cold weather, and when power gets disrupted, it can be for a long time. Also, in the South, the price of grid-tied electricity is relatively inexpensive in most of the southern half of the US.
Battery backup always makes sense when the house is built in a non-grid area, and/or bringing grid-power to the site costs too much.
Batteries last between 5-15 years. Expect to replace them once or more times during the 25-30+ year PV system lifespan.
Typical Equipment in a residential PV system:
Solar Panels – Monocrystalline panels are made from crystalline silicon.
- Racks – Racking systems allow mounting of panels to the roof.
- AC Inverter (string, micro inverters or power optimizers) – Inverters convert the direct current (DC) produced by the solar panels into the alternating current (AC) that homes use.
- AC Battery Charger – A standalone charging unit that plugs into a standard power outlet (or a generator), to replenish a battery bank that is low on charge.
- Automatic Transfer Switch (when grid power is available) – Is used to power your devices until the solar battery is low, then it will switch to grid power or batteries.
- Circuit Breakers – In addition to fuses, protection of photovoltaic modules is provided by string circuit-breakers. They protect photovoltaic modules from fault currents.
- Solar Monitoring – It allows you to see how much power is generated per hour, day or year.
- Batteries – The majority of new home energy storage technologies use some form of lithium ion chemical composition. Lithium ion batteries are lighter, more compact, and have a higher depth of discharge and longer lifespan compared to lead acid batteries. However, lithium ion batteries are more expensive than lead acid batteries.
- Net Meter (if available) – Net Energy Metering, NEM, is a solar incentive that allows you to store energy in the electric grid. When your solar panels produce more electricity than you need, that energy is sent to the grid in exchange for credits. Then, at night or other times when your solar panels are not or under producing, you get energy from the grid and use these credits to offset the costs of that energy.
SunPower Equinox is a one company design and engineered all around system, with the typical great warranty of SunPower. This all-in-one packaged system adds storage when backup power is needed during an outage, and also helps reduce peak-time charges by utility companies.
ROOF DESIGN FOR SOLAR PANEL SYSTEM
This guidelines apply to roof mounted Solar Panels. These systems have been installed for a while, usually installed over a shingled roof, increasing the likelihood of premature leaks, shortening the short life of most roof systems. Metal, Tile and Membrane roofs are a good option with some house designs.
- Study NREL & NRCA Guidelines for Rooftop-mounted Photovoltaic System Installations.
- Not all PV systems are the same. Contact your systems manufacturers for design guidelines and recommendations.
- Integrate system design between house designer, framer, roofer, electrician and PV contractor.
- Design roof pitch and orientation for maximum solar efficiency at your location.
- South facing panels produce 10-20% more electricity year-round than West facing PV.
- Design trusses or framing members with additional 10-25 psf dead load.
- There is no need to increase dead load for a roof if using solar shingles or thin-films.
- If panels are exposed to high winds and/or seismic zones, consult with a Structural Engineer in your area.
- Install clear area, roughly double the total panel area to accommodate fire clearance codes.
- Roofing material must have economic life for 25-30+ year’s warranty. Metal, copper, tile and slate are usually 50+ year roofs. Think of reroof before installing PV, costs of reroof once PV is installed go way up.
- No shade is better. Using micro-inverters instead of string inverters help increase panel performance between 4-14%, per NREL.
- Install additional bracing
- Install 5/8”-3/4” roof sheathing, screwed.
- Many mounting solutions that penetrate or not the roofing materials, like flashing mounts, clamps, racks, tilt-modules, or rail-based mounts
- Check your codes with your local building and fire departments.
- Panels have minimum distances from ridges, valleys, edges, penetrations, skylights, hatches, ventilation hatches, and roof access pathways.
- Check panel height and clearance for snow accumulation.
- Panels may not be installed reflecting neighbors fenestration or house cladding, like vinyl siding.
- You can’t make a neighbor cut a tree down to install your PV array.
PITCHED ROOF – Metal
- Standing seam is most compatible of all roofing systems for PV installations
- Requires pitches greater than 2/12.
- Cost savings, sturdiness and durability are best attributes.
- Clamps makes it easier to grab seams and install racks or rail-based mounts.
- Saves petroleum based old shingles from going to the city dump.
PITCH ROOF – Asphalt Shingle
- Well detailed installation is crucial. Too many penetrations.
- Requires pitches greater than 2/12.
- Asphalt shingle roofs lifespan of 20 years is much shorter than a PV system lifespan. So, reroofs are common, with high expenses.
- Most all reroofs require removing and reinstalling the PV array, adding costs.
- Most shingle roof manufacturers do not cover PV system damages.
- Penetrations to the roofing system create moisture problems, and add to shorter roof life.
PITCHED ROOF – Clay, Slate or Concrete Tile
- Expensive, but cost can be offset by long life of tile roofs, up to 100 years.
- Requires pitches greater than 4/12.
- Very heavy. Add 15-25 psf minimum to roof structural design.
- Tiles are brittle. Many broken tiles delivered and many are broken on installation, plan to order additional tiles.
- Difficult installation. It requires expert and experienced installers.
- I’ve found it works best to install the “Rack Area” with standing seam metal or asphalt single roof and the rest in tile, called “strip and go” installation.
PITCH ROOF – Solar Shingles
- Requires pitches greater than 2/12.
- Fairly new technology made to integrate with regular shingles or as stand alone, but still unrefined and have many issues to work out. Stay tuned!
- Light weight and easy to install but costs are high.
- More aesthetically pleasant than solar panels, especially on the front elevation.
- Solar efficiency is around 10-12%. Some Mono-Si cell have efficiency of 15-20%, but are very expensive.
- Life expectancy is between 20-30 years, far less than a solar panel on a metal roof.
FLAT ROOFS – Roof Membranes
- Requires pitches greater than .25/12.
- Install membrane with higher mil thickness for protection.
- White thermoplastics TPO, adhered and highly reflective membrane provide higher heat resistance and UV protection. It increases efficiency by 10%-15%, compared to dark membranes, like EPDM, with low reflectivity.
- Installing 2 layers, staggered and taped, high compression rigid foam and cover board under TPO.
- Ballasted systems are most common PV installs. Easier, less costs, but must protect roofing from metal and ballast systems, like concrete blocks, etc.
- Be careful that ballast trays can block and/or inhibit proper drainage.
- Non-ballasted systems shall be attached observing good moisture management with non-penetrating membrane attachment systems, like the U-Anchor by Anchor Products.
- Panels may need to be install several feet away from parapets. Check parapet height for shade.
FILM LAMINATE PV SYSTEMS
- Thin-film solar PV systems are technology that aims to simplify installations. Light-weight laminates, that install by peeling a backing membrane and sticking the thin-film PV to a standing seam roof.
- Efficiency for thin-film modules are between 11-17%, so roof space is at a premium.
- Lab testing shows faster significant degradation compared to conventional PV panels.
- Hard to find in the USA.
- I’ve no experience with these systems, so I can’t say much more.
GROUND-MOUNT PV SYSTEMS
- Standard ground-mount use a steel or wood frame on concrete slab, footings or piers to install panels. Usually they are stationary.
- Some metal standard mounts can be adjusted manually for season efficiency.
- Pole-mount systems support several panels and usually have tracking systems.
- Tracking systems maximizes solar productivity, usually 25% more, so less panels are required, but are more expensive.
- Single-axis systems track the sun throughout the day as it moves through the sky.
- Dual- axis systems track the sun seasonal variations of the sun position.