Solar System Design: What to Know Before You Start

1 / 2
Having a grid-tied solar system design allows you to sell excess energy back to the utility company, banking that energy for overcast days when your system may not be able to keep up with the house load.
2 / 2
Doug and Jennie Ostgaard designed and built their own solar power system, and in "DIY Photovoltaic Solar Power for Homeowners," they share the details of the design and build process as well as the wiring schematics and solar energy calculations they used for their DIY solar power system.

The sun’s energy is free and abundant, but planning a solar power system to utilize this energy can be a daunting task. DIY Photovoltaic Solar Power for Homeowners(CreateSpace, 2014), by Doug and Jennie Ostgaard, is a resource for any homeowner interested in custom solar system design. Plenty of photos and schematics illustrate the entire design and build process, along with the solar energy and electrical load calculations the Ostgaards used to determine the necessary scope of the project. The following excerpt is from chapter 3, “Designing for Solar Energy.”

Before beginning our DIY solar power system, an evaluation needed to be done to determine the amount of solar energy available at our site and our power requirements. We needed to calculate what size system would get us close to our target and be the most cost efficient with optimum payback period. There were many design requirements and options in PV systems that needed to be evaluated. For example, should we use wood or steel frames? We found it is best to layout questions and variables first. After that, we could proceed in a logical path forward to capture the best system for the lowest price and fastest payback.

Solar System Design Requirements:

Listed below are the basic design requirements we used for our system. While we will explain our requirements, we also include comments to you the reader — to help with your own requirements. We believe these requirements are best for a robust survivable system. With these requirements, even when the power grid goes down, the system will be up and running.

1: Reliable and renewable energy.

Renewable free energy was the key driver for us. The alternative was to use carbon fuel reserves whose price will skyrocket as dwindling supplies are exhausted. Using renewable resources also allows us to do our part to take care of the earth (Genesis 2:15).

2: Off-grid with grid-tied option.

Off-grid has no connection to power grid. If we were off-grid, we would need to produce all of our electricity.  Being off-grid would save us 11 cents per kWh which is the cost of purchasing our electricity from the local utility company. However, if we were on-grid we would receive all of our power from the power grid and sell our PV produced power to the utility company. The utility company would then subtract what we produced from what we purchased from them.

Originally we intended to do a mix with part of our home off-grid, and part of our home on-grid. However, the utility company representative explained that this was inefficient. With this mixed/partitioned system, in the summer we would have all the electricity we needed. However, any excess power would be wasted. In the winter we would not have enough power and would need to purchase electricity. He suggested we be totally on-grid with off-grid capability. Our excess power could then be banked at the utility company and withdrawn (from our power account) when our production was low and our demand was higher. We appreciated the explanation and decided to go with their recommendation.

An added benefit to being on-grid with off-grid capability was that if the power grid went down, we could quickly throw a transfer switch to go off-grid and use our own power stored in our battery bank. Being on-grid also made us candidates for Washington State tax free incentives which would pay us for producing electricity.

Based on our current analysis we’ve realized a savings per year of $950 in reduced energy cost from 2012 (just by being aware and conserving our energy usage); plus $985 for solar produced energy at 11 cents per kWh (our local utility rate); plus a Washington State incentive at 18 cents per kWh for all the green energy generated. This gave us about $1,615 (tax free). The net result of using on-grid with off-grid capability is $2,600 savings per year. With the incentives, we believe on-grid with off-grid capability was an excellent choice.

3: Determine large critical electrical loads.

Large critical electrical loads may have a loss of life impact. We are on a water well. It was critical that our deep well pump run, even when the power grid failed. We had two options to run the deep well pump. One was to use our PV system (using the underground wiring from our house to the well pump house). However, the PV system would only work for two days in some low solar power conditions (i.e. dark cloudy days) — because we would be running off of our own battery backup with two days of storage. Therefore we needed a secondary option. For this, we chose a diesel generator (which we house in a Faraday cage)

4: Identify off-grid loads.

House electrical loads such as water tanks, pool pump, etc. from our house subpanels A & B were identified and used to design the house wiring for the PV system.

Additionally, we identified what loads would be imperative to work should the power grid fail. These included light loads which were needed to maintain life year around. In the summer additional available solar power could expand this list. PV power is expensive to install, so it is not practical to oversize. It is better to reduce loads/needs.

5: Design solar power system to satisfy the following criteria:

5.1: To run light house loads all year long 365 days.

5.2: To run about 50% of yearly power bill.

The light load list we came up with shows 20 kWh are needed. Our 8 kW system will produce about 25 kWh (299 kWh / 12 = 25 kWh). Yearly estimates for our 8 kW system, was 299 kWh per year which is well over 244 kWh.

Every kW was and is significant — reducing electrical usage is an ongoing job. Our estimated power reduction amounted to 288 kWh a year from life changes.

6: Design battery back-up to last for two days.

Design your battery back-up system to store power in the event you experience two days with no sun. Your batteries should be sized to power 2,000 watts of your critical electric loads for at least 6 hours. Energy storage can be accomplished in a variety of ways. Since PV systems generate excess power on sunny days, this excess power can be stored in batteries. Lithium batteries are good, but expensive. Sealed 12 volt batteries are convenient but don’t have enough amp hours. Golf cart 6 volt batteries are good and inexpensive, but they do not have the cycle endurance of 6 volt marine batteries. Large lead acid unsealed 6 volt marine batteries offer an inexpensive deep cycle capability that should last for over ten years if regularly given an equalization voltage to de-sulfate.

Storage capability is necessary in order to have electricity available for emergency situations when the sun is down or the PV system is not producing. With storage capability, power is readily available even when the power grid fails or, of course, if one is choosing to be off-grid.

Note: If one is on a reliable power grid and has a reliable generator for back-up, it is possible to run on only one of the two battery banks connected in parallel. This would provide a 48 VAC and 370 Ah instead of 740 Ah — a savings of over $2,000.

7: Solar system design is a function of, and must address the following variables.

• Tree interference
• Air temperature
• Cloud cover
• Solar energy available at site
• Panel tilt angle and azimuth angle (should these be automated)
• PV panel efficiency, system efficiency, age degradation, debris on glass
• Battery storage system performance and maintenance — must match inverter and charge controllers. Overheating and freezing prohibited.

8: Add net meter and production meter with adjoining external disconnect switch to run on-grid in sell mode.

The utility company allows us to bank power with them to store excess summer power and to withdraw power if needed (such as in the cold winter months).

Washington State incentive payment is 18 cents for every green kWh we produce. The utility company manages these incentive payments and they send us an incentive check once a year. The incentive payment should generate about $1,600 and helps to get a short 8.5 year system payback.

Correct sizing is important in order to have excess power to bank for winter usage. We monitor our PV energy production using net and production meters from the utility company. The utility monitors these meters and provides us with an online chart that show us daily what we are producing and using.

9: Design life should be at least 25 years.

It is always a good idea to design a reasonable system lifetime so materials and parts will not fail prematurely and cause high maintenance issues. PV panels should have a lifetime of 50 plus years with some degradation of performance, but likely will not need replacement. If corrosion were an issue for us we would have used stainless steel and corrosion resistant materials, instead of aluminum.

10: Design for wind speed.

Wind speed of 80 plus MPH is a recommended standard for most locations. We designed for 40 MPH because our site is not windy due to 120 foot fir trees surrounding it. Your site may be in a location that requires a design for higher wind speeds.

11: Design should consider limited snow loads.

In winter our panels are rotated vertically and snow loads are not possible. The frame we use is basically a truss in vertical orientation and has a lot of inherent strength.

12: Must vent battery cabinet automatically with a backup system to protect from hydrogen explosion.

The cabinet fan system must run continuously due to the lead acid batteries. A fail safe fan with alerting system was installed for safety. We were aware that this was a critical inspection issue.

13: Plan on inverter noise, humming, fan noise, and heat requiring cooling.

Be aware that the inverter produces significant humming noise, there also is fan noise, and excess heat which requires cooling. Design a fan system capable of removing system heating.  We used a screened door on our cabinet to cool the battery cabinet room off on hot summer days.

14: Choose solar panels manufactured in the U.S. to receive rebates.

United States manufactured panels are required by law for the federal 30% rebate. We also found a Washington State incentive for 18 cents per kWh. Check your state for other incentives in your area. Sales tax rebates also may be in effect.

15: Satisfy all building and electrical codes.

The PV system must meet local building and electrical codes. These include International Residential Codes (IRC) and National Electrical Codes (NEC) with specific solar requirements. Check for local electrical codes.

16: Identify all electrical inspections needed.

Solar energy systems must meet not only the NEC and IRC codes, but also local codes. Inspections were a key concern. Permits must be purchased before any work is started.

17: Cost limits and goals need to be set and understood, and system size adjusted as needed.

Costs must be reasonable and within budget. Less than a 10 year system payback was our target — we came in at 8.5 year payback. The PV panels needed to cost less than $1 per watt and our overall cost was $4.12 per watt. Our system could cost approximately $70k if done with outside contractors but we came in at $33,000.00

18: Schedule needs to be set and understood.

The federal rebate for us could only be taken after the system was operational. We kept a tight self-imposed six month schedule to keep our project on track. The last two months went from cable bury to final inspection and operation.

Reprinted with permission from DIY Photovoltaic Solar Power for Homeowners: How We Designed and Built Our Own 8kW Systemby Doug and Jennie Ostgaard and published through CreateSpace, 2014.