This 2021 project involved two members of the ModBESS team who worked with a local installer. The project involved the redesign and modification of an existing large home in Hawaii to meet the objectives of the owner to attain greater cost reduction and most importantly greater reliability than those attained by the original design. The typical monthly consumption varied between around 2,400 kWh/mo while off-site to over 4,000 kWh while on-site, resulting in electricity bills in excess of $1,500 per month.
As originally designed, the project consisted of 60 solar panels, 24 KWh of batteries and three Outback inverters. It was designed to operate the system using a “critical loads” panel to retain service to a few of the loads, selected by the initial installer without proper consultation with the owner. The owner’s objectives were to maintain power to the entire house as much as possible, not just to pre-selected critical loads. The second objective of the owner was to see a significant reduction in their utility costs. The owner was under a NEM CGS+ rate, which required energy-related payments of between $0.29 and $0.39 per kWh, depending on the month, and paid the customer back $0.11 per kWh for energy sent back.
The initial installation reduced the costs of operating the house to around $800 per month. More important to the owner, however, was that during the one and only significant outage during the first few months of operation the backup capabilities of the system proved inadequate. The only loads served during the outage were some garden lights, the electrical gates and a few outlets. The most critical load, a refrigeration unit, was not served during the outage.
As a result, we undertook a redesign of the home electrical system. We concluded that a few additional solar panels were needed (30 additional panels were recommended), that additional batteries were needed (the battery capacity was increased to 80 KWh), that new inverters were required (the Outback inverters were replaced with SolArk inverters), and that, because of the size of the load and the large utility supply, a separate transfer switch was required to handle outages and still be able to serve the entire house. Then, beyond the hardware redesign, there was a need to consider the operating parameters for the charging and discharging of the batteries to attain the dual objectives of saving in the electrical bills and retaining enough power in the batteries to tolerate an extended outage.
The system as it stands will require further controls to fully attain the objective of allowing the house to operate off-the-grid indefinitely in emergencies with a possible power outage at the worst possible time. This will require the active management of loads in the house as well as a more sophisticated and predictive battery charging and discharging control that will take into consideration usage as well as expected solar production (this is what HomeOptimizer is designed to do).
For now, we can report the following:
The cost of operating the house has been further reduced to under $200 per month average.
The entire house is currently served in an emergency.
If the outage occurs in the early morning hours following a sunny day when the battery was able to charge to 100%, the house is able to operate at its full load until the sun comes up.
In case of a not-fully-charged battery as a result of a previous cloudy day and an outage occurring at an early morning time, the battery charge may be insufficient to carry out the load until the sun comes up. This is being addressed by the HomeOptimizer software.
In case of a fully charged battery the system should carry the full house load until the next day. However, for a multi-day power outage, the system would need to use the HomeOptimizer predictive control capabilities to do load management to be able to operate in a “survival off-the-grid” mode indefinitely.
Below we illustrate “before and after” details for the home in question. First we illustrate the electricity bill summary by the customer for the period prior to installation of the retrofitted system and after the retrofit.
As of September 2022 there is considerable reduction in the energy consumption resulting not only from the slightly larger solar but mainly from the greater and smarter utilization of the solar and battery systems.
Next we illustrate the typical battery status and power flows for two 3-day sets in March 2022. The first set illustrates three sunny days under more-or-less ideal conditions:
During this period the solar (green) is almost ideal, the batteries charge to 100% and by morning they are down to around 25%, and the excess power that is not needed to charge the batteries (yellow) is sent to the utility every afternoon. In this scenario, the house can operate off-the-grid indefinitely with no intervention.
Next we illustrate a different three-day sequence in the same month:
For this less-than-fully-sunny period, the battery is unable to charge to 100% and the customer must rely on utility power for many of the nighttime hours, and is unable to charge the batteries back to 100%. The only ways to attain full off-the-grid capability would be either to expand both the solar and the batteries, to add a backup generator, or to do load management. HomeOptimizer is designed to optimally integrate all of the above for each situation as it evolves.
