Invoice Financial savings vs. Backup Energy: Evaluating Operational Tradeoffs for House Photo voltaic+Storage Programs

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Research reveals that, in most circumstances, clients ought to preserve low reserves, prioritizing invoice financial savings over backup energy

House battery storage paired with photo voltaic photovoltaic (PV) techniques can scale back utility payments beneath sure fee buildings whereas additionally offering backup energy throughout energy interruptions. For energy interruptions that include some warning (resembling these brought on by hurricanes and different pure disasters), clients are sometimes in a position to cost up their battery upfront. However many energy interruptions come with out warning. To protect in opposition to this kind of outage, most battery storage techniques come geared up with a “reserve setting” that permits the client to take care of some minimal degree of storage capability in reserve. But, this function comes with a tradeoff: the extra storage capability that’s held in reserve, the larger the client’s skill to journey by way of unpredictable energy interruptions, however much less capability is then accessible to handle utility payments on a day-to-day foundation.

The brand new Berkeley Lab report, entitled Invoice Financial savings vs. Backup Energy: Evaluating operational tradeoffs for dwelling photo voltaic+storage techniques, reveals how clients can strategy this tradeoff given their specific situations. The research quantifies how utility invoice financial savings and the reliability worth from mitigated energy interruptions fluctuate with reserve ranges. The evaluation leverages Berkeley Lab’s PRESTO mannequin, which stochastically simulates energy interruption occasions primarily based on historic patterns in every U.S. county. The research focuses on ten consultant counties and considers a variety of frequent utility fee buildings, in addition to different key components, together with electrical energy costs, native reliability ranges, and the client’s worth of misplaced load (VoLL). The research isn’t a cost-effectiveness analysis, however moderately examines the slim query of how the worth of battery storage to the host buyer is impacted by the reserve settings. Different essential methodological particulars and limitations are summarized within the report.

Key findings from the report are highlighted beneath and shall be mentioned in an upcoming webinar on October twenty third at 11:00 am Pacific. Please register for the webinar right here: https://lbnl.zoom.us/webinar/register/WN_xFm0GAPuRHSnoeoGq40tiQ

Reliability worth is (surprisingly) insensitive to order degree beneath most situations. The reliability worth of the backup energy offered by the photo voltaic+storage system will increase with reserve degree, however solely modestly in most circumstances. For instance, as proven within the left-hand panel of Determine 1, the common annual reliability worth for a typical buyer in Memphis rose by simply $1-10/yr when rising reserves from 20% to 80% of the battery’s capability. All different research areas confirmed related traits as effectively, as proven within the report. The final “flatness” of those traits partly displays the restricted quantity of backup load for the battery to serve throughout a mean yr, given the everyday frequency and length of energy interruptions in most areas. The research additionally contains sensitivities exploring extra frequent energy interruptions, given the various reliability ranges that particular person clients could expertise inside any given county.

Invoice financial savings drop precipitously with greater reserves. Storage generates invoice financial savings by arbitraging between high and low costs, beneath both time-of-use (TOU) charges with peak and off-peak costs, or internet billing charges that compensate photo voltaic exports at costs beneath retail charges. As proven within the right-hand panel of Determine 1, the invoice financial savings from storage arbitrage decline considerably with greater reserves, albeit to differing levels relying on the particular tariff construction. For instance, beneath TOU charges the place grid discharging isn’t allowed (the 2 backside traces), the decline in invoice financial savings is kind of gradual for reserve ranges as much as roughly 40%, because the battery would hardly ever discharge beneath that degree. To make certain, the instance proven in Determine 1 is predicated on a pricing differential of simply $0.05/kWh. With the next pricing differential, the invoice financial savings at low reserve ranges could be greater, and the drop in invoice financial savings with rising reserves could be much more pronounced.

Complete buyer worth tends to be best when reserves are set as little as allowed. Given the 2 findings above, whole buyer worth—that’s, the sum of buyer reliability worth and invoice financial savings—tends to say no with greater reserves, similar to the drop in invoice financial savings. Determine 2 reveals this development throughout all ten research areas, for the 2 “bookend” tariff buildings: internet billing and TOU self-consumption. All areas present monotonically declining buyer worth as reserves rise. That is true for each tariff buildings, although the decline in whole buyer worth beneath the TOU self-consumption tariff is significantly much less pronounced, and so clients on that sort of tariff could also be considerably detached to modifications in reserve degree (not less than as much as some level).

Increased reserve ranges could improve whole buyer worth for patrons with an exceptionally excessive worth of misplaced load, dwelling in areas with exceptionally frequent energy interruptions. The research included situations the place clients have the next worth of misplaced load (VoLL) and/or reside in areas with extra frequent energy interruptions (SAIFI), in comparison with their county common. Determine 3 reveals how the whole buyer worth modifications throughout these sensitivities because the reserve degree is elevated from 20% to 80%: that’s, a unfavorable worth signifies that whole buyer worth declines with rising reserves, whereas a constructive worth signifies that whole buyer worth will increase. As proven in Determine 3, solely within the case the place the client has ten occasions the base-case VoLL ($50/kWh) and likewise ten occasions the common SAIFI (roughly 10 interruptions per yr, for Memphis) do the outcomes flip, and whole buyer worth rises with reserves. Nevertheless, even that exception is restricted to specific tariff buildings (internet billing and TOU charges that permit solely grid discharging). Moreover, as proven within the report, bigger value differentials can shift the economics again towards decrease reserve ranges.

The complete report supplies a wealth of further element and outcomes, together with extra detailed quantitative findings for every location analyzed, in addition to sensitivity circumstances for battery measurement (i.e., 10 kWh vs. 30 kWh battery), alternate fee ranges (together with greater value arbitrage ranges and completely different base costs), and buyer utilization ranges (together with a low consumption and excessive consumption case). Whereas the report usually means that clients will maximize their worth by sustaining the bottom degree of reserves allowed, the findings additionally present how the particular circumstances of any particular person buyer must be thought-about when making selections about reserve degree settings.

The research builds on earlier work printed by the Berkeley Lab staff (right here and right here), evaluating the capabilities of photo voltaic+storage in offering backup energy over lengthy multi-day interruptions.

We thank the U.S. Division of Vitality Photo voltaic Vitality Applied sciences Workplace for his or her assist of this work, in addition to members of the exterior technical advisory group and different exterior reviewers who offered invaluable steerage and suggestions on this evaluation.

Courtesy of Galen Barbose, Will Gorman, and the remainder of the research’s authors.

The views expressed right here don’t essentially characterize the views of the U.S. Division of Vitality or the U.S. Authorities.