Transparency about assumptions, stakeholder perspectives and basic methodologies will be vital in the deployment of residential solar under policies such as net energy metering (NEM), according to an influential US sustainability and energy think-tank yesterday.

A Review of Solar PV Benefit and Cost Studies is the first report from the recently formed Electricity Innovation Lab (e-Lab) at the Rocky Mountain Institute in Snowmass, Colorado, which aims to find common ground between utilities and the solar industry on policies such as net energy metering.

The authors had originally planned a meta-analysis across 15 major assessments exploring the economic value of distributed photovoltaics (DPV) but the input assessments were so heavily influenced by the sponsor of the report that only a review was possible.

Virginia Lacy, senior consultant at RMI and co-author of the report, said: "We wanted to pull together the studies out there and pull out the best practices or conclusions and see what the state of the industry is. The methodologies that have been employed so far and the basic input assumptions are also driving some significant differences and that came out loud and clear.

"What we found is that there are a lot of inconsistencies, it's not easy to compare a lot of the results across studies primarily because the local context is critically important and often lost. You can't just pull out a number and say the value of solar across the US is 9c/kwh. It's impossible to say for a variety of reasons that make the local context so different across different territories."

In 2012, the US added 2GW of solar PV to the nation’s generation mix, of which approximately 50% were customer-sited solar, net-metered projects. About 80% of customer-sited PV is concentrated in states with either ample solar resource and/ or especially solar-friendly policies: California, New Jersey, Arizona, Hawaii and Massachusetts.

The report found that under today’s regulatory and pricing structures, multiple misalignments along economic, social and technical dimensions are emerging. Pricing mechanisms are not in place to recognise or reward service that is being provided by either the utility or customer, it said.

Lacy said that greater granularity of data would be essential in assessing the costs and benefits of NEM and that reports should be very clear about stakeholder perspectives.

"Sponsorship needs to be disclosed very clearly as to who sponsored the study because the results tend to be correlated.

"We are trying to be an arbiter in some sense to recognise that the volley of press releases back and forth between utilities and the solar industry DG PV that's probably not going to lead to an optimal result. At some point we need to take a step back and start to have some conversations around real data.

"There were inconsistencies in the report, but we also found agreement. On energy there's much more agreement, but as you start to move out to things like capacity value, there's a less agreement how much transmission and distribution could you obviate if you were to integrate PV in a thoughtful way, a lot less agreement. When we get into social value we're talking about pretty polar opposites."

Lacy pointed to a study conducted in California by consultants E3, the Technical Potential of Local Distributed Photovoltaics in California (March 2012), as a good example of helpful granular data.

"It reflects a trend that will be necessary in the evaluation of distributed PV: employing the granular data necessary to explore the locational and temporal values of DPV," she said.

The California Public Utilities Commission has commissioned E3 to conduct a report on costs and benefits of NEM due for release in October.

Last week, the Solar Electric Power Association issued a new report on NEM, called Ratemaking, Solar Value and Solar Net Energy Metering - A Primer.

"The report highlights the different approaches taken by state regulators for ratemaking, and underscores that there is no 'one-size-fits-all' approach for all situations," said SEPA President and CEO Julia Hamm.