So that would be another outlier that if this is the year we get more on that, we maybe had in the original, our original plan, that could be a help, too.A decline in usage per customer, mostly related to mild weather that impacted residential and commercial energy used for heating purposes and, to a lesser extent, due to a stay-at-home-related declines in sales to commercial customers, decreased operating income by $6.1 million. Steve, as far as the guidance goes, can you give us a little color about what you're anticipating in terms of the fourth quarter? Lisa, you mentioned that the irrigation draw as part of your reduced hydro expectations for the year.
And just to be clear, what is your sales forecast for 2020? But the fact that that's continuing to go and it hopefully brings some revenue growth that we have some in our plan, we hope it brings a little more.Lisa Grow - President, Idaho Power CompanyDarrel Anderson - President & Chief Executive OfficerJustin Forsberg - Director of Investor Relations & TreasuryThanks, Justin. Is that accurate?That is the Bureau of Reclamation in the state that runs all the water. This is Darrel. But for the most part, again, we believe the news has been positive and favorable.Thanks, Jamie, and good afternoon, everyone. The ones we've been able to chat with are moving ahead. They're pretty optimistic in that regard. We may -- and we have booked credits in first quarter. Most of them kind of full steam ahead. I would say, it's more that we are on a steady, we do this rollout. It's safe to say we've identified things that we think cover that sort of an approach, and there are levers that go beyond that, as you say, if it worsens. But that's the part where we have had a lot of growth. So things are associated with safety, we're not going to cut corners on where I can do those things as it relates to reliability.
But if it's caused by the weather, and again, just a normal summer would be really nice to have. Yeah, no, I think you're right. You guys do your quarters. But that's the part. However, given that April and May are off-peak months, we do not expect a significant increase unless the crisis extends into summer, when customers run their air conditioners. So we'll watch all that and balance it to the best outcome.And the other thing, as you see in the 10-Q, we also made a filing with the Oregon and Idaho commissions around the potential to recover any excess costs that we might incur as a result of this COVID. As Darrel noted, we estimated the effect of COVID-19 on commercial customer revenues was less than $1 million in this past quarter. However, as you would expect, it is difficult to predict the full long-term impact of evolving economic conditions on Idaho Power's customers and suppliers and how that could impact the upper end of the earnings guidance range or the use of tax credits if the pandemic worsens or is prolonged.But as far as the farmers knowing that there's a really -- that there's a high-quality resource available to them, are they just picking that up from the reporting websites that they can look at? Usually, there's some costs that come along with having those extra sales then, too.
And I'm just curious, outside of O&M, which you just discussed, and sales, I guess, what are the other assumptions that you can repeat that track record this year and going forward?Today, we also affirm our full year 2020 IDACORP earnings guidance estimate to be in the range of $4.45 to $4.65 per diluted share, with our expectation that Idaho Power will not need to utilize any of the tax credits in 2020 that are available to support earnings in Idaho under its settlement stipulation with the Idaho Public Utilities Commission. Investor Relations.