Sizing a BESS for demand charges

Hi Paul,
Thank you for the reply and looking into the requests. I also got in touch with Emma to get some clarity on REopt, who was extremely helpful.

I think that there is a manual way to size solar + BESS without using REopt. Emma recommended I direct this to the SAM forum.

I wanted to get your thoughts on the following.

In the attached file, for the solar only case, the NPV is $73,794.4(after -tax), project titled ‘solar only’.
When I model solar with a 250kW/250kWh Battery and peak shaving dispatch option, the NPV reduces to - $86,222.5. This is project titled ‘Battery_case1’.The negative NPV means, that the 250kW/250kWh BESS for demand charge savings does not make a financial case.

I then look at the ‘electricity to load from the battery (kW)’in the profiles tab and notice that the BESS only dispatches 15/20kW (max). This ‘kW’ dispatch appears to line up with the assumption that BESS is incorrectly sized. I also read this as that the BESS only charges/dispatches a certain ‘kW’ when it makes a financial sense to do so.

In the profiles tab(30mins) I review the ‘electrical load(kW)’ and ‘electricity battery power target profiles for automated dispatch(kW)‘. Based on these profiles, I estimate that a decrease in BESS’spower rating and an increase in the energy rating could yield better outcomes. I nominated a new BESS rated at 50kW/250kWh. The NPV changes to +$17,621 with a payback of 9.9years.The savings are approximately the same for solar and solar+BESS (50kW/250kWh) case, the exception being the cash flow is rather negative in the later half of the solar +BESS case.

Anyways, my takeaway is that I do not need REopt to size BESS, I can look at the financial indicators and play with standard BESS sizes to create some options.

Also, do I need to enter anything in the ‘battery dispatch' field under ‘custom dispatch’ option in system design tab? (please ignore project titled BESSsizing with dispatch).

Many thanks!

Hi Ash,

That sounds like a reasonable approach. Using SAM in this way gives you the option of adjusting other assumptions to refine the sizing process.

There are many factors that influence the NPV such as installation and operating costs, incentives, size of debt, discount and inflation rates, etc., so be aware that your results are based on those factors.

For the PVWatts Battery model, if you choose a peak shaving dispatch option, you do not need to provide a custom dispatch profile.

Best regards,
Paul.

Thanks Paul.