# Modeling BESS + Reserve Margin

**URL:** <https://forum.u4ria.org/t/modeling-bess-reserve-margin/593>\
**Category:** Troubleshooting\
**Created:** [March 16, 2026, 11:06am UTC](https://forum.u4ria.org/t/modeling-bess-reserve-margin/593 "2026-03-16T11:06:18Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![CorentinAmi](https://avatars.discourse-cdn.com/v4/letter/c/3da27b/32.png) [@CorentinAmi](https://forum.u4ria.org/u/CorentinAmi)\
**Post date:** [March 16, 2026, 11:06am UTC](https://forum.u4ria.org/t/modeling-bess-reserve-margin/593/1 "2026-03-16T11:06:18Z")

</div>

Hello everyone,

I have completed the OSeMOSYS ‘Flatpack’ and Hands-on 9. I am now developing a national model for an African country and I’m facing an issue with **BESS and Reserve Margin (RM)**.

I have decoupled my storage into two technologies:

1. **BATTPWR** (Inverters) at **80 €/kW**.

2. **BESS\_Cells** (Storage) at **300 €/kWh**.

Even with these costs, the model installs ‘power-only’ batteries (huge GW of inverters with almost no GWh of cells) just to satisfy the RM requirement at the lowest cost.

Is there a standard way to enforce a fixed **Energy-to-Power ratio** (e.g., forcing 4 GWh of cells for every 1 GW of inverter). will it be using a **User Defined Constraint (UDC)**?

Edit : the issue should be somewhere else: if I remove the BESS from the UDC, then the model installs tons of Diesel to cover the RM requirement… (corresponding ridiculous investment appears in the results)

 ![image](https://europe1.discourse-cdn.com/loughborough/original/1X/9f56c671375307a23e56d76ed3815f95a82c8da3.png)

Thanks for your help!

---

<div class="post-metadata">

**Author:** ![CorentinAmi](https://avatars.discourse-cdn.com/v4/letter/c/3da27b/32.png) [@CorentinAmi](https://forum.u4ria.org/u/CorentinAmi)\
**Post date:** [March 17, 2026, 1:11pm UTC](https://forum.u4ria.org/t/modeling-bess-reserve-margin/593/2 "2026-03-17T13:11:00Z")

</div>

It seems the error was to had modeled this RM through the “UDC M TOTAL Capacity” instead of “UDC Multiplier Activity”

Now it is solved

However I still have too much “BATTPWR” to ‘stabilize’ my grid because I cannot link the BATTPWR to the BATTSTO through an UDC. Is it correct that UDC can only make constraints among technologies, and cannot include ‘Storage’ assets ?

I want to model that for 1GW Inverter I want 4GWh of cell storage. I don’t find how to model that in OSEMOSYS MUIO5.5

Thank you for your help