diff --git a/esbo_etc/classes/SpectralQty.py b/esbo_etc/classes/SpectralQty.py
new file mode 100644
index 0000000..21fcf4b
--- /dev/null
+++ b/esbo_etc/classes/SpectralQty.py
@@ -0,0 +1,75 @@
+from esbo_etc.lib.helpers import error
+import numpy as np
+from scipy.integrate import cumtrapz
+import astropy.units as u
+
+
+class SpectralQty:
+    """
+    A class to hold and work with spectral quantities
+    """
+    def __init__(self, wl: u.Quantity, qty: u.Quantity):
+        """
+        Initialize a new spectral quantity
+
+        Parameters
+        ----------
+        wl : Quantity
+            The binned wavelengths
+        qty : Quantity
+            The quantity values corresponding to the binned wavelengths
+        """
+        self.wl = wl
+        self.qty = qty
+
+    def add(self, sqty: "SpectralQty"):
+        pass
+
+    def multiply(self, sqty: "SpectralQty"):
+        pass
+
+    def rebin(self, wl: u.Quantity):
+        """
+        Resample the spectral quantity sqty(wl) over the new grid wl, rebinning if necessary, otherwise interpolates.
+        Copied from ExoSim (https://github.com/ExoSim/ExoSimPublic).
+
+        Parameters
+        ----------
+        wl : Quantity
+            new binned wavelengths
+
+        Returns
+        -------
+        """
+
+        if wl.unit != self.wl.unit:
+            error("Mismatching units for rebinning: " + wl.unit + ", " + self.wl.unit)
+
+        idx = np.where(np.logical_and(self.wl > 0.9 * wl.min(), self.wl < 1.1 * wl.max()))[0]
+        wl_old = self.wl[idx]
+        qty_old = self.qty[idx]
+
+        if np.diff(wl_old).min() < np.diff(wl).min():
+            # Binning
+            c = cumtrapz(qty_old, x=wl_old) * qty_old.unit * wl_old.unit
+            xpc = wl_old[1:]
+
+            delta = np.gradient(wl)
+            new_c_1 = np.interp(wl - 0.5 * delta, xpc, c, left=0.0, right=0.0) * c.unit
+            new_c_2 = np.interp(wl + 0.5 * delta, xpc, c, left=0.0, right=0.0) * c.unit
+            qty = (new_c_2 - new_c_1) / delta
+        else:
+            # Interpolation
+            qty = np.interp(wl, wl_old, qty_old, left=0.0, right=0.0) * qty_old.unit
+
+        self.wl = wl
+        self.qty = qty
+
+        # import matplotlib.pyplot as plt
+        # plt.plot(wl_old, qty_old, '-')
+        # plt.plot(wl, qty, '.-')
+        # plt.show()
+        # # check
+        # print(np.trapz(qty, wl))
+        # idx = np.where(np.logical_and(wl_old >= wl.min(), wl_old <= wl.max()))
+        # print(np.trapz(qty_old[idx], wl_old[idx]))