With a little creativity, you can use groupsummary. Since you haven't shared enough of your code to use that for an example, here's one modified from an example on the histcount2 documentation page.
% Set up the data
x = randn(100,1);
y = randn(100,1);
[N,~,~,bx,by] = histcounts2(x,y);
% create a 3rd vector, pwr
pwr = randn(100,1);
% Sum the data by grouping pwr by bin numbers by and bx.
% Sum data in the same group.
% Include data for all possible bins
[P,G,C] = groupsummary(pwr,[by,bx],'sum',"IncludeEmptyGroups",true,"IncludeMissingGroups",true);
% Reshape the data to be the same size an N
p = reshape(P,size(N))
N % compare to verify that when N==0, P==0.
