app/views/energy_tariffs/energy_tariff_differential_prices/new.html.erb
<% url = energy_tariffs_path(@energy_tariff, [:energy_tariff_differential_prices]) %>
<%= simple_form_for @energy_tariff_price, url: url, remote: true do |f| %>
<%= render 'form', energy_tariff: @energy_tariff, energy_tariff_price: @energy_tariff_price, f: f %>
<% end %>