First off, your slider will never work because you cannot reach 400, 900, 1400, 1900, 2400, 2900, 3400, 3900, 4400, 4900 etc. Sliders will require all numbers from 100 to 15800 in increments of 100. Also, this can get very complicated because the on/off switch combinations can result with the same answer for different combinations. Example:
5000 on, 3000 on, and 1000 on result in 9000
and
5000 on, 4000 on, result in 9000.
All in all, your slider will never work when sliding without making a very complicated script. Jinja (the templates) won’t be able to support it easily.
If you used a different number combination, you could pontially get this to work with a slider, but your combinations would need to be:
100, 200, 400, 800, 1600, 3200, 6400, 12800.
Here is a simple template that will work with your configuration (but not a slider):
sensor:
- platform: template
sensors:
load_bank_1:
friendly_name: "Load Bank 1"
unit_of_measurement: 'W'
value_template: >
{% set a = 100 if is_state('input_boolean.loadbank_1_100', 'on') else 0 %}
{% set b = 200 if is_state('input_boolean.loadbank_1_200', 'on') else 0 %}
{% set c = 500 if is_state('input_boolean.loadbank_1_500', 'on') else 0 %}
{% set d = 1000 if is_state('input_boolean.loadbank_1_1000', 'on') else 0 %}
{% set e = 2000 if is_state('input_boolean.loadbank_1_2000', 'on') else 0 %}
{% set f = 3000 if is_state('input_boolean.loadbank_1_3000', 'on') else 0 %}
{% set g = 4000 if is_state('input_boolean.loadbank_1_4000', 'on') else 0 %}
{% set h = 5000 if is_state('input_boolean.loadbank_1_5000', 'on') else 0 %}
{{ a+b+c+d+e+f+g+h }}