update:
<script>
// when the DOM is ready
$(document).ready(function() {
let usageInput;
let panelSelected;
let inverterSelected;
let batteryIncluded;
let upgradeSelected;
//
$('#usageForm').submit(function(e){
usageInput = $('#usage-input-field').val();
console.log(usageInput);
});
$('#panelForm').submit(function(e){
panelSelected = $('#panel-select-field').val();
console.log(panelSelected);
});
$('#inverterForm').submit(function(e){
inverterSelected = $('#inverter-select-field').val();
console.log(inverterSelected);
});
$('#batteryForm').submit(function(e){
batteryIncluded = $('#battery-checkbox').val();
console.log(batteryIncluded);
});
$('#upgradeForm').submit(function(e){
upgradeSelected = $('#upgrade-select-field').val();
console.log(upgradeSelected);
});
});
</script>