function update_price(form,price_for_first_unit,price_for_each_additional_unit) {
	
	var total_quantity=0;
	var total_price=0;
	
	
	if (!isNaN(parseInt(form.num_units_ac.value)))
		q1=form.num_units_ac.value;
	else 
		q1='0';
	
	if (!isNaN(parseInt(form.num_units_gasheater.value)))
		q2=form.num_units_gasheater.value;
	else 
		q2='0';
		
	if (!isNaN(parseInt(form.num_units_heatpump.value)))
		q3=form.num_units_heatpump.value;
	else 
		q3='0';
		
	if (!isNaN(parseInt(form.num_units_evap.value)))
		q4=form.num_units_evap.value;
	else 
		q4='0';		

	total_quantity = eval(q1 + ' + ' + q2 + ' + ' + q3 + ' + ' + q4);

	
	if (total_quantity == 1)
		total_price = price_for_first_unit;
	else if (total_quantity > 1)
		total_price = price_for_first_unit + (price_for_each_additional_unit * (total_quantity-1));
	else
		total_price = 0;

	quant.innerHTML = total_quantity;
	price.innerHTML = total_price;
	quantity_error.innerHTML = ' ';	// clear any error that may currently be displayed.  it's ok to 
																// do this because if the quantity is not greater than zero, 
																// then the error will show up again when they click continue 
																// to submit the form.

} // end function update_price


function QuantityIsGreaterThanZero (form) {
	
	var total_quantity=0;	
	
	if (!isNaN(parseInt(form.num_units_ac.value)))
		q1=form.num_units_ac.value;
	else 
		q1='0';
	
	if (!isNaN(parseInt(form.num_units_gasheater.value)))
		q2=form.num_units_gasheater.value;
	else 
		q2='0';
		
	if (!isNaN(parseInt(form.num_units_heatpump.value)))
		q3=form.num_units_heatpump.value;
	else 
		q3='0';
		
	if (!isNaN(parseInt(form.num_units_evap.value)))
		q4=form.num_units_evap.value;
	else 
		q4='0';		

	total_quantity = eval(q1 + ' + ' + q2 + ' + ' + q3 + ' + ' + q4);
	
	if (total_quantity > 0 )
		return true;
	else
		quantity_error.innerHTML = 'Quantity must be at least 1';
		return false;

} // end function QuantityIsGreaterThanZero