codice:
<script>
function somma(obj) {
 t = 0
 e = obj.elements;
 for (i=0;i<e.lenght-1;i++) {
  if e[i].name.substr(0,7)=="Consumo") t += parseFloat(e[i].value)
 }
 obj.CampoTotale.vaue=t
}
</script>

<input type="text" name="Consumo1" size="11" maxlength="15" class="form" onblur="somma(this.form)">
ciao