# How to multiply two arrays (one for energy and one for price) to calculate cost energy?

Dears,

I store in InfluxdB one array with the energy of my system by kwh at each hour) and one array with the price in € each kwh.

To calculate the cost I need to read the data, and I have two arrays, but I don’t know how I can multiply for each index of arrays the energy and price and finally calculate the total cost.

Anyone can help me?. Thanks.

Like this?

``````var arrayEnergy = msg.energy;
var arrayPrice = msg.price;
var pricePerHour = [];
var minLength = 0;
var totalCost = 0;

// only use length of shortest array so we don't end up multiplying x*none
if (arrayEnergy.length <= arrayPrice.length){
minLength = arrayEnergy.length;
}
else{
minLength = arrayPrice.length;
}

// multiply corresponding indexes of each array
for(var i=0; i<minLength; i++) {
pricePerHour[i] = arrayEnergy[i] * arrayPrice[i];
}

// add all entrys in pricePerHour together
pricePerHour.forEach(entry => totalCost += entry)

// set output message
msg = {"pricePerHour":pricePerHour, "totalCost":totalCost}

return msg;
``````

You have to pass in the two arrays of msg.energy and msg.price in one message

Thanks a lot Syntox.