trigger UpdateInverterQuantity on OpportunityLineItem (before insert, before update) {
    Map<Id, Product2> panelProdMap = new Map<Id, Product2>([
        SELECT Id, Name 
          FROM Product2
         WHERE Name = 'Heliene 60M-BLK 310W' 
            OR Name = 'Heliene 60M-HBLK 300W'
    ]); 
    Map<Id, Product2> invProdMap = new Map<Id, Product2>([
        SELECT Id, Name 
          FROM Product2
         WHERE Name = 'APSystems QS1' 
            OR Name = 'YC600'
    ]); 

    list<String> oppIds = new List<String>();
    for (OpportunityLineItem oli : Trigger.new ) {
        oppIds.add(oli.OpportunityId);
    }
    list<OpportunityLineItem> panelOLIs = new list<OpportunityLineItem>();
    panelOLIs = [SELECT Id, OpportunityId, Quantity, Product2.Name 
                    FROM OpportunityLineItem
                    WHERE OpportunityId IN :oppIds
                    AND Product2Id IN :panelProdMap.keySet()];
    Map<Id,OpportunityLineItem> panelOLImap = new Map<Id, OpportunityLineItem>();
    for (OpportunityLineItem oli : panelOLIs) {
        panelOLImap.put(oli.opportunityId, oli);
    }

    list<OpportunityLineItem> invOLIs = new list<OpportunityLineItem>();
    invOLIs = [SELECT Id, OpportunityId, Quantity, Product2.Name  
                    FROM OpportunityLineItem
                    WHERE OpportunityId IN :oppIds
                    AND Product2Id IN :invProdMap.keySet()];
    Map<Id,OpportunityLineItem> invOLImap = new Map<Id, OpportunityLineItem>();
    for (OpportunityLineItem oli : invOLIs) {
        invOLImap.put(oli.opportunityId, oli);
    }

    Integer divisor;
    Integer panelQty;
    OpportunityLineItem tempOLI;
    List<OpportunityLineItem> toUpdate = new List<OpportunityLineItem>();

    for (OpportunityLineItem oli : Trigger.new ) {
        System.debug('DEBUG - FOR LOOP');
        if (panelProdMap.ContainsKey(oli.Product2Id )) {   //inserting/updating a panel oli
            System.debug('DEBUG - insert/update Panel');
            if ( panelProdMap.get(oli.Product2Id).Name == 'Heliene 60M-HBLK 300W' ) {
                divisor = 4;
            } else if ( panelProdMap.get(oli.Product2Id).Name == 'Heliene 60M-BLK 310W' ) {
                divisor = 2;
            } else {
                divisor = 1;
            }
            if (invOLImap.ContainsKey(oli.OpportunityId )) {
                System.debug('DEBUG - found previous invertors');
                tempOLI = invOLImap.get(oli.OpportunityId );     //we have a matching panel OLI
                Decimal inverterQuantity = (oli.Quantity / divisor); 
                inverterQuantity = inverterQuantity.round(System.RoundingMode.CEILING); 
                if (tempOLI.Quantity <> inverterQuantity ) {
                     tempOLI.Quantity = inverterQuantity;
                     toUpdate.add(tempOLI);
                }
            } 
        }
        if (invProdMap.ContainsKey(oli.Product2Id )) {     //inserting/updating a invertor oli
            if (trigger.isUpdate && (oli.Quantity <> trigger.oldMap.get(oli.Id).Quantity) ) {
                System.debug('DEBUG - update Invertor with old map');
                //quantity has been changed on the invertor - do something else
            } else {
                System.debug('DEBUG - insert Invertor');
                if ( invProdMap.get(oli.Product2Id).Name == 'APSystems QS1' ) {
                    divisor = 4;
                } else if ( invProdMap.get(oli.Product2Id).Name == 'YC600' ) {
                    divisor = 2;
                } else {
                    divisor = 1;
                }
                if (panelOLImap.ContainsKey(oli.OpportunityId )) {
                    System.debug('DEBUG - found previous panels');
                    tempOLI = panelOLImap.get(oli.OpportunityId );     //we have a matching panel OLI
                    Decimal inverterQuantity = (tempOLI.Quantity / divisor); 
                    inverterQuantity = inverterQuantity.round(System.RoundingMode.CEILING); 
                    if (oli.Quantity <> inverterQuantity ) {
                         oli.Quantity = inverterQuantity;
                    }
                }
            }
        }
    }
    if (toUpdate.size() > 0  ) {
        update toUpdate;
    }
}

@isTest
private class UpdateInverterQuantityTest {
    @isTest static void updatePanelQuantity() {
        // create an opportunity
        Opportunity myOpp  = new Opportunity();
        myOpp.Name         = 'Test';
        myOpp.StageName    = 'Closed Won';
        myOpp.CloseDate    = Date.today();
        myOpp.Pricebook2Id = Test.getStandardPricebookId();
        insert myOpp;

        Id pricebookId = Test.getStandardPricebookId();

        //Create your panel
        Product2 panel = new Product2(
            Name        = 'Heliene 60M-HBLK 300W',
            ProductCode = 'H300W',
            isActive    = true
        );
        insert panel;

        //Create your inverter
        Product2 firstInverter = new Product2(
            Name        = 'APSystems QS1',
            ProductCode = 'APS',
            isActive    = true
        );
        insert firstInverter;

        //Create your pricebook entry for panel
        PricebookEntry  panelEntry = new PricebookEntry(
            Pricebook2Id = pricebookId,
            Product2Id   = panel.Id,
            UnitPrice    = 100.00,
            IsActive     = true
        );
        insert panelEntry;

        //Create your pricebook entry for inverter
        PricebookEntry inverterEntry = new PricebookEntry(
            Pricebook2Id = pricebookId,
            Product2Id   = firstInverter.Id,
            UnitPrice    = 100.00,
            IsActive     = true
        );
        insert inverterEntry;

         // add panel to opp products
        OpportunityLineItem oli = new OpportunityLineItem(
            OpportunityId    = myOpp.Id,
            Quantity         = 20,
            Product2Id = panelEntry.Id,
            TotalPrice       = 20 * panelEntry.UnitPrice
        );
        insert oli;
        System.debug('TEST DEBUG - Inserted panel oli');

        // add inverter to opp products
        OpportunityLineItem oli2 = new OpportunityLineItem(
            OpportunityId    = myOpp.Id,
            Quantity         = 20,
            Product2Id = inverterEntry.Id,
            TotalPrice       = 20 * inverterEntry.UnitPrice
        );
        insert oli2;
        System.debug('TEST DEBUG - Inserted inverter oli');

        // Get updated inverter quantity
        OpportunityLineItem inverter = [
            SELECT Quantity
              FROM OpportunityLineItem
             WHERE OpportunityId = :oli.OpportunityId
               AND Id = :oli2.Id
             LIMIT 1
        ];

        System.assertEquals(5, inverter.Quantity);

        // Update quantity of panels
        oli.Quantity = 40;
        update oli;

        // Get updated quantity of inverters
        OpportunityLineItem updatedInverter = [
            SELECT Quantity
              FROM OpportunityLineItem
             WHERE OpportunityId = :oli.OpportunityId
               AND Id = :oli2.Id
             LIMIT 1
        ];

        System.assertEquals(10, updatedInverter.Quantity);
    }
}