Add excel button in Grid.ts file.

namespace Serene.Northwind {

    @Serenity.Decorators.registerClass()
    @Serenity.Decorators.filterable()
    export class CustomerGrid extends Serenity.EntityGrid<CustomerRow, any> {
        protected getColumnsKey() { return "Northwind.Customer"; }
        protected getDialogType() { return <any>CustomerDialog; }
        protected getIdProperty() { return CustomerRow.idProperty; }
        protected getLocalTextPrefix() { return CustomerRow.localTextPrefix; }
        protected getService() { return CustomerService.baseUrl; }

        constructor(container: JQuery) {
            super(container);
        }

        getButtons() {
            var buttons = super.getButtons();

            buttons.push(Serene.Common.ExcelExportHelper.createToolButton({
                grid: this,
                onViewSubmit: () => this.onViewSubmit(),
                service: 'Northwind/Customer/ListExcel',
                separator: true
            }));

            buttons.push(Serene.Common.PdfExportHelper.createToolButton({
                grid: this,
                onViewSubmit: () => this.onViewSubmit()
            }));

            return buttons;
        }

    }
}

Add excel export server side mothis in Endpoint.cs.

namespace Serene.Northwind.Endpoints
{
    using Serenity.Data;
    using Serenity.Reporting;
    using Serenity.Services;
    using Serenity.Web;
    using System;
    using System.Data;
    using System.Web.Mvc;
    using MyRepository = Repositories.CustomerRepository;
    using MyRow = Entities.CustomerRow;

    [RoutePrefix("Services/Northwind/Customer"), Route("{action}")]
    [ConnectionKey(typeof(MyRow)), ServiceAuthorize(typeof(MyRow))]
    public class CustomerController : ServiceEndpoint
    {
        [HttpPost, AuthorizeCreate(typeof(MyRow))]
        public SaveResponse Create(IUnitOfWork uow, SaveRequest<MyRow> request)
        {
            return new MyRepository().Create(uow, request);
        }

        [HttpPost, AuthorizeUpdate(typeof(MyRow))]
        public SaveResponse Update(IUnitOfWork uow, SaveRequest<MyRow> request)
        {
            return new MyRepository().Update(uow, request);
        }

        [HttpPost, AuthorizeDelete(typeof(MyRow))]
        public DeleteResponse Delete(IUnitOfWork uow, DeleteRequest request)
        {
            return new MyRepository().Delete(uow, request);
        }

        public GetNextNumberResponse GetNextNumber(IDbConnection connection, GetNextNumberRequest request)
        {
            return new MyRepository().GetNextNumber(connection, request);
        }

        public RetrieveResponse<MyRow> Retrieve(IDbConnection connection, RetrieveRequest request)
        {
            return new MyRepository().Retrieve(connection, request);
        }

        public ListResponse<MyRow> List(IDbConnection connection, ListRequest request)
        {
            return new MyRepository().List(connection, request);
        }

        public FileContentResult ListExcel(IDbConnection connection, ListRequest request)
        {
            var data = List(connection, request).Entities;
            var report = new DynamicDataReport(data, request.IncludeColumns, typeof(Columns.CustomerColumns));
            var bytes = new ReportRepository().Render(report);
            return ExcelContentResult.Create(bytes, "CustomerList_" +
                DateTime.Now.ToString("yyyyMMdd_HHmmss") + ".xlsx");
        }
    }
}

Another way for add excel export button.

    buttons.push(Common.ExcelExportHelper.createToolButton({
                grid: this,
                service: CustomerGrossSalesService.baseUrl + '/ListExcel',
                onViewSubmit: () => this.onViewSubmit(),
                separator: true
            }));