Solar panels that would supply your electricity needs at your residence cost $60,000 to install. The government subsidizes your expenditure on the investment up to 45% so that your net investment is $33,000. Currently you spend $280 a month on electricity, and you expect to go on consuming the same amount of electricity. Taking the risk of the investment into account, you think that a discount rate of 0.54% per month is justified. Assuming a maintenance cost of $10 per month, how long would the panels have to last for the purchase of solar panels to be a positive NPV investment for you?