Government bonds in foreign currencies. Of course this exposes you to whatever that country does with their central bank and to foreign exchange rates.
Corporate bonds, obviously riskier than government bonds but at least you can get positive interest rates in your preferred currency
Preferred stock, which are similar to bonds. Before you invest in these, make sure you know how they work. They can be called back by the issuer
Other asset classes which are less safe but could help you diversify: crypto, commodities and precious metals, collector's items and art, foreign currencies. Wouldn't really recommend putting too much money in this
Put your upfront costs at the first year's row (you're buying the system).
Calculate how much the system will save you in the current year (365 days), and put that in another column ("energy income") on the first row.
Each future energy income row gets discounted by ~7% multiplicatively: (1/1.07) * prev_row. This is to account for your ability to earn money from other investments, and to discount the future energy flows because of uncertainty. You can tweak this number up or down if you think the regulatory or technical value of solar energy is riskier than I do, or have different opinions about the stock market.
If the sum of the energy income column is bigger than the sum of the expense column, it's a good idea to buy solar. Shop around for the best deal you can get.
Google Sunroof: https://www.google.com/get/sunroof
Always buy the panels (no PPA), don't buy storage unless your net metering agreement with the utility is terrible (likely kills the ROI, storage isn't cheap enough yet). Ensure your tax liability is enough to capture the full 30% federal tax credit. Check for state, utility, and SREC benefits in your area. Panels are 25 year warranty, stay away from SolarEdge inverters; their mortality rate has skyrocketed over the last 12-18 months. Paying cash is best, otherwise find the cheapest debt you can find to finance whatever remains after incentives.
So your ROI is pretty much calculated with:
How much do the panels cost (installed)
How long do they last, whats expected maintenance, etc
How much energy are they expected to produce
Whats your electricity bill.
From there its pretty simple math. The hardest part is how much energy they are expected to produce, and no online calculator will help figuring out how much sun hits your roof.
There's also no limit to the amount you can buy.
Bottom line is that if there are still positive yielding bonds in a market dominated by negatives (we're not yet there) the risk of default is priced in.
Bitcoin makes zero sense as an investment. Doubly so during a recession.