The Homestead Calculator

This is the guide that started the whole site for us.

When we began seriously planning our off-grid structure, we kept hitting the same wall. Every resource on rainwater collection treated the water system as if it existed independently of electricity. Every resource on solar power listed "well pump" as a load item without explaining what happens when your water source is a tank and a pressure pump rather than a drilled well. The two systems were always discussed separately, even though on an actual off-grid homestead they are completely dependent on each other.

Your water pump runs on your solar power. Your solar panels need to be cleaned with your collected water. Your filtration system needs power to operate. Your irrigation timer draws from your battery bank. These aren't parallel systems — they're one integrated system with two visible parts.

Here's how to plan them together.

How the Two Systems Connect

A typical off-grid rainwater-to-tap system flows like this:

Every step marked with a power indicator is a load on your solar system. Getting that load wrong during solar sizing is the most common reason off-grid water systems fail in practice — the solar system was spec'd without fully accounting for the water infrastructure.

The Water Loads You Need to Account For in Solar Sizing

The pump is the big variable. A 750W pump running 45 minutes a day is adding 560 Wh to your daily solar load — almost as much as a refrigerator. Choosing an efficient DC pump vs. an AC pump, or a 12V submersible vs. a surface pump, can cut that number dramatically.

Gravity — The Free Alternative to Pump Power

Before you size your pump load, ask whether gravity can do some of the work. If your storage tank can be positioned above your end use points — even just 10–15 feet of elevation — gravity pressure alone may be sufficient for garden irrigation, livestock watering, and outdoor washing without any pump at all.

10 feet of elevation provides approximately 4.3 PSI of pressure. That's marginal for household taps (which typically need 20–60 PSI) but perfectly adequate for gravity-fed drip irrigation and livestock troughs. A gravity system for the garden and a pump only for household use cuts your pump runtime significantly.

Seasonal Mismatch: The Core Planning Challenge

Here's the problem that catches most people: your peak water demand and your peak solar production don't align with your peak rainfall.

In Tennessee:

• Peak rainfall: Winter and spring (December–April)
• Peak solar production: Summer (May–August)
• Peak water demand: Summer (garden irrigation, animal heat stress)

This means you're collecting the most water when you need it least and need it most when you're collecting less of it. Your tank is your buffer — it stores the winter and spring collection to carry you through the summer demand. Your solar system is producing well exactly when your pump needs to work hardest.

But in winter, when solar production drops to 60–70% of summer levels, your heated livestock waterers are running constantly. The loads shift seasonally in opposing directions, which means your system needs to be designed for both peaks, not just the comfortable average.

DC vs. AC Pumps — Why It Matters for Solar

Most residential water pumps are AC — they run on standard 120V household current and require your inverter to operate. This adds inverter inefficiency (typically 10–15% loss) to every pumping cycle.

DC pumps run directly on battery voltage (12V or 24V) and bypass the inverter entirely, which means more efficient operation and the ability to pump directly from solar panels during daylight hours without drawing from batteries. For a cistern pump that runs a few cycles per day, the efficiency difference is meaningful over a year's operation.

The trade-off is that DC pumps typically have lower flow rates and lower maximum pressure than comparably priced AC pumps. For a rainwater system feeding garden irrigation and livestock, DC is usually more than adequate. For full household pressure (showers, toilet flushing, dishwasher), an AC pump with a pressure tank is often the more practical choice.

The System Sizing Sequence

Here's the order we'd follow to size both systems together correctly:

1. Calculate your daily water use by end use: household gallons per day, garden gallons per week, livestock gallons per day. Convert everything to a daily average for each season.
2. Size your storage tank for your longest dry stretch — the gap between significant rainfall events in your driest period. Use the rainwater calculator to find this.
3. Choose your pump type and size based on flow rate needed and whether you'll use DC or AC. Calculate daily pump runtime at your peak demand.
4. Add water system electrical loads to your solar load list — pump, UV, heated waterers. These are often the most overlooked items.
5. Size your solar array and battery bank with the full load, including water system loads, using the solar calculator.
6. Check the seasonal alignment — does your solar production in December cover your winter pump and heated waterer loads? Does your tank capacity bridge your summer demand gap?
7. Adjust one or both systems until they work together — more tank capacity, more panels, gravity assistance, or seasonal behavioral adjustments.

What a Combined System Looks Like in Practice

For a small off-grid homestead in middle Tennessee — a family of four, a modest garden, a small flock, and a couple of goats — a combined system might look like:

• Roof collection area: 1,500–2,000 sq ft
• Storage: 5,000–7,500 gallon tank (concrete cistern or linked polyethylene tanks)
• Pump: 12V DC submersible, 3–5 GPM, gravity-fed to garden, pressure pump for household
• Solar array: 2,400–3,200W (6–8 panels at 400W)
• Battery bank: 15–20 kWh LFP (covers 3 days autonomy including pump loads)
• Winter supplementation: Small propane backup for heated waterers, or oversized battery bank

That system, designed together, works. The same components designed separately often don't — either the tank is undersized for the solar-powered pump's capacity, or the solar system is spec'd without the pump load and falls short in summer.

The Honest Starting Point for Families Still Planning

If you're in the same place we are — planning ahead before the land is ready — the most useful thing you can do right now is document your current water use. Track it for a week. Note how many times you run the washing machine, how long showers run, how many times you fill a pot for cooking. That baseline tells you what you're trying to replicate off-grid, and where the opportunities to reduce load are before you ever build anything.

Most families find, when they look honestly at the numbers, that they're using 50–80 gallons per person per day on municipal water — and that a thoughtful off-grid household can get to 10–20 gallons per person per day without meaningful sacrifice. That reduction cuts your pump size, your tank size, and your solar load simultaneously. The planning pays for itself before you spend a dollar on hardware.