From Your Budget to Battery and Panel Size

You've got a daily watt-hour total. Now what? This page turns it into two rough numbers: how much battery to hold, and how many panels to refill it. They're for planning and for talking to a supplier or an electrician. They aren't a design.

Why you need a battery at all

Demand from your loads doesn't always match what the panels make at that moment, so stand-alone systems generally use battery banks¹. The battery stores energy when there's extra and gives it back when it's needed¹.

Step 1: Start from the corrected budget

Use the total from your budget, then allow for what the inverter wastes, as covered on the page about how inverters waste energy. Low efficiency loads can increase energy consumption too¹, which is one more reason to measure.

Step 2: Decide how many days of backup

This is your call, not a formula. Count the cloudy days you want to ride through without recharging. One day is thin. More days means a bigger, dearer battery.

Step 3: Allow for the usable share

You can't drain a battery to zero without shortening its life. Depth of discharge (DoD) is how much of the capacity you take out. For almost all rechargeable types there's a link between depth of discharge and cycle life².

Two sources give examples:

These are examples, not rules for your battery. Use the limits in the maker's manual for your model.

Worked example

Say your corrected budget is 1,000 Wh a day. That round number is invented for the demonstration⁴. You want two days of backup.

• Energy to store for backup: 1,000 Wh × 2 = 2,000 Wh⁴.
• Lead-acid at a 50% depth of discharge: 2,000 ÷ 0.5 = 4,000 Wh of battery³.
• Lithium at a 70% depth of discharge: 2,000 ÷ 0.7 = about 2,860 Wh of battery².

Many batteries are sold in amp-hours, so convert. One watt is what you get from one amp across one volt⁴, and an amp-hour is one amp flowing for one hour⁵. So watt-hours ÷ volts = amp-hours. At 12 V, 4,000 Wh is about 333 Ah, and 2,860 Wh is about 238 Ah. The 12 V figure is just for the example.

The lithium bank comes out smaller on paper for the same backup. That's a property of the depth-of-discharge figures, not a verdict on either type. Price, weight, temperature limits and the maker's charging rules all matter too.

Step 4: Size the panels

Panels have to refill what you use each day. A solar panel's rating is in watts, and the energy it gives is watts times hours of full sun. Sunshine is measured in kilowatt hours per square metre⁶, and it varies by place and season. Get a local figure for your site and for your worst month.

As a made-up example, say your site gets the equivalent of 4 hours of full sun a day in winter. Then 1,000 Wh ÷ 4 h = 250 W of panels, before any losses⁴. Real systems lose some on the way, so plan for more than that.

Safety: this is where a licensed electrician comes in

Batteries can deliver very high current. Before you connect or disconnect a battery cable, isolate the battery first. Then work to the maker's manual for the battery, inverter or fuse involved. If you're unsure, call a licensed electrician. This is never routine DIY.

Lithium batteries need extra care. They can be a safety hazard because they contain a flammable electrolyte, and a damaged one may become pressurized⁷. Failing to follow current and voltage limits can result in excessive heating of the battery⁷. A faulty battery can cause a serious fire⁷, and a short circuit can make a cell overheat and possibly catch fire⁷. So don't use or charge a damaged battery. The maker's charging limits (current and voltage) and storage limits apply to your battery. Look them up in the maker's manual for your model, and don't use numbers from a web page instead.

Fixed mains wiring, switchboards and any grid connection are never DIY. In Australia, that's licensed electrical work. Panels and batteries that feed a building's wiring need a licensed electrician to design and connect them.